diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 5e8bd655..446371ee 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -154,7 +154,7 @@ target_link_libraries(vitastor-nbd
 
 # vitastor-cli
 add_executable(vitastor-cli
-	cli.cpp cli_alloc_osd.cpp cli_simple_offsets.cpp cli_status.cpp cli_df.cpp
+	cli.cpp cli_common.cpp cli_alloc_osd.cpp cli_simple_offsets.cpp cli_status.cpp cli_df.cpp
 	cli_ls.cpp cli_create.cpp cli_modify.cpp cli_flatten.cpp cli_merge.cpp cli_rm_data.cpp cli_rm.cpp
 )
 target_link_libraries(vitastor-cli
diff --git a/src/cli.cpp b/src/cli.cpp
index 78f646fa..b238fee3 100644
--- a/src/cli.cpp
+++ b/src/cli.cpp
@@ -2,8 +2,7 @@
 // License: VNPL-1.1 (see README.md for details)
 
 /**
- * CLI tool
- * Currently can (a) remove inodes and (b) merge snapshot/clone layers
+ * CLI tool and also a library for administrative tasks
  */
 
 #include <vector>
@@ -17,7 +16,9 @@
 
 static const char *exe_name = NULL;
 
-json11::Json::object cli_tool_t::parse_args(int narg, const char *args[])
+static void help();
+
+static json11::Json::object parse_args(int narg, const char *args[])
 {
     json11::Json::object cfg;
     json11::Json::array cmd;
@@ -79,7 +80,7 @@ json11::Json::object cli_tool_t::parse_args(int narg, const char *args[])
     return cfg;
 }
 
-void cli_tool_t::help()
+static void help()
 {
     printf(
         "Vitastor command-line tool\n"
@@ -164,224 +165,126 @@ void cli_tool_t::help()
     exit(0);
 }
 
-void cli_tool_t::change_parent(inode_t cur, inode_t new_parent)
-{
-    auto cur_cfg_it = cli->st_cli.inode_config.find(cur);
-    if (cur_cfg_it == cli->st_cli.inode_config.end())
-    {
-        fprintf(stderr, "Inode 0x%lx disappeared\n", cur);
-        exit(1);
-    }
-    inode_config_t new_cfg = cur_cfg_it->second;
-    std::string cur_name = new_cfg.name;
-    std::string cur_cfg_key = base64_encode(cli->st_cli.etcd_prefix+
-        "/config/inode/"+std::to_string(INODE_POOL(cur))+
-        "/"+std::to_string(INODE_NO_POOL(cur)));
-    new_cfg.parent_id = new_parent;
-    json11::Json::object cur_cfg_json = cli->st_cli.serialize_inode_cfg(&new_cfg);
-    waiting++;
-    cli->st_cli.etcd_txn_slow(json11::Json::object {
-        { "compare", json11::Json::array {
-            json11::Json::object {
-                { "target", "MOD" },
-                { "key", cur_cfg_key },
-                { "result", "LESS" },
-                { "mod_revision", new_cfg.mod_revision+1 },
-            },
-        } },
-        { "success", json11::Json::array {
-            json11::Json::object {
-                { "request_put", json11::Json::object {
-                    { "key", cur_cfg_key },
-                    { "value", base64_encode(json11::Json(cur_cfg_json).dump()) },
-                } }
-            },
-        } },
-    }, [this, new_parent, cur, cur_name](std::string err, json11::Json res)
-    {
-        if (err != "")
-        {
-            fprintf(stderr, "Error changing parent of %s: %s\n", cur_name.c_str(), err.c_str());
-            exit(1);
-        }
-        if (!res["succeeded"].bool_value())
-        {
-            fprintf(stderr, "Inode %s was modified during snapshot deletion\n", cur_name.c_str());
-            exit(1);
-        }
-        if (new_parent)
-        {
-            auto new_parent_it = cli->st_cli.inode_config.find(new_parent);
-            std::string new_parent_name = new_parent_it != cli->st_cli.inode_config.end()
-                ? new_parent_it->second.name : "<unknown>";
-            printf(
-                "Parent of layer %s (inode %lu in pool %u) changed to %s (inode %lu in pool %u)\n",
-                cur_name.c_str(), INODE_NO_POOL(cur), INODE_POOL(cur),
-                new_parent_name.c_str(), INODE_NO_POOL(new_parent), INODE_POOL(new_parent)
-            );
-        }
-        else
-        {
-            printf(
-                "Parent of layer %s (inode %lu in pool %u) detached\n",
-                cur_name.c_str(), INODE_NO_POOL(cur), INODE_POOL(cur)
-            );
-        }
-        waiting--;
-        ringloop->wakeup();
-    });
-}
-
-void cli_tool_t::etcd_txn(json11::Json txn)
-{
-    waiting++;
-    cli->st_cli.etcd_txn_slow(txn, [this](std::string err, json11::Json res)
-    {
-        waiting--;
-        if (err != "")
-        {
-            fprintf(stderr, "Error reading from etcd: %s\n", err.c_str());
-            exit(1);
-        }
-        etcd_result = res;
-        ringloop->wakeup();
-    });
-}
-
-inode_config_t* cli_tool_t::get_inode_cfg(const std::string & name)
-{
-    for (auto & ic: cli->st_cli.inode_config)
-    {
-        if (ic.second.name == name)
-        {
-            return &ic.second;
-        }
-    }
-    fprintf(stderr, "Layer %s not found\n", name.c_str());
-    exit(1);
-}
-
-void cli_tool_t::run(json11::Json cfg)
+static int run(cli_tool_t *p, json11::Json cfg)
 {
+    p->parse_config(cfg);
     json11::Json::array cmd = cfg["command"].array_items();
+    std::function<bool(cli_result_t &)> action_cb;
     if (!cmd.size())
     {
         fprintf(stderr, "command is missing\n");
-        exit(1);
+        return EINVAL;
     }
     else if (cmd[0] == "status")
     {
         // Show cluster status
-        action_cb = start_status(cfg);
+        action_cb = p->start_status(cfg);
     }
     else if (cmd[0] == "df")
     {
         // Show pool space stats
-        action_cb = start_df(cfg);
+        action_cb = p->start_df(cfg);
     }
     else if (cmd[0] == "ls")
     {
         // List images
-        action_cb = start_ls(cfg);
+        action_cb = p->start_ls(cfg);
     }
     else if (cmd[0] == "create" || cmd[0] == "snap-create")
     {
         // Create image/snapshot
-        action_cb = start_create(cfg);
+        action_cb = p->start_create(cfg);
     }
     else if (cmd[0] == "modify")
     {
         // Modify image
-        action_cb = start_modify(cfg);
+        action_cb = p->start_modify(cfg);
     }
     else if (cmd[0] == "rm-data")
     {
         // Delete inode data
-        action_cb = start_rm(cfg);
+        action_cb = p->start_rm(cfg);
     }
     else if (cmd[0] == "merge-data")
     {
         // Merge layer data without affecting metadata
-        action_cb = start_merge(cfg);
+        action_cb = p->start_merge(cfg);
     }
     else if (cmd[0] == "flatten")
     {
         // Merge layer data without affecting metadata
-        action_cb = start_flatten(cfg);
+        action_cb = p->start_flatten(cfg);
     }
     else if (cmd[0] == "rm")
     {
         // Remove multiple snapshots and rebase their children
-        action_cb = start_snap_rm(cfg);
+        action_cb = p->start_snap_rm(cfg);
     }
     else if (cmd[0] == "alloc-osd")
     {
         // Allocate a new OSD number
-        action_cb = start_alloc_osd(cfg);
+        action_cb = p->start_alloc_osd(cfg);
     }
     else if (cmd[0] == "simple-offsets")
     {
         // Calculate offsets for simple & stupid OSD deployment without superblock
-        action_cb = simple_offsets(cfg);
+        action_cb = p->simple_offsets(cfg);
     }
     else
     {
         fprintf(stderr, "unknown command: %s\n", cmd[0].string_value().c_str());
-        exit(1);
+        return EINVAL;
     }
     if (action_cb == NULL)
     {
-        return;
+        return 0;
     }
-    color = !cfg["no-color"].bool_value();
-    json_output = cfg["json"].bool_value();
-    iodepth = cfg["iodepth"].uint64_value();
-    if (!iodepth)
-        iodepth = 32;
-    parallel_osds = cfg["parallel_osds"].uint64_value();
-    if (!parallel_osds)
-        parallel_osds = 4;
-    log_level = cfg["log_level"].int64_value();
-    progress = cfg["progress"].uint64_value() ? true : false;
-    list_first = cfg["wait-list"].uint64_value() ? true : false;
     // Create client
-    ringloop = new ring_loop_t(512);
-    epmgr = new epoll_manager_t(ringloop);
-    cli = new cluster_client_t(ringloop, epmgr->tfd, cfg);
+    p->ringloop = new ring_loop_t(512);
+    p->epmgr = new epoll_manager_t(p->ringloop);
+    p->cli = new cluster_client_t(p->ringloop, p->epmgr->tfd, cfg);
     // Smaller timeout by default for more interactiveness
-    cli->st_cli.etcd_slow_timeout = cli->st_cli.etcd_quick_timeout;
-    cli->on_ready([this]()
+    p->cli->st_cli.etcd_slow_timeout = p->cli->st_cli.etcd_quick_timeout;
+    ring_consumer_t consumer;
+    cli_result_t result;
+    p->cli->on_ready([&]()
     {
         // Initialize job
-        consumer.loop = [this]()
+        consumer.loop = [&]()
         {
             if (action_cb != NULL)
             {
-                bool done = action_cb();
+                bool done = action_cb(result);
                 if (done)
                 {
                     action_cb = NULL;
                 }
             }
-            ringloop->submit();
+            p->ringloop->submit();
         };
-        ringloop->register_consumer(&consumer);
+        p->ringloop->register_consumer(&consumer);
         consumer.loop();
     });
     // Loop until it completes
     while (action_cb != NULL)
     {
-        ringloop->loop();
+        p->ringloop->loop();
         if (action_cb != NULL)
-            ringloop->wait();
+            p->ringloop->wait();
+    }
+    // Print result
+    if (result.text != "")
+    {
+        fprintf(stderr, "%s\n", result.text.c_str());
     }
     // Destroy the client
-    delete cli;
-    delete epmgr;
-    delete ringloop;
-    cli = NULL;
-    epmgr = NULL;
-    ringloop = NULL;
+    delete p->cli;
+    delete p->epmgr;
+    delete p->ringloop;
+    p->cli = NULL;
+    p->epmgr = NULL;
+    p->ringloop = NULL;
+    return result.err;
 }
 
 int main(int narg, const char *args[])
@@ -390,7 +293,7 @@ int main(int narg, const char *args[])
     setvbuf(stderr, NULL, _IONBF, 0);
     exe_name = args[0];
     cli_tool_t *p = new cli_tool_t();
-    p->run(cli_tool_t::parse_args(narg, args));
+    int r = run(p, parse_args(narg, args));
     delete p;
-    return 0;
+    return r;
 }
diff --git a/src/cli.h b/src/cli.h
index f7a9b8f3..53f10c5d 100644
--- a/src/cli.h
+++ b/src/cli.h
@@ -19,6 +19,13 @@ class epoll_manager_t;
 class cluster_client_t;
 struct inode_config_t;
 
+struct cli_result_t
+{
+    int err;
+    std::string text;
+    json11::Json data;
+};
+
 class cli_tool_t
 {
 public:
@@ -35,33 +42,28 @@ public:
 
     int waiting = 0;
     json11::Json etcd_result;
-    ring_consumer_t consumer;
-    std::function<bool(void)> action_cb;
 
-    void run(json11::Json cfg);
+    void parse_config(json11::Json cfg);
 
     void change_parent(inode_t cur, inode_t new_parent);
     inode_config_t* get_inode_cfg(const std::string & name);
 
-    static json11::Json::object parse_args(int narg, const char *args[]);
-    static void help();
-
     friend struct rm_inode_t;
     friend struct snap_merger_t;
     friend struct snap_flattener_t;
     friend struct snap_remover_t;
 
-    std::function<bool(void)> start_status(json11::Json cfg);
-    std::function<bool(void)> start_df(json11::Json);
-    std::function<bool(void)> start_ls(json11::Json);
-    std::function<bool(void)> start_create(json11::Json);
-    std::function<bool(void)> start_modify(json11::Json);
-    std::function<bool(void)> start_rm(json11::Json);
-    std::function<bool(void)> start_merge(json11::Json);
-    std::function<bool(void)> start_flatten(json11::Json);
-    std::function<bool(void)> start_snap_rm(json11::Json);
-    std::function<bool(void)> start_alloc_osd(json11::Json cfg, uint64_t *out = NULL);
-    std::function<bool(void)> simple_offsets(json11::Json cfg);
+    std::function<bool(cli_result_t &)> start_status(json11::Json);
+    std::function<bool(cli_result_t &)> start_df(json11::Json);
+    std::function<bool(cli_result_t &)> start_ls(json11::Json);
+    std::function<bool(cli_result_t &)> start_create(json11::Json);
+    std::function<bool(cli_result_t &)> start_modify(json11::Json);
+    std::function<bool(cli_result_t &)> start_rm(json11::Json);
+    std::function<bool(cli_result_t &)> start_merge(json11::Json);
+    std::function<bool(cli_result_t &)> start_flatten(json11::Json);
+    std::function<bool(cli_result_t &)> start_snap_rm(json11::Json);
+    std::function<bool(cli_result_t &)> start_alloc_osd(json11::Json cfg);
+    std::function<bool(cli_result_t &)> simple_offsets(json11::Json cfg);
 
     void etcd_txn(json11::Json txn);
 };
diff --git a/src/cli_alloc_osd.cpp b/src/cli_alloc_osd.cpp
index 676de69b..ad6bc509 100644
--- a/src/cli_alloc_osd.cpp
+++ b/src/cli_alloc_osd.cpp
@@ -102,20 +102,20 @@ struct alloc_osd_t
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_alloc_osd(json11::Json cfg, uint64_t *out)
+std::function<bool(cli_result_t &)> cli_tool_t::start_alloc_osd(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto alloc_osd = new alloc_osd_t();
     alloc_osd->parent = this;
-    return [alloc_osd, out]()
+    return [alloc_osd](cli_result_t & result)
     {
         alloc_osd->loop();
         if (alloc_osd->is_done())
         {
-            if (out)
-                *out = alloc_osd->new_id;
-            else if (alloc_osd->new_id)
-                printf("%lu\n", alloc_osd->new_id);
+            result = (cli_result_t){
+                .text = std::to_string(alloc_osd->new_id),
+                .data = json11::Json(alloc_osd->new_id),
+            };
             delete alloc_osd;
             return true;
         }
diff --git a/src/cli_common.cpp b/src/cli_common.cpp
new file mode 100644
index 00000000..518cfb2b
--- /dev/null
+++ b/src/cli_common.cpp
@@ -0,0 +1,118 @@
+// Copyright (c) Vitaliy Filippov, 2019+
+// License: VNPL-1.1 (see README.md for details)
+
+#include "base64.h"
+#include "cluster_client.h"
+#include "cli.h"
+
+void cli_tool_t::change_parent(inode_t cur, inode_t new_parent)
+{
+    auto cur_cfg_it = cli->st_cli.inode_config.find(cur);
+    if (cur_cfg_it == cli->st_cli.inode_config.end())
+    {
+        fprintf(stderr, "Inode 0x%lx disappeared\n", cur);
+        exit(1);
+    }
+    inode_config_t new_cfg = cur_cfg_it->second;
+    std::string cur_name = new_cfg.name;
+    std::string cur_cfg_key = base64_encode(cli->st_cli.etcd_prefix+
+        "/config/inode/"+std::to_string(INODE_POOL(cur))+
+        "/"+std::to_string(INODE_NO_POOL(cur)));
+    new_cfg.parent_id = new_parent;
+    json11::Json::object cur_cfg_json = cli->st_cli.serialize_inode_cfg(&new_cfg);
+    waiting++;
+    cli->st_cli.etcd_txn_slow(json11::Json::object {
+        { "compare", json11::Json::array {
+            json11::Json::object {
+                { "target", "MOD" },
+                { "key", cur_cfg_key },
+                { "result", "LESS" },
+                { "mod_revision", new_cfg.mod_revision+1 },
+            },
+        } },
+        { "success", json11::Json::array {
+            json11::Json::object {
+                { "request_put", json11::Json::object {
+                    { "key", cur_cfg_key },
+                    { "value", base64_encode(json11::Json(cur_cfg_json).dump()) },
+                } }
+            },
+        } },
+    }, [this, new_parent, cur, cur_name](std::string err, json11::Json res)
+    {
+        if (err != "")
+        {
+            fprintf(stderr, "Error changing parent of %s: %s\n", cur_name.c_str(), err.c_str());
+            exit(1);
+        }
+        if (!res["succeeded"].bool_value())
+        {
+            fprintf(stderr, "Inode %s was modified during snapshot deletion\n", cur_name.c_str());
+            exit(1);
+        }
+        if (new_parent)
+        {
+            auto new_parent_it = cli->st_cli.inode_config.find(new_parent);
+            std::string new_parent_name = new_parent_it != cli->st_cli.inode_config.end()
+                ? new_parent_it->second.name : "<unknown>";
+            printf(
+                "Parent of layer %s (inode %lu in pool %u) changed to %s (inode %lu in pool %u)\n",
+                cur_name.c_str(), INODE_NO_POOL(cur), INODE_POOL(cur),
+                new_parent_name.c_str(), INODE_NO_POOL(new_parent), INODE_POOL(new_parent)
+            );
+        }
+        else
+        {
+            printf(
+                "Parent of layer %s (inode %lu in pool %u) detached\n",
+                cur_name.c_str(), INODE_NO_POOL(cur), INODE_POOL(cur)
+            );
+        }
+        waiting--;
+        ringloop->wakeup();
+    });
+}
+
+void cli_tool_t::etcd_txn(json11::Json txn)
+{
+    waiting++;
+    cli->st_cli.etcd_txn_slow(txn, [this](std::string err, json11::Json res)
+    {
+        waiting--;
+        if (err != "")
+        {
+            fprintf(stderr, "Error reading from etcd: %s\n", err.c_str());
+            exit(1);
+        }
+        etcd_result = res;
+        ringloop->wakeup();
+    });
+}
+
+inode_config_t* cli_tool_t::get_inode_cfg(const std::string & name)
+{
+    for (auto & ic: cli->st_cli.inode_config)
+    {
+        if (ic.second.name == name)
+        {
+            return &ic.second;
+        }
+    }
+    fprintf(stderr, "Layer %s not found\n", name.c_str());
+    exit(1);
+}
+
+void cli_tool_t::parse_config(json11::Json cfg)
+{
+    color = !cfg["no-color"].bool_value();
+    json_output = cfg["json"].bool_value();
+    iodepth = cfg["iodepth"].uint64_value();
+    if (!iodepth)
+        iodepth = 32;
+    parallel_osds = cfg["parallel_osds"].uint64_value();
+    if (!parallel_osds)
+        parallel_osds = 4;
+    log_level = cfg["log_level"].int64_value();
+    progress = cfg["progress"].uint64_value() ? true : false;
+    list_first = cfg["wait-list"].uint64_value() ? true : false;
+}
diff --git a/src/cli_create.cpp b/src/cli_create.cpp
index 2896c602..aedf0a88 100644
--- a/src/cli_create.cpp
+++ b/src/cli_create.cpp
@@ -33,6 +33,7 @@ struct image_creator_t
     uint64_t max_id_mod_rev = 0, cfg_mod_rev = 0, idx_mod_rev = 0;
 
     int state = 0;
+    cli_result_t result;
 
     bool is_done()
     {
@@ -43,13 +44,27 @@ struct image_creator_t
     {
         if (state >= 1)
             goto resume_1;
+        if (image_name == "")
+        {
+            // FIXME: EINVAL -> specific codes for every error
+            result = (cli_result_t){ .err = EINVAL, .text = "Image name is missing" };
+            state = 100;
+            return;
+        }
+        if (image_name.find('@') != std::string::npos)
+        {
+            result = (cli_result_t){ .err = EINVAL, .text = "Image name can't contain @ character" };
+            state = 100;
+            return;
+        }
         if (new_pool_id)
         {
             auto & pools = parent->cli->st_cli.pool_config;
             if (pools.find(new_pool_id) == pools.end())
             {
-                fprintf(stderr, "Pool %u does not exist\n", new_pool_id);
-                exit(1);
+                result = (cli_result_t){ .err = ENOENT, .text = "Pool "+std::to_string(new_pool_id)+" does not exist" };
+                state = 100;
+                return;
             }
         }
         else if (new_pool_name != "")
@@ -64,8 +79,9 @@ struct image_creator_t
             }
             if (!new_pool_id)
             {
-                fprintf(stderr, "Pool %s does not exist\n", new_pool_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = ENOENT, .text = "Pool "+new_pool_name+" does not exist" };
+                state = 100;
+                return;
             }
         }
         else if (parent->cli->st_cli.pool_config.size() == 1)
@@ -91,8 +107,9 @@ struct image_creator_t
         {
             if (ic.second.name == image_name)
             {
-                fprintf(stderr, "Image %s already exists\n", image_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EEXIST, .text = "Image "+image_name+" already exists" };
+                state = 100;
+                return;
             }
             if (ic.second.name == new_parent)
             {
@@ -109,18 +126,21 @@ struct image_creator_t
         }
         if (new_parent != "" && !new_parent_id)
         {
-            fprintf(stderr, "Parent image not found\n");
-            exit(1);
+            result = (cli_result_t){ .err = ENOENT, .text = "Parent image "+new_parent+" not found" };
+            state = 100;
+            return;
         }
         if (!new_pool_id)
         {
-            fprintf(stderr, "Pool name or ID is missing\n");
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Pool name or ID is missing" };
+            state = 100;
+            return;
         }
         if (!size)
         {
-            fprintf(stderr, "Image size is missing\n");
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Image size is missing" };
+            state = 100;
+            return;
         }
         do
         {
@@ -140,14 +160,12 @@ resume_3:
             if (!parent->etcd_result["succeeded"].bool_value() &&
                 parent->etcd_result["responses"][0]["response_range"]["kvs"].array_items().size() > 0)
             {
-                fprintf(stderr, "Image %s already exists\n", image_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EEXIST, .text = "Image "+image_name+" already exists" };
+                state = 100;
+                return;
             }
         } while (!parent->etcd_result["succeeded"].bool_value());
-        if (parent->progress)
-        {
-            printf("Image %s created\n", image_name.c_str());
-        }
+        result = (cli_result_t){ .err = 0, .text = "Image "+image_name+" created" };
         state = 100;
     }
 
@@ -163,14 +181,16 @@ resume_3:
         {
             if (ic.second.name == image_name+"@"+new_snap)
             {
-                fprintf(stderr, "Snapshot %s@%s already exists\n", image_name.c_str(), new_snap.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EEXIST, .text = "Snapshot "+image_name+"@"+new_snap+" already exists" };
+                state = 100;
+                return;
             }
         }
         if (new_parent != "")
         {
-            fprintf(stderr, "--parent can't be used with snapshots\n");
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Parent can't be specified for snapshots" };
+            state = 100;
+            return;
         }
         do
         {
@@ -182,8 +202,9 @@ resume_3:
                 return;
             if (!old_id)
             {
-                fprintf(stderr, "Image %s does not exist\n", image_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = ENOENT, .text = "Image "+image_name+" does not exist" };
+                state = 100;
+                return;
             }
             if (!new_pool_id)
             {
@@ -198,14 +219,12 @@ resume_4:
             if (!parent->etcd_result["succeeded"].bool_value() &&
                 parent->etcd_result["responses"][0]["response_range"]["kvs"].array_items().size() > 0)
             {
-                fprintf(stderr, "Snapshot %s@%s already exists\n", image_name.c_str(), new_snap.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EEXIST, .text = "Snapshot "+image_name+"@"+new_snap+" already exists" };
+                state = 100;
+                return;
             }
         } while (!parent->etcd_result["succeeded"].bool_value());
-        if (parent->progress)
-        {
-            printf("Snapshot %s@%s created\n", image_name.c_str(), new_snap.c_str());
-        }
+        result = (cli_result_t){ .err = 0, .text = "Snapshot "+image_name+"@"+new_snap+" created" };
         state = 100;
     }
 
@@ -288,8 +307,9 @@ resume_2:
                 idx_mod_rev = kv.mod_revision;
                 if (!old_id || !old_pool_id || old_pool_id >= POOL_ID_MAX)
                 {
-                    fprintf(stderr, "Invalid pool or inode ID in etcd key %s\n", kv.key.c_str());
-                    exit(1);
+                    result = (cli_result_t){ .err = ENOENT, .text = "Invalid pool or inode ID in etcd key "+kv.key };
+                    state = 100;
+                    return;
                 }
             }
             parent->etcd_txn(json11::Json::object {
@@ -469,13 +489,12 @@ uint64_t parse_size(std::string size_str)
     uint64_t size = json11::Json(size_str).uint64_value() * mul;
     if (size == 0 && size_str != "0" && (size_str != "" || mul != 1))
     {
-        fprintf(stderr, "Invalid syntax for size: %s\n", size_str.c_str());
-        exit(1);
+        return UINT64_MAX;
     }
     return size;
 }
 
-std::function<bool(void)> cli_tool_t::start_create(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_create(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto image_creator = new image_creator_t();
@@ -492,8 +511,12 @@ std::function<bool(void)> cli_tool_t::start_create(json11::Json cfg)
         int p = image_creator->image_name.find('@');
         if (p == std::string::npos || p == image_creator->image_name.length()-1)
         {
-            fprintf(stderr, "Please specify new snapshot name after @\n");
-            exit(1);
+            delete image_creator;
+            return [](cli_result_t & result)
+            {
+                result = (cli_result_t){ .err = EINVAL, .text = "Please specify new snapshot name after @" };
+                return true;
+            };
         }
         image_creator->new_snap = image_creator->image_name.substr(p + 1);
         image_creator->image_name = image_creator->image_name.substr(0, p);
@@ -502,32 +525,39 @@ std::function<bool(void)> cli_tool_t::start_create(json11::Json cfg)
     if (cfg["size"].string_value() != "")
     {
         image_creator->size = parse_size(cfg["size"].string_value());
+        if (image_creator->size == UINT64_MAX)
+        {
+            return [size = cfg["size"].string_value()](cli_result_t & result)
+            {
+                result = (cli_result_t){ .err = EINVAL, .text = "Invalid syntax for size: "+size };
+                return true;
+            };
+        }
         if (image_creator->size % 4096)
         {
-            fprintf(stderr, "Size should be a multiple of 4096\n");
-            exit(1);
+            delete image_creator;
+            return [](cli_result_t & result)
+            {
+                result = (cli_result_t){ .err = EINVAL, .text = "Size should be a multiple of 4096" };
+                return true;
+            };
         }
         if (image_creator->new_snap != "")
         {
-            fprintf(stderr, "--size can't be specified for snapshots\n");
-            exit(1);
+            delete image_creator;
+            return [](cli_result_t & result)
+            {
+                result = (cli_result_t){ .err = EINVAL, .text = "Size can't be specified for snapshots" };
+                return true;
+            };
         }
     }
-    if (image_creator->image_name == "")
-    {
-        fprintf(stderr, "Image name is missing\n");
-        exit(1);
-    }
-    if (image_creator->image_name.find('@') != std::string::npos)
-    {
-        fprintf(stderr, "Image name can't contain @ character\n");
-        exit(1);
-    }
-    return [image_creator]()
+    return [image_creator](cli_result_t & result)
     {
         image_creator->loop();
         if (image_creator->is_done())
         {
+            result = image_creator->result;
             delete image_creator;
             return true;
         }
diff --git a/src/cli_df.cpp b/src/cli_df.cpp
index 423bd2f2..887cf53a 100644
--- a/src/cli_df.cpp
+++ b/src/cli_df.cpp
@@ -12,6 +12,7 @@ struct pool_lister_t
 
     int state = 0;
     json11::Json space_info;
+    cli_result_t result;
     std::map<pool_id_t, json11::Json::object> pool_stats;
 
     bool is_done()
@@ -150,10 +151,10 @@ resume_1:
         get_stats();
         if (parent->waiting > 0)
             return;
+        result.data = to_list();
         if (parent->json_output)
         {
             // JSON output
-            printf("%s\n", json11::Json(to_list()).dump().c_str());
             state = 100;
             return;
         }
@@ -206,21 +207,22 @@ resume_1:
                 : 100)+"%";
             kv.second["eff_fmt"] = format_q(kv.second["space_efficiency"].number_value()*100)+"%";
         }
-        printf("%s", print_table(to_list(), cols, parent->color).c_str());
+        result.text = print_table(result.data, cols, parent->color);
         state = 100;
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_df(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_df(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto lister = new pool_lister_t();
     lister->parent = this;
-    return [lister]()
+    return [lister](cli_result_t & result)
     {
         lister->loop();
         if (lister->is_done())
         {
+            result = lister->result;
             delete lister;
             return true;
         }
diff --git a/src/cli_flatten.cpp b/src/cli_flatten.cpp
index bcd97484..b18c11b3 100644
--- a/src/cli_flatten.cpp
+++ b/src/cli_flatten.cpp
@@ -22,7 +22,8 @@ struct snap_flattener_t
     std::string top_parent_name;
     inode_t target_id = 0;
     int state = 0;
-    std::function<bool(void)> merger_cb;
+    std::function<bool(cli_result_t &)> merger_cb;
+    cli_result_t result;
 
     void get_merge_parents()
     {
@@ -37,23 +38,34 @@ struct snap_flattener_t
             auto it = parent->cli->st_cli.inode_config.find(cur->parent_id);
             if (it == parent->cli->st_cli.inode_config.end())
             {
-                fprintf(stderr, "Parent inode of layer %s (id %ld) not found\n", cur->name.c_str(), cur->parent_id);
-                exit(1);
+                result = (cli_result_t){
+                    .err = ENOENT,
+                    .text = "Parent inode of layer "+cur->name+" (id "+std::to_string(cur->parent_id)+") does not exist",
+                    .data = json11::Json::object {
+                        { "error", "parent-not-found" },
+                        { "inode_id", cur->num },
+                        { "inode_name", cur->name },
+                        { "parent_id", cur->parent_id },
+                    },
+                };
+                state = 100;
+                return;
             }
             cur = &it->second;
             chain_list.push_back(cur->num);
         }
         if (cur->parent_id != 0)
         {
-            fprintf(stderr, "Layer %s has a loop in parents\n", target_name.c_str());
-            exit(1);
+            result = (cli_result_t){ .err = EBADF, .text = "Layer "+target_name+" has a loop in parents" };
+            state = 100;
+            return;
         }
         top_parent_name = cur->name;
     }
 
     bool is_done()
     {
-        return state == 5;
+        return state == 100;
     }
 
     void loop()
@@ -64,8 +76,16 @@ struct snap_flattener_t
             goto resume_2;
         else if (state == 3)
             goto resume_3;
+        if (target_name == "")
+        {
+            result = (cli_result_t){ .err = EINVAL, .text = "Layer to flatten not specified" };
+            state = 100;
+            return;
+        }
         // Get parent layers
         get_merge_parents();
+        if (state == 100)
+            return;
         // Start merger
         merger_cb = parent->start_merge(json11::Json::object {
             { "command", json11::Json::array{ "merge-data", top_parent_name, target_name } },
@@ -76,12 +96,17 @@ struct snap_flattener_t
         });
         // Wait for it
 resume_1:
-        while (!merger_cb())
+        while (!merger_cb(result))
         {
             state = 1;
             return;
         }
         merger_cb = NULL;
+        if (result.err)
+        {
+            state = 100;
+            return;
+        }
         // Change parent
         parent->change_parent(target_id, 0);
         // Wait for it to complete
@@ -92,31 +117,27 @@ resume_2:
         state = 3;
 resume_3:
         // Done
-        return;
+        state = 100;
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_flatten(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_flatten(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto flattener = new snap_flattener_t();
     flattener->parent = this;
     flattener->target_name = cmd.size() > 1 ? cmd[1].string_value() : "";
-    if (flattener->target_name == "")
-    {
-        fprintf(stderr, "Layer to flatten argument is missing\n");
-        exit(1);
-    }
     flattener->fsync_interval = cfg["fsync-interval"].uint64_value();
     if (!flattener->fsync_interval)
         flattener->fsync_interval = 128;
     if (!cfg["cas"].is_null())
         flattener->use_cas = cfg["cas"].uint64_value() ? 2 : 0;
-    return [flattener]()
+    return [flattener](cli_result_t & result)
     {
         flattener->loop();
         if (flattener->is_done())
         {
+            result = flattener->result;
             delete flattener;
             return true;
         }
diff --git a/src/cli_ls.cpp b/src/cli_ls.cpp
index 4e15aba2..585f61b8 100644
--- a/src/cli_ls.cpp
+++ b/src/cli_ls.cpp
@@ -24,6 +24,7 @@ struct image_lister_t
     int state = 0;
     std::map<inode_t, json11::Json::object> stats;
     json11::Json space_info;
+    cli_result_t result;
 
     bool is_done()
     {
@@ -44,8 +45,9 @@ struct image_lister_t
             }
             if (!list_pool_id)
             {
-                fprintf(stderr, "Pool %s does not exist\n", list_pool_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = ENOENT, .text = "Pool "+list_pool_name+" does not exist" };
+                state = 100;
+                return;
             }
         }
         for (auto & ic: parent->cli->st_cli.inode_config)
@@ -246,10 +248,10 @@ resume_1:
             if (parent->waiting > 0)
                 return;
         }
+        result.data = to_list();
         if (parent->json_output)
         {
             // JSON output
-            printf("%s\n", json11::Json(to_list()).dump().c_str());
             state = 100;
             return;
         }
@@ -359,7 +361,7 @@ resume_1:
             kv.second["size_fmt"] = format_size(kv.second["size"].uint64_value());
             kv.second["ro"] = kv.second["readonly"].bool_value() ? "RO" : "-";
         }
-        printf("%s", print_table(to_list(), cols, parent->color).c_str());
+        result.text = print_table(to_list(), cols, parent->color);
         state = 100;
     }
 };
@@ -546,7 +548,7 @@ back:
     return true;
 }
 
-std::function<bool(void)> cli_tool_t::start_ls(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_ls(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto lister = new image_lister_t();
@@ -562,11 +564,12 @@ std::function<bool(void)> cli_tool_t::start_ls(json11::Json cfg)
     {
         lister->only_names.insert(cmd[i].string_value());
     }
-    return [lister]()
+    return [lister](cli_result_t & result)
     {
         lister->loop();
         if (lister->is_done())
         {
+            result = lister->result;
             delete lister;
             return true;
         }
diff --git a/src/cli_merge.cpp b/src/cli_merge.cpp
index 51c947fc..373f7c03 100644
--- a/src/cli_merge.cpp
+++ b/src/cli_merge.cpp
@@ -12,6 +12,9 @@ struct snap_rw_op_t
     cluster_op_t op;
     int todo = 0;
     uint32_t start = 0, end = 0;
+    int error_code = 0;
+    uint64_t error_offset = 0;
+    bool error_read = false;
 };
 
 // Layer merge is the base for multiple operations:
@@ -54,17 +57,27 @@ struct snap_merger_t
     uint64_t last_written_offset = 0;
     int deleted_unsynced = 0;
     uint64_t processed = 0, to_process = 0;
+    std::string rwo_error;
+
+    cli_result_t result;
 
     void start_merge()
     {
+        if (from_name == "" || to_name == "")
+        {
+            result = (cli_result_t){ .err = EINVAL, .text = "Beginning or end of the merge sequence is missing" };
+            state = 100;
+            return;
+        }
         check_delete_source = delete_source || check_delete_source;
         inode_config_t *from_cfg = parent->get_inode_cfg(from_name);
         inode_config_t *to_cfg = parent->get_inode_cfg(to_name);
         inode_config_t *target_cfg = target_name == "" ? from_cfg : parent->get_inode_cfg(target_name);
         if (to_cfg->num == from_cfg->num)
         {
-            fprintf(stderr, "Only one layer specified, nothing to merge\n");
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Only one layer specified, nothing to merge" };
+            state = 100;
+            return;
         }
         // Check that to_cfg is actually a child of from_cfg and target_cfg is somewhere between them
         std::vector<inode_t> chain_list;
@@ -78,8 +91,18 @@ struct snap_merger_t
             auto it = parent->cli->st_cli.inode_config.find(cur->parent_id);
             if (it == parent->cli->st_cli.inode_config.end())
             {
-                fprintf(stderr, "Parent inode of layer %s (id %ld) not found\n", cur->name.c_str(), cur->parent_id);
-                exit(1);
+                result = (cli_result_t){
+                    .err = ENOENT,
+                    .text = "Parent inode of layer "+cur->name+" (id "+std::to_string(cur->parent_id)+") does not exist",
+                    .data = json11::Json::object {
+                        { "error", "parent-not-found" },
+                        { "inode_id", cur->num },
+                        { "inode_name", cur->name },
+                        { "parent_id", cur->parent_id },
+                    },
+                };
+                state = 100;
+                return;
             }
             cur = &it->second;
             chain_list.push_back(cur->num);
@@ -87,8 +110,9 @@ struct snap_merger_t
         }
         if (cur->parent_id != from_cfg->num)
         {
-            fprintf(stderr, "Layer %s is not a child of %s\n", to_name.c_str(), from_name.c_str());
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Layer "+to_name+" is not a child of "+from_name };
+            state = 100;
+            return;
         }
         chain_list.push_back(from_cfg->num);
         layer_block_size[from_cfg->num] = get_block_size(from_cfg->num);
@@ -99,8 +123,9 @@ struct snap_merger_t
         }
         if (sources.find(target_cfg->num) == sources.end())
         {
-            fprintf(stderr, "Layer %s is not between %s and %s\n", target_name.c_str(), to_name.c_str(), from_name.c_str());
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Layer "+target_name+" is not between "+to_name+" and "+from_name };
+            state = 100;
+            return;
         }
         target = target_cfg->num;
         target_rank = sources.at(target);
@@ -130,14 +155,15 @@ struct snap_merger_t
                     int parent_rank = it->second;
                     if (parent_rank < to_rank && (parent_rank >= target_rank || check_delete_source))
                     {
-                        fprintf(
-                            stderr, "Layers at or above %s, but below %s are not allowed"
-                                " to have other children, but %s is a child of %s\n",
-                            (check_delete_source ? from_name.c_str() : target_name.c_str()),
-                            to_name.c_str(), ic.second.name.c_str(),
-                            parent->cli->st_cli.inode_config.at(ic.second.parent_id).name.c_str()
-                        );
-                        exit(1);
+                        result = (cli_result_t){
+                            .err = EINVAL,
+                            .text = "Layers at or above "+(check_delete_source ? from_name : target_name)+
+                                ", but below "+to_name+" are not allowed to have other children, but "+
+                                ic.second.name+" is a child of "+
+                                parent->cli->st_cli.inode_config.at(ic.second.parent_id).name,
+                        };
+                        state = 100;
+                        return;
                     }
                     if (parent_rank >= to_rank)
                     {
@@ -152,11 +178,14 @@ struct snap_merger_t
             use_cas = 0;
         }
         sources.erase(target);
-        printf(
-            "Merging %ld layer(s) into target %s%s (inode %lu in pool %u)\n",
-            sources.size(), target_cfg->name.c_str(),
-            use_cas ? " online (with CAS)" : "", INODE_NO_POOL(target), INODE_POOL(target)
-        );
+        if (parent->progress)
+        {
+            printf(
+                "Merging %ld layer(s) into target %s%s (inode %lu in pool %u)\n",
+                sources.size(), target_cfg->name.c_str(),
+                use_cas ? " online (with CAS)" : "", INODE_NO_POOL(target), INODE_POOL(target)
+            );
+        }
         target_block_size = get_block_size(target);
     }
 
@@ -253,7 +282,8 @@ struct snap_merger_t
         oit = merge_offsets.begin();
     resume_5:
         // Now read, overwrite and optionally delete offsets one by one
-        while (in_flight < parent->iodepth*parent->parallel_osds && oit != merge_offsets.end())
+        while (in_flight < parent->iodepth*parent->parallel_osds &&
+            oit != merge_offsets.end() && !rwo_error.size())
         {
             in_flight++;
             read_and_write(*oit);
@@ -264,6 +294,15 @@ struct snap_merger_t
                 printf("\rOverwriting blocks: %lu/%lu", processed, to_process);
             }
         }
+        if (in_flight == 0 && rwo_error.size())
+        {
+            result = (cli_result_t){
+                .err = EIO,
+                .text = rwo_error,
+            };
+            state = 100;
+            return;
+        }
         if (in_flight > 0 || oit != merge_offsets.end())
         {
             // Wait until overwrites finish
@@ -396,8 +435,9 @@ struct snap_merger_t
         {
             if (op->retval != op->len)
             {
-                fprintf(stderr, "error reading target at offset %lx: %s\n", op->offset, strerror(-op->retval));
-                exit(1);
+                rwo->error_code = -op->retval;
+                rwo->error_offset = op->offset;
+                rwo->error_read = true;
             }
             next_write(rwo);
         };
@@ -410,7 +450,7 @@ struct snap_merger_t
         // FIXME: Allow to use single write with "holes" (OSDs don't allow it yet)
         uint32_t gran = parent->cli->get_bs_bitmap_granularity();
         uint64_t bitmap_size = target_block_size / gran;
-        while (rwo->end < bitmap_size)
+        while (rwo->end < bitmap_size && !rwo->error_code)
         {
             auto bit = ((*((uint8_t*)rwo->op.bitmap_buf + (rwo->end >> 3))) & (1 << (rwo->end & 0x7)));
             if (!bit)
@@ -434,7 +474,7 @@ struct snap_merger_t
                 rwo->end++;
             }
         }
-        if (rwo->end > rwo->start)
+        if (rwo->end > rwo->start && !rwo->error_code)
         {
             // write start->end
             rwo->todo++;
@@ -473,8 +513,9 @@ struct snap_merger_t
                     delete subop;
                     return;
                 }
-                fprintf(stderr, "error writing target at offset %lx: %s\n", subop->offset, strerror(-subop->retval));
-                exit(1);
+                rwo->error_code = -subop->retval;
+                rwo->error_offset = subop->offset;
+                rwo->error_read = false;
             }
             // Increment CAS version
             rwo->op.version++;
@@ -510,11 +551,12 @@ struct snap_merger_t
     {
         if (!rwo->todo)
         {
-            if (last_written_offset < rwo->op.offset+target_block_size)
+            if (!rwo->error_code &&
+                last_written_offset < rwo->op.offset+target_block_size)
             {
                 last_written_offset = rwo->op.offset+target_block_size;
             }
-            if (delete_source)
+            if (!rwo->error_code && delete_source)
             {
                 deleted_unsynced++;
                 if (deleted_unsynced >= fsync_interval)
@@ -545,13 +587,20 @@ struct snap_merger_t
             }
             free(rwo->buf);
             delete rwo;
+            if (rwo->error_code)
+            {
+                char buf[1024];
+                snprintf(buf, 1024, "Error %s target at offset %lx: %s",
+                    rwo->error_read ? "reading" : "writing", rwo->error_offset, strerror(rwo->error_code));
+                rwo_error = std::string(buf);
+            }
             in_flight--;
             continue_merge_reent();
         }
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_merge(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_merge(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto merger = new snap_merger_t();
@@ -559,22 +608,18 @@ std::function<bool(void)> cli_tool_t::start_merge(json11::Json cfg)
     merger->from_name = cmd.size() > 1 ? cmd[1].string_value() : "";
     merger->to_name = cmd.size() > 2 ? cmd[2].string_value() : "";
     merger->target_name = cfg["target"].string_value();
-    if (merger->from_name == "" || merger->to_name == "")
-    {
-        fprintf(stderr, "Beginning or end of the merge sequence is missing\n");
-        exit(1);
-    }
     merger->delete_source = cfg["delete-source"].string_value() != "";
     merger->fsync_interval = cfg["fsync-interval"].uint64_value();
     if (!merger->fsync_interval)
         merger->fsync_interval = 128;
     if (!cfg["cas"].is_null())
         merger->use_cas = cfg["cas"].uint64_value() ? 2 : 0;
-    return [merger]()
+    return [merger](cli_result_t & result)
     {
         merger->continue_merge_reent();
         if (merger->is_done())
         {
+            result = merger->result;
             delete merger;
             return true;
         }
diff --git a/src/cli_modify.cpp b/src/cli_modify.cpp
index aeb0e839..6939a7b6 100644
--- a/src/cli_modify.cpp
+++ b/src/cli_modify.cpp
@@ -23,7 +23,8 @@ struct image_changer_t
     bool has_children = false;
 
     int state = 0;
-    std::function<bool(void)> cb;
+    std::function<bool(cli_result_t &)> cb;
+    cli_result_t result;
 
     bool is_done()
     {
@@ -36,6 +37,18 @@ struct image_changer_t
             goto resume_1;
         else if (state == 2)
             goto resume_2;
+        if (image_name == "")
+        {
+            result = (cli_result_t){ .err = EINVAL, .text = "Image name is missing" };
+            state = 100;
+            return;
+        }
+        if (new_size != 0 && (new_size % 4096))
+        {
+            result = (cli_result_t){ .err = EINVAL, .text = "Image size should be a multiple of 4096" };
+            state = 100;
+            return;
+        }
         for (auto & ic: parent->cli->st_cli.inode_config)
         {
             if (ic.second.name == image_name)
@@ -46,14 +59,16 @@ struct image_changer_t
             }
             if (new_name != "" && ic.second.name == new_name)
             {
-                fprintf(stderr, "Image %s already exists\n", new_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EEXIST, .text = "Image "+new_name+" already exists" };
+                state = 100;
+                return;
             }
         }
         if (!inode_num)
         {
-            fprintf(stderr, "Image %s does not exist\n", image_name.c_str());
-            exit(1);
+            result = (cli_result_t){ .err = ENOENT, .text = "Image "+image_name+" does not exist" };
+            state = 100;
+            return;
         }
         for (auto & ic: parent->cli->st_cli.inode_config)
         {
@@ -68,7 +83,7 @@ struct image_changer_t
             (!new_size || cfg.size == new_size) &&
             (new_name == "" || new_name == image_name))
         {
-            printf("No change\n");
+            result = (cli_result_t){ .text = "No change" };
             state = 100;
             return;
         }
@@ -79,8 +94,9 @@ struct image_changer_t
                 // Check confirmation when trimming an image with children
                 if (has_children && !force)
                 {
-                    fprintf(stderr, "Image %s has children. Refusing to shrink it without --force\n", image_name.c_str());
-                    exit(1);
+                    result = (cli_result_t){ .err = EINVAL, .text = "Image "+image_name+" has children. Refusing to shrink it without --force" };
+                    state = 100;
+                    return;
                 }
                 // Shrink the image first
                 cb = parent->start_rm(json11::Json::object {
@@ -90,12 +106,17 @@ struct image_changer_t
                     { "min-offset", new_size },
                 });
 resume_1:
-                while (!cb())
+                while (!cb(result))
                 {
                     state = 1;
                     return;
                 }
                 cb = NULL;
+                if (result.err)
+                {
+                    state = 100;
+                    return;
+                }
             }
             cfg.size = new_size;
         }
@@ -109,8 +130,9 @@ resume_1:
             // Check confirmation when making an image with children read-write
             if (has_children && !force)
             {
-                fprintf(stderr, "Image %s has children. Refusing to make it read-write without --force\n", image_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EINVAL, .text = "Image "+image_name+" has children. Refusing to make it read-write without --force" };
+                state = 100;
+                return;
             }
         }
         if (new_name != "")
@@ -180,32 +202,23 @@ resume_2:
             return;
         if (!parent->etcd_result["succeeded"].bool_value())
         {
-            fprintf(stderr, "Image %s was modified by someone else, please repeat your request\n", image_name.c_str());
-            exit(1);
+            result = (cli_result_t){ .err = EAGAIN, .text = "Image "+image_name+" was modified by someone else, please repeat your request" };
+            state = 100;
+            return;
         }
-        printf("Image %s modified\n", image_name.c_str());
+        result = (cli_result_t){ .err = 0, .text = "Image "+image_name+" modified" };
         state = 100;
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_modify(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_modify(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto changer = new image_changer_t();
     changer->parent = this;
     changer->image_name = cmd.size() > 1 ? cmd[1].string_value() : "";
-    if (changer->image_name == "")
-    {
-        fprintf(stderr, "Image name is missing\n");
-        exit(1);
-    }
     changer->new_name = cfg["rename"].string_value();
     changer->new_size = parse_size(cfg["resize"].string_value());
-    if (changer->new_size != 0 && (changer->new_size % 4096))
-    {
-        fprintf(stderr, "Image size should be a multiple of 4096\n");
-        exit(1);
-    }
     changer->force = cfg["force"].bool_value();
     changer->set_readonly = cfg["readonly"].bool_value();
     changer->set_readwrite = cfg["readwrite"].bool_value();
@@ -213,11 +226,12 @@ std::function<bool(void)> cli_tool_t::start_modify(json11::Json cfg)
     if (!changer->fsync_interval)
         changer->fsync_interval = 128;
     // FIXME Check that the image doesn't have children when shrinking
-    return [changer]()
+    return [changer](cli_result_t & result)
     {
         changer->loop();
         if (changer->is_done())
         {
+            result = changer->result;
             delete changer;
             return true;
         }
diff --git a/src/cli_rm.cpp b/src/cli_rm.cpp
index a7d1ba4f..ca33943b 100644
--- a/src/cli_rm.cpp
+++ b/src/cli_rm.cpp
@@ -63,7 +63,9 @@ struct snap_remover_t
     inode_t new_parent = 0;
     int state = 0;
     int current_child = 0;
-    std::function<bool(void)> cb;
+    std::function<bool(cli_result_t &)> cb;
+
+    cli_result_t result;
 
     bool is_done()
     {
@@ -90,11 +92,25 @@ struct snap_remover_t
             goto resume_8;
         else if (state == 9)
             goto resume_9;
+        if (from_name == "")
+        {
+            result = (cli_result_t){ .err = EINVAL, .text = "Layer to remove argument is missing" };
+            state = 100;
+            return;
+        }
+        if (to_name == "")
+        {
+            to_name = from_name;
+        }
         // Get children to merge
         get_merge_children();
+        if (state == 100)
+            return;
         // Try to select an inode for the "inverse" optimized scenario
         // Read statistics from etcd to do it
         read_stats();
+        if (state == 100)
+            return;
         state = 1;
 resume_1:
         if (parent->waiting > 0)
@@ -106,13 +122,20 @@ resume_1:
             if (merge_children[current_child] == inverse_child)
                 continue;
             start_merge_child(merge_children[current_child], merge_children[current_child]);
+            if (state == 100)
+                return;
 resume_2:
-            while (!cb())
+            while (!cb(result))
             {
                 state = 2;
                 return;
             }
             cb = NULL;
+            if (result.err)
+            {
+                state = 100;
+                return;
+            }
             parent->change_parent(merge_children[current_child], new_parent);
             state = 3;
 resume_3:
@@ -123,25 +146,41 @@ resume_3:
         if (inverse_child != 0)
         {
             start_merge_child(inverse_child, inverse_parent);
+            if (state == 100)
+                return;
 resume_4:
-            while (!cb())
+            while (!cb(result))
             {
                 state = 4;
                 return;
             }
             cb = NULL;
+            if (result.err)
+            {
+                state = 100;
+                return;
+            }
             // Delete "inverse" child data
             start_delete_source(inverse_child);
+            if (state == 100)
+                return;
 resume_5:
-            while (!cb())
+            while (!cb(result))
             {
                 state = 5;
                 return;
             }
             cb = NULL;
+            if (result.err)
+            {
+                state = 100;
+                return;
+            }
             // Delete "inverse" child metadata, rename parent over it,
             // and also change parent links of the previous "inverse" child
             rename_inverse_parent();
+            if (state == 100)
+                return;
             state = 6;
 resume_6:
             if (parent->waiting > 0)
@@ -154,13 +193,20 @@ resume_6:
                 continue;
             start_delete_source(chain_list[current_child]);
 resume_7:
-            while (!cb())
+            while (!cb(result))
             {
                 state = 7;
                 return;
             }
             cb = NULL;
+            if (result.err)
+            {
+                state = 100;
+                return;
+            }
             delete_inode_config(chain_list[current_child]);
+            if (state == 100)
+                return;
             state = 8;
 resume_8:
             if (parent->waiting > 0)
@@ -186,16 +232,19 @@ resume_9:
             auto it = parent->cli->st_cli.inode_config.find(cur->parent_id);
             if (it == parent->cli->st_cli.inode_config.end())
             {
-                fprintf(stderr, "Parent inode of layer %s (id %ld) not found\n", cur->name.c_str(), cur->parent_id);
-                exit(1);
+                char buf[1024];
+                snprintf(buf, 1024, "Parent inode of layer %s (id 0x%lx) not found", cur->name.c_str(), cur->parent_id);
+                state = 100;
+                return;
             }
             cur = &it->second;
             chain_list.push_back(cur->num);
         }
         if (cur->num != from_cfg->num)
         {
-            fprintf(stderr, "Layer %s is not a child of %s\n", to_name.c_str(), from_name.c_str());
-            exit(1);
+            result = (cli_result_t){ .err = EINVAL, .text = "Layer "+to_name+" is not a child of "+from_name };
+            state = 100;
+            return;
         }
         new_parent = from_cfg->parent_id;
         // Calculate ranks
@@ -263,8 +312,9 @@ resume_9:
             parent->waiting--;
             if (err != "")
             {
-                fprintf(stderr, "Error reading layer statistics from etcd: %s\n", err.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EIO, .text = "Error reading layer statistics from etcd: "+err };
+                state = 100;
+                return;
             }
             for (auto inode_result: data["responses"].array_items())
             {
@@ -275,14 +325,16 @@ resume_9:
                 sscanf(kv.key.c_str() + parent->cli->st_cli.etcd_prefix.length()+13, "%u/%lu%c", &pool_id, &inode, &null_byte);
                 if (!inode || null_byte != 0)
                 {
-                    fprintf(stderr, "Bad key returned from etcd: %s\n", kv.key.c_str());
-                    exit(1);
+                    result = (cli_result_t){ .err = EIO, .text = "Bad key returned from etcd: "+kv.key };
+                    state = 100;
+                    return;
                 }
                 auto pool_cfg_it = parent->cli->st_cli.pool_config.find(pool_id);
                 if (pool_cfg_it == parent->cli->st_cli.pool_config.end())
                 {
-                    fprintf(stderr, "Pool %u does not exist\n", pool_id);
-                    exit(1);
+                    result = (cli_result_t){ .err = ENOENT, .text = "Pool "+std::to_string(pool_id)+" does not exist" };
+                    state = 100;
+                    return;
                 }
                 inode = INODE_WITH_POOL(pool_id, inode);
                 auto & pool_cfg = pool_cfg_it->second;
@@ -324,14 +376,20 @@ resume_9:
         auto child_it = parent->cli->st_cli.inode_config.find(inverse_child);
         if (child_it == parent->cli->st_cli.inode_config.end())
         {
-            fprintf(stderr, "Inode %ld disappeared\n", inverse_child);
-            exit(1);
+            char buf[1024];
+            snprintf(buf, 1024, "Inode 0x%lx disappeared", inverse_child);
+            result = (cli_result_t){ .err = EIO, .text = std::string(buf) };
+            state = 100;
+            return;
         }
         auto target_it = parent->cli->st_cli.inode_config.find(inverse_parent);
         if (target_it == parent->cli->st_cli.inode_config.end())
         {
-            fprintf(stderr, "Inode %ld disappeared\n", inverse_parent);
-            exit(1);
+            char buf[1024];
+            snprintf(buf, 1024, "Inode 0x%lx disappeared", inverse_parent);
+            result = (cli_result_t){ .err = EIO, .text = std::string(buf) };
+            state = 100;
+            return;
         }
         inode_config_t *child_cfg = &child_it->second;
         inode_config_t *target_cfg = &target_it->second;
@@ -422,16 +480,19 @@ resume_9:
             parent->waiting--;
             if (err != "")
             {
-                fprintf(stderr, "Error renaming %s to %s: %s\n", target_name.c_str(), child_name.c_str(), err.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EIO, .text = "Error renaming "+target_name+" to "+child_name+": "+err };
+                state = 100;
+                return;
             }
             if (!res["succeeded"].bool_value())
             {
-                fprintf(
-                    stderr, "Parent (%s), child (%s), or one of its children"
-                    " configuration was modified during rename\n", target_name.c_str(), child_name.c_str()
-                );
-                exit(1);
+                result = (cli_result_t){
+                    .err = EIO,
+                    .text = "Parent ("+target_name+"), child ("+child_name+"), or one of its children"
+                        " configuration was modified during rename",
+                };
+                state = 100;
+                return;
             }
             printf("Layer %s renamed to %s\n", target_name.c_str(), child_name.c_str());
             parent->ringloop->wakeup();
@@ -443,8 +504,11 @@ resume_9:
         auto cur_cfg_it = parent->cli->st_cli.inode_config.find(cur);
         if (cur_cfg_it == parent->cli->st_cli.inode_config.end())
         {
-            fprintf(stderr, "Inode 0x%lx disappeared\n", cur);
-            exit(1);
+            char buf[1024];
+            snprintf(buf, 1024, "Inode 0x%lx disappeared", cur);
+            result = (cli_result_t){ .err = EIO, .text = std::string(buf) };
+            state = 100;
+            return;
         }
         inode_config_t *cur_cfg = &cur_cfg_it->second;
         std::string cur_name = cur_cfg->name;
@@ -480,13 +544,15 @@ resume_9:
             parent->waiting--;
             if (err != "")
             {
-                fprintf(stderr, "Error deleting %s: %s\n", cur_name.c_str(), err.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EIO, .text = "Error deleting "+cur_name+": "+err };
+                state = 100;
+                return;
             }
             if (!res["succeeded"].bool_value())
             {
-                fprintf(stderr, "Layer %s configuration was modified during deletion\n", cur_name.c_str());
-                exit(1);
+                result = (cli_result_t){ .err = EIO, .text = "Layer "+cur_name+" was modified during deletion" };
+                state = 100;
+                return;
             }
             printf("Layer %s deleted\n", cur_name.c_str());
             parent->ringloop->wakeup();
@@ -498,14 +564,20 @@ resume_9:
         auto child_it = parent->cli->st_cli.inode_config.find(child_inode);
         if (child_it == parent->cli->st_cli.inode_config.end())
         {
-            fprintf(stderr, "Inode %ld disappeared\n", child_inode);
-            exit(1);
+            char buf[1024];
+            snprintf(buf, 1024, "Inode 0x%lx disappeared", child_inode);
+            result = (cli_result_t){ .err = EIO, .text = std::string(buf) };
+            state = 100;
+            return;
         }
         auto target_it = parent->cli->st_cli.inode_config.find(target_inode);
         if (target_it == parent->cli->st_cli.inode_config.end())
         {
-            fprintf(stderr, "Inode %ld disappeared\n", target_inode);
-            exit(1);
+            char buf[1024];
+            snprintf(buf, 1024, "Inode 0x%lx disappeared", target_inode);
+            result = (cli_result_t){ .err = EIO, .text = std::string(buf) };
+            state = 100;
+            return;
         }
         cb = parent->start_merge(json11::Json::object {
             { "command", json11::Json::array{ "merge-data", from_name, child_it->second.name } },
@@ -521,8 +593,11 @@ resume_9:
         auto source = parent->cli->st_cli.inode_config.find(inode);
         if (source == parent->cli->st_cli.inode_config.end())
         {
-            fprintf(stderr, "Inode %ld disappeared\n", inode);
-            exit(1);
+            char buf[1024];
+            snprintf(buf, 1024, "Inode 0x%lx disappeared", inode);
+            result = (cli_result_t){ .err = EIO, .text = std::string(buf) };
+            state = 100;
+            return;
         }
         cb = parent->start_rm(json11::Json::object {
             { "inode", inode },
@@ -532,22 +607,13 @@ resume_9:
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_snap_rm(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_snap_rm(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto snap_remover = new snap_remover_t();
     snap_remover->parent = this;
     snap_remover->from_name = cmd.size() > 1 ? cmd[1].string_value() : "";
     snap_remover->to_name = cmd.size() > 2 ? cmd[2].string_value() : "";
-    if (snap_remover->from_name == "")
-    {
-        fprintf(stderr, "Layer to remove argument is missing\n");
-        exit(1);
-    }
-    if (snap_remover->to_name == "")
-    {
-        snap_remover->to_name = snap_remover->from_name;
-    }
     snap_remover->fsync_interval = cfg["fsync-interval"].uint64_value();
     if (!snap_remover->fsync_interval)
         snap_remover->fsync_interval = 128;
@@ -555,11 +621,12 @@ std::function<bool(void)> cli_tool_t::start_snap_rm(json11::Json cfg)
         snap_remover->use_cas = cfg["cas"].uint64_value() ? 2 : 0;
     if (!cfg["writers_stopped"].is_null())
         snap_remover->writers_stopped = true;
-    return [snap_remover]()
+    return [snap_remover](cli_result_t & result)
     {
         snap_remover->loop();
         if (snap_remover->is_done())
         {
+            result = snap_remover->result;
             delete snap_remover;
             return true;
         }
diff --git a/src/cli_rm_data.cpp b/src/cli_rm_data.cpp
index 5cff8fd7..3671a455 100644
--- a/src/cli_rm_data.cpp
+++ b/src/cli_rm_data.cpp
@@ -32,6 +32,9 @@ struct rm_inode_t
     uint64_t pgs_to_list = 0;
     bool lists_done = false;
     int state = 0;
+    int error_count = 0;
+
+    cli_result_t result;
 
     void start_delete()
     {
@@ -74,8 +77,13 @@ struct rm_inode_t
         });
         if (!lister)
         {
-            fprintf(stderr, "Failed to list inode %lu from pool %u objects\n", INODE_NO_POOL(inode), INODE_POOL(inode));
-            exit(1);
+            result = (cli_result_t){
+                .err = EIO,
+                .text = "Failed to list objects of inode "+std::to_string(INODE_NO_POOL(inode))+
+                    " from pool "+std::to_string(INODE_POOL(inode)),
+            };
+            state = 100;
+            return;
         }
         pgs_to_list = parent->cli->list_pg_count(lister);
         parent->cli->list_inode_next(lister, parent->parallel_osds);
@@ -118,6 +126,7 @@ struct rm_inode_t
                         fprintf(stderr, "Failed to remove object %lx:%lx from PG %u (OSD %lu) (retval=%ld)\n",
                             op->req.rw.inode, op->req.rw.offset,
                             cur_list->pg_num, cur_list->rm_osd_num, op->reply.hdr.retval);
+                        error_count++;
                     }
                     delete op;
                     cur_list->obj_done++;
@@ -161,31 +170,41 @@ struct rm_inode_t
         }
         if (lists_done && !lists.size())
         {
-            printf("Done, inode %lu in pool %u data removed\n", INODE_NO_POOL(inode), pool_id);
-            state = 2;
+            result = (cli_result_t){
+                .err = error_count > 0 ? EIO : 0,
+                .text = error_count > 0 ? "Some blocks were not removed" : (
+                    "Done, inode "+std::to_string(INODE_NO_POOL(inode))+" from pool "+
+                    std::to_string(pool_id)+" removed"),
+            };
+            state = 100;
         }
     }
 
-    bool loop()
+    bool is_done()
     {
-        if (state == 0)
+        return state == 100;
+    }
+
+    void loop()
+    {
+        if (state == 1)
+            goto resume_1;
+        if (!pool_id)
         {
-            start_delete();
-            state = 1;
+            result = (cli_result_t){ .err = EINVAL, .text = "Pool is not specified" };
+            state = 100;
+            return;
         }
-        else if (state == 1)
-        {
-            continue_delete();
-        }
-        else if (state == 2)
-        {
-            return true;
-        }
-        return false;
+        start_delete();
+        if (state == 100)
+            return;
+        state = 1;
+    resume_1:
+        continue_delete();
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_rm(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_rm(json11::Json cfg)
 {
     auto remover = new rm_inode_t();
     remover->parent = this;
@@ -196,16 +215,13 @@ std::function<bool(void)> cli_tool_t::start_rm(json11::Json cfg)
         remover->inode = (remover->inode & (((uint64_t)1 << (64-POOL_ID_BITS)) - 1)) | (((uint64_t)remover->pool_id) << (64-POOL_ID_BITS));
     }
     remover->pool_id = INODE_POOL(remover->inode);
-    if (!remover->pool_id)
-    {
-        fprintf(stderr, "pool is missing\n");
-        exit(1);
-    }
     remover->min_offset = cfg["min-offset"].uint64_value();
-    return [remover]()
+    return [remover](cli_result_t & result)
     {
-        if (remover->loop())
+        remover->loop();
+        if (remover->is_done())
         {
+            result = remover->result;
             delete remover;
             return true;
         }
diff --git a/src/cli_simple_offsets.cpp b/src/cli_simple_offsets.cpp
index 6751f506..f4bdd9ca 100644
--- a/src/cli_simple_offsets.cpp
+++ b/src/cli_simple_offsets.cpp
@@ -11,7 +11,7 @@
 #include <sys/stat.h>
 
 // Calculate offsets for a block device and print OSD command line parameters
-std::function<bool(void)> cli_tool_t::simple_offsets(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::simple_offsets(json11::Json cfg)
 {
     std::string device = cfg["command"][1].string_value();
     uint64_t object_size = parse_size(cfg["object_size"].string_value());
diff --git a/src/cli_status.cpp b/src/cli_status.cpp
index 0fd8a5df..7984bb01 100644
--- a/src/cli_status.cpp
+++ b/src/cli_status.cpp
@@ -277,16 +277,17 @@ resume_2:
     }
 };
 
-std::function<bool(void)> cli_tool_t::start_status(json11::Json cfg)
+std::function<bool(cli_result_t &)> cli_tool_t::start_status(json11::Json cfg)
 {
     json11::Json::array cmd = cfg["command"].array_items();
     auto printer = new status_printer_t();
     printer->parent = this;
-    return [printer]()
+    return [printer](cli_result_t & result)
     {
         printer->loop();
         if (printer->is_done())
         {
+            result = { .err = 0 };
             delete printer;
             return true;
         }