doc: usage: Add general rule for `$?`

For nearly all commands in U-Boot the '?' variable is handled the same
way with 0 meaning success, 1 meaning any failure.  Explain this in the
general rules section of the cmdline documentation (with a link to a
counter example) and then remove the redundant wording from most
commands. We retain a section about the return value in a number of
places where we are doing something such as always returning a specific
value or we have useful additional information to go along with the
normal return codes.

Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Heinrich Schuchardt <[email protected]>

45 files changed, 6 insertions, 245 deletions

diff --git a/doc/usage/cmd/armffa.rst b/doc/usage/cmd/armffa.rst
index 4f41e3393fd..dbe4d5bc842 100644
--- a/doc/usage/cmd/armffa.rst
+++ b/doc/usage/cmd/armffa.rst
@@ -90,8 +90,3 @@ Configuration
 -------------
 
 The command is available if CONFIG_CMD_ARMFFA=y and CONFIG_ARM_FFA_TRANSPORT=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) on failure.
diff --git a/doc/usage/cmd/bdinfo.rst b/doc/usage/cmd/bdinfo.rst
index a21fbc83ccf..a139c9d855c 100644
--- a/doc/usage/cmd/bdinfo.rst
+++ b/doc/usage/cmd/bdinfo.rst
@@ -115,8 +115,3 @@ Configuration
 -------------
 
 The bdinfo command is available if CONFIG_CMD_BDI=y.
-
-Return code
------------
-
-The return code $? is 0 (true).
diff --git a/doc/usage/cmd/bind.rst b/doc/usage/cmd/bind.rst
index 67a0405bab5..d2bef15508d 100644
--- a/doc/usage/cmd/bind.rst
+++ b/doc/usage/cmd/bind.rst
@@ -99,8 +99,3 @@ Configuration
 -------------
 
 The bind command is only available if CONFIG_CMD_BIND=y.
-
-Return code
------------
-
-The return code $? is 0 (true) on success and 1 (false) on failure.
diff --git a/doc/usage/cmd/blkcache.rst b/doc/usage/cmd/blkcache.rst
index 0329261ba9a..764a4903ca0 100644
--- a/doc/usage/cmd/blkcache.rst
+++ b/doc/usage/cmd/blkcache.rst
@@ -69,9 +69,3 @@ Configuration
 -------------
 
 The blkcache command is only available if CONFIG_CMD_BLOCK_CACHE=y.
-
-Return code
------------
-
-If the command succeeds, the return code $? is set 0 (true). In case of an
-error the return code is set to 1 (false).
diff --git a/doc/usage/cmd/cat.rst b/doc/usage/cmd/cat.rst
index b22dc6184a2..fa70e5f6a09 100644
--- a/doc/usage/cmd/cat.rst
+++ b/doc/usage/cmd/cat.rst
@@ -45,8 +45,3 @@ Configuration
 -------------
 
 The cat command is only available if CONFIG_CMD_CAT=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the file is readable, otherwise it returns a non-zero error code.
diff --git a/doc/usage/cmd/cli.rst b/doc/usage/cmd/cli.rst
index 23e5ee7a902..97de5331d1b 100644
--- a/doc/usage/cmd/cli.rst
+++ b/doc/usage/cmd/cli.rst
@@ -70,8 +70,3 @@ compiled::
     modern
     => cli set old
     Want to set current parser to old, but its code was not compiled!
-
-Return value
-------------
-
-The return value $? indicates whether the command succeeded.
diff --git a/doc/usage/cmd/cls.rst b/doc/usage/cmd/cls.rst
index 828276742b9..b8fa335079e 100644
--- a/doc/usage/cmd/cls.rst
+++ b/doc/usage/cmd/cls.rst
@@ -22,8 +22,3 @@ Configuration
 -------------
 
 The cls command is only available if CONFIG_CMD_CLS=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success and 1 (false) on failure.
diff --git a/doc/usage/cmd/cp.rst b/doc/usage/cmd/cp.rst
index 434dfedfc2b..7bf2f468421 100644
--- a/doc/usage/cmd/cp.rst
+++ b/doc/usage/cmd/cp.rst
@@ -79,9 +79,3 @@ Configuration
 The cp command is available if CONFIG_CMD_MEMORY=y. Support for 64 bit words
 (cp.q) is only available on 64-bit targets. Copying to flash depends on
 CONFIG_MTD_NOR_FLASH=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the command was successfully,
-1 (false) otherwise.
diff --git a/doc/usage/cmd/cpu.rst b/doc/usage/cmd/cpu.rst
index 8b0b7d5a699..705fb2dd691 100644
--- a/doc/usage/cmd/cpu.rst
+++ b/doc/usage/cmd/cpu.rst
@@ -93,9 +93,3 @@ Configuration
 -------------
 
 The cpu command is available if CONFIG_CMD_CPU=y.
-
-Return code
------------
-
-The return value $? is set to 0 (true) if the command is successful,
-1 (false) otherwise.
diff --git a/doc/usage/cmd/cpuid.rst b/doc/usage/cmd/cpuid.rst
index cccf9262ed4..568c7e86ca1 100644
--- a/doc/usage/cmd/cpuid.rst
+++ b/doc/usage/cmd/cpuid.rst
@@ -25,11 +25,6 @@ Configuration
 
 The cpuid command is only available on x86.
 
-Return value
-------------
-
-The return value $? is 0 (true).
-
 Example
 -------
 
diff --git a/doc/usage/cmd/dmareset.rst b/doc/usage/cmd/dmareset.rst
index ab06979251b..56180595363 100644
--- a/doc/usage/cmd/dmareset.rst
+++ b/doc/usage/cmd/dmareset.rst
@@ -47,9 +47,3 @@ Configuration
 
 The dmareset command is only available if CONFIG_CMD_C5_PL330_DMA=y in
 "Shell scripting commands".
-
-Return value
-------------
-
-If the command succeeds, the return value $? is set to 0 (true).
-If an error occurs, the return value $? is set to 1 (false).
diff --git a/doc/usage/cmd/ebtupdate.rst b/doc/usage/cmd/ebtupdate.rst
index 22415ee07b4..5102e6090a4 100644
--- a/doc/usage/cmd/ebtupdate.rst
+++ b/doc/usage/cmd/ebtupdate.rst
@@ -64,9 +64,3 @@ Configuration
 
 The ebtupdate command is only available if CONFIG_CMD_EBTUPDATE=y and
 only on Tegra 2 and Tegra 3 configurations.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if everything went successfully. If an
-error occurs, the return value $? is set to 1 (false).
diff --git a/doc/usage/cmd/fatinfo.rst b/doc/usage/cmd/fatinfo.rst
index 2e05ab8bece..e5a89f37ba6 100644
--- a/doc/usage/cmd/fatinfo.rst
+++ b/doc/usage/cmd/fatinfo.rst
@@ -46,9 +46,3 @@ Configuration
 -------------
 
 The fatinfo command is only available if CONFIG_CMD_FAT=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the partition is a FAT partition.
-Otherwise it is set to 1 (false).
diff --git a/doc/usage/cmd/fdt.rst b/doc/usage/cmd/fdt.rst
index 71a9fc627e5..493ed895bc7 100644
--- a/doc/usage/cmd/fdt.rst
+++ b/doc/usage/cmd/fdt.rst
@@ -66,8 +66,3 @@ address and expand it to 0xf000 in size::
     Working FDT set to 10000
     => md 10000 4
     00010000: edfe0dd0 00f00000 78000000 7c270000  ...........x..'|
-
-Return value
-------------
-
-The return value $? indicates whether the command succeeded.
diff --git a/doc/usage/cmd/font.rst b/doc/usage/cmd/font.rst
index 44a04f5d075..a1860466ac1 100644
--- a/doc/usage/cmd/font.rst
+++ b/doc/usage/cmd/font.rst
@@ -54,9 +54,3 @@ Configuration
 -------------
 
 The command is only available if CONFIG_CONSOLE_TRUETYPE=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) if the command completes.
-The return value is 1 (false) if the command fails.
diff --git a/doc/usage/cmd/fuse.rst b/doc/usage/cmd/fuse.rst
index 42c6e209f99..50cf5a4b657 100644
--- a/doc/usage/cmd/fuse.rst
+++ b/doc/usage/cmd/fuse.rst
@@ -168,9 +168,3 @@ Configuration
 
 The fuse commands are available if CONFIG_CMD_FUSE=y.
 The fuse writebuff command is available if CONFIG_CMD_FUSE_WRITEBUFF=y.
-
-Return code
------------
-
-The return value $? is set to 0 (true) if the command is successful,
-1 (false) otherwise.
diff --git a/doc/usage/cmd/gpio.rst b/doc/usage/cmd/gpio.rst
index 4b0dc2716e5..0159d3478c2 100644
--- a/doc/usage/cmd/gpio.rst
+++ b/doc/usage/cmd/gpio.rst
@@ -127,9 +127,3 @@ Configuration
 
 The *gpio* command is only available if CONFIG_CMD_GPIO=y.
 The *gpio read* command is only available if CONFIG_CMD_GPIO_READ=y.
-
-Return value
-------------
-
-If the command succeds the return value $? is set to 0. If an error occurs, the
-return value $? is set to 1.
diff --git a/doc/usage/cmd/host.rst b/doc/usage/cmd/host.rst
index a70a432b6f2..672d8f2112b 100644
--- a/doc/usage/cmd/host.rst
+++ b/doc/usage/cmd/host.rst
@@ -111,9 +111,3 @@ Unbind a device::
     => host info
     dev       blocks label           path
       1         2048 fat             1MB.fat32.img
-
-
-Return value
-------------
-
-The return value $? indicates whether the command succeeded.
diff --git a/doc/usage/cmd/i3c.rst b/doc/usage/cmd/i3c.rst
index 922fa3ea37c..f49fcd4e766 100644
--- a/doc/usage/cmd/i3c.rst
+++ b/doc/usage/cmd/i3c.rst
@@ -129,12 +129,6 @@ Configuration
 
 The ``i3c`` command is only available if CONFIG_CMD_I3C=y.
 
-Return value
-------------
-
-If the command succeeds, the return value ``$?`` is set to 0. If an error
-occurs, the return value ``$?`` is set to 1.
-
 Note
 ----
 
diff --git a/doc/usage/cmd/imxtract.rst b/doc/usage/cmd/imxtract.rst
index 235d15e445b..1621a4a68b2 100644
--- a/doc/usage/cmd/imxtract.rst
+++ b/doc/usage/cmd/imxtract.rst
@@ -76,9 +76,3 @@ Configuration
 The imxtract command is only available if CONFIG_CMD_XIMG=y. Support for FIT
 images requires CONFIG_FIT=y. Support for legacy U-Boot images requires
 CONFIG_LEGACY_IMAGE_FORMAT=y.
-
-Return value
-------------
-
-On success the return value $? of the command is 0 (true). On failure the
-return value is 1 (false).
diff --git a/doc/usage/cmd/loadb.rst b/doc/usage/cmd/loadb.rst
index 4f9a52c793f..9e6a41e7fa0 100644
--- a/doc/usage/cmd/loadb.rst
+++ b/doc/usage/cmd/loadb.rst
@@ -66,8 +66,3 @@ Configuration
 -------------
 
 The command is only available if CONFIG_CMD_LOADB=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) on error.
diff --git a/doc/usage/cmd/loadm.rst b/doc/usage/cmd/loadm.rst
index 005840a27bb..722690be2c5 100644
--- a/doc/usage/cmd/loadm.rst
+++ b/doc/usage/cmd/loadm.rst
@@ -43,10 +43,3 @@ Configuration
 -------------
 
 The command is only available if CONFIG_CMD_LOADM=y.
-
-Return value
-------------
-
-The return value $? is set 0 (true) if the loading is succefull, and
-is set to 1 (false) in case of error.
-
diff --git a/doc/usage/cmd/loads.rst b/doc/usage/cmd/loads.rst
index 0a2ac14acfe..278d17850d7 100644
--- a/doc/usage/cmd/loads.rst
+++ b/doc/usage/cmd/loads.rst
@@ -92,8 +92,3 @@ Configuration
 
 The command is only available if CONFIG_CMD_LOADS=y. The parameter to set the
 baud rate is only available if CONFIG_SYS_LOADS_BAUD_CHANGE=y
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) otherwise.
diff --git a/doc/usage/cmd/loadx.rst b/doc/usage/cmd/loadx.rst
index 661b36723c3..50182eaa9d3 100644
--- a/doc/usage/cmd/loadx.rst
+++ b/doc/usage/cmd/loadx.rst
@@ -73,8 +73,3 @@ The command is only available if CONFIG_CMD_LOADB=y.
 Initial timeout in seconds while waiting for transfer is configured by
 config option CMD_LOADXY_TIMEOUT or by env variable $loadxy_timeout.
 Setting it to 0 means infinite timeout.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) otherwise.
diff --git a/doc/usage/cmd/loady.rst b/doc/usage/cmd/loady.rst
index 8367759471e..8d21d7c3267 100644
--- a/doc/usage/cmd/loady.rst
+++ b/doc/usage/cmd/loady.rst
@@ -70,8 +70,3 @@ The command is only available if CONFIG_CMD_LOADB=y.
 Initial timeout in seconds while waiting for transfer is configured by
 config option CMD_LOADXY_TIMEOUT or by env variable $loadxy_timeout.
 Setting it to 0 means infinite timeout.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) otherwise.
diff --git a/doc/usage/cmd/msr.rst b/doc/usage/cmd/msr.rst
index 04ee52cc1c7..8bb1212cdc1 100644
--- a/doc/usage/cmd/msr.rst
+++ b/doc/usage/cmd/msr.rst
@@ -38,11 +38,6 @@ Configuration
 
 The msr command is only available on x86.
 
-Return value
-------------
-
-The return value $? is 0 (true).
-
 Example
 -------
 
diff --git a/doc/usage/cmd/mtest.rst b/doc/usage/cmd/mtest.rst
index e01f2a6d575..a5705076af7 100644
--- a/doc/usage/cmd/mtest.rst
+++ b/doc/usage/cmd/mtest.rst
@@ -62,8 +62,3 @@ Configuration
 -------------
 
 The mtest command is enabled by CONFIG_CMD_MEMTEST=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) if the command succeeds, 1 (false) otherwise.
diff --git a/doc/usage/cmd/optee.rst b/doc/usage/cmd/optee.rst
index 46c569a105f..caefdceeb7b 100644
--- a/doc/usage/cmd/optee.rst
+++ b/doc/usage/cmd/optee.rst
@@ -63,8 +63,3 @@ Configuration
 -------------
 
 The optee command is enabled by CONFIG_OPTEE=y and CONFIG_CMD_OPTEE=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) if the command succeeds, 1 (false) otherwise.
diff --git a/doc/usage/cmd/part.rst b/doc/usage/cmd/part.rst
index e7f6e54ecea..72f5d8b8de7 100644
--- a/doc/usage/cmd/part.rst
+++ b/doc/usage/cmd/part.rst
@@ -223,9 +223,3 @@ This shows looking at a device with multiple partition tables::
             type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
             guid:	a0891d7e-b930-4513-94da-f629dbd637b2
     =>
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the command succededd. If an
-error occurs, the return value $? is set to 1 (false).
diff --git a/doc/usage/cmd/pause.rst b/doc/usage/cmd/pause.rst
index 6cdd83d3163..b55825f5d93 100644
--- a/doc/usage/cmd/pause.rst
+++ b/doc/usage/cmd/pause.rst
@@ -48,9 +48,3 @@ Note that complex prompts require proper quoting:
     
     Usage:
     pause [prompt] - Wait until users presses any key. [prompt] can be used to customize the message.
-
-Return value
-------------
-
-The return value $? is always set to 0 (true), unless invoked in an invalid
-manner.
diff --git a/doc/usage/cmd/pinmux.rst b/doc/usage/cmd/pinmux.rst
index 30c5eb16a68..c066db1770c 100644
--- a/doc/usage/cmd/pinmux.rst
+++ b/doc/usage/cmd/pinmux.rst
@@ -90,9 +90,3 @@ Configuration
 -------------
 
 The pinmux command is only available if CONFIG_CMD_PINMUX=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the command succeded and to 1 (false)
-otherwise.
diff --git a/doc/usage/cmd/pwm.rst b/doc/usage/cmd/pwm.rst
index 522acb5afa3..6f7daba0736 100644
--- a/doc/usage/cmd/pwm.rst
+++ b/doc/usage/cmd/pwm.rst
@@ -83,9 +83,3 @@ Configuration
 -------------
 
 The ``pwm`` command is only available if CONFIG_CMD_PWM=y.
-
-Return value
-------------
-
-If the command succeeds, the return value ``$?`` is set to 0. If an error occurs, the
-return value ``$?`` is set to 1.
diff --git a/doc/usage/cmd/saves.rst b/doc/usage/cmd/saves.rst
index b380a4feb6f..d429eeceddf 100644
--- a/doc/usage/cmd/saves.rst
+++ b/doc/usage/cmd/saves.rst
@@ -84,8 +84,3 @@ Configuration
 
 The command is only available if CONFIG_CMD_SAVES=y. The parameter to set the
 baud rate is only available if CONFIG_SYS_LOADS_BAUD_CHANGE=y
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) otherwise.
diff --git a/doc/usage/cmd/scmi.rst b/doc/usage/cmd/scmi.rst
index 9591cdc07a5..c6e80a56a60 100644
--- a/doc/usage/cmd/scmi.rst
+++ b/doc/usage/cmd/scmi.rst
@@ -119,11 +119,4 @@ Configuration
 The scmi command is only available if CONFIG_CMD_SCMI=y.
 Default n because this command is mainly for debug purpose.
 
-Return value
-------------
-
-The return value ($?) is set to 0 if the operation succeeded,
-1 if the operation failed or -1 if the operation failed due to
-a syntax error.
-
 .. _`SCMI specification`: https://developer.arm.com/documentation/den0056/e/?lang=en
diff --git a/doc/usage/cmd/setexpr.rst b/doc/usage/cmd/setexpr.rst
index 5bc37ae50fc..96894a2754f 100644
--- a/doc/usage/cmd/setexpr.rst
+++ b/doc/usage/cmd/setexpr.rst
@@ -147,10 +147,3 @@ Configuration
 * The *setexpr gsub* and *setexpr sub* sub-commands are only available
   if CONFIG_REGEX=y. For an overview of the supported regex syntax,
   see :doc:`test`.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the operation was successful.
-
-If an error occurs, the return value $? is set to 1 (false).
diff --git a/doc/usage/cmd/size.rst b/doc/usage/cmd/size.rst
index 306fcba0ba4..42e2dec6323 100644
--- a/doc/usage/cmd/size.rst
+++ b/doc/usage/cmd/size.rst
@@ -35,9 +35,3 @@ Configuration
 -------------
 
 The size command is only available if CONFIG_CMD_FS_GENERIC=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the command succeded and to 1 (false)
-otherwise.
diff --git a/doc/usage/cmd/smbios.rst b/doc/usage/cmd/smbios.rst
index 1ffd706d7de..3c98795698e 100644
--- a/doc/usage/cmd/smbios.rst
+++ b/doc/usage/cmd/smbios.rst
@@ -86,8 +86,3 @@ Configuration
 -------------
 
 The command is only available if CONFIG_CMD_SMBIOS=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success, 1 (false) otherwise.
diff --git a/doc/usage/cmd/sound.rst b/doc/usage/cmd/sound.rst
index 97d610f3745..4474ce95420 100644
--- a/doc/usage/cmd/sound.rst
+++ b/doc/usage/cmd/sound.rst
@@ -56,8 +56,3 @@ Configuration
 -------------
 
 The sound command is enabled by CONFIG_CMD_SOUND=y.
-
-Return value
-------------
-
-The return value $? is 0 (true) if the command succeeds, 1 (false) otherwise.
diff --git a/doc/usage/cmd/source.rst b/doc/usage/cmd/source.rst
index 0de5f33390e..bd585f37414 100644
--- a/doc/usage/cmd/source.rst
+++ b/doc/usage/cmd/source.rst
@@ -188,9 +188,3 @@ The FIT image file format requires CONFIG_FIT=y.#
 The legacy U-Boot image file format requires CONFIG_LEGACY_IMAGE_FORMAT=y.
 On hardened systems support for the legacy U-Boot image format should be
 disabled as these images cannot be signed and verified.
-
-Return value
-------------
-
-If the scripts is executed successfully, the return value $? is 0 (true).
-Otherwise it is 1 (false).
diff --git a/doc/usage/cmd/temperature.rst b/doc/usage/cmd/temperature.rst
index 945bc8204ac..d5ea1196f45 100644
--- a/doc/usage/cmd/temperature.rst
+++ b/doc/usage/cmd/temperature.rst
@@ -45,9 +45,3 @@ Configuration
 -------------
 
 The *temperature* command is only available if CONFIG_CMD_TEMPERATURE=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the command succeeded and to 1 (false)
-otherwise.
diff --git a/doc/usage/cmd/tftpput.rst b/doc/usage/cmd/tftpput.rst
index 2bcb3032cb2..bdffa3af463 100644
--- a/doc/usage/cmd/tftpput.rst
+++ b/doc/usage/cmd/tftpput.rst
@@ -79,8 +79,3 @@ after which an ACK response is required. The window size defaults to 1.
 
 If CONFIG_TFTP_TSIZE=y, the progress bar is limited to 50 '#' characters.
 Otherwise an '#' is written per UDP package which may decrease performance.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success and 1 (false) otherwise.
diff --git a/doc/usage/cmd/unbind.rst b/doc/usage/cmd/unbind.rst
index 1ae9c1b172c..5a165cfee23 100644
--- a/doc/usage/cmd/unbind.rst
+++ b/doc/usage/cmd/unbind.rst
@@ -91,8 +91,3 @@ Configuration
 -------------
 
 The unbind command is only available if CONFIG_CMD_BIND=y.
-
-Return code
------------
-
-The return code $? is 0 (true) on success and 1 (false) on failure.
diff --git a/doc/usage/cmd/wdt.rst b/doc/usage/cmd/wdt.rst
index f48b8840907..711b74d20da 100644
--- a/doc/usage/cmd/wdt.rst
+++ b/doc/usage/cmd/wdt.rst
@@ -73,8 +73,3 @@ Configuration
 -------------
 
 The command is only available if CONFIG_CMD_WDT=y.
-
-Return value
-------------
-
-The return value $? is 0 if the command succeeds, 1 upon failure.
diff --git a/doc/usage/cmd/wget.rst b/doc/usage/cmd/wget.rst
index 8feda0248b2..e6c42f967b9 100644
--- a/doc/usage/cmd/wget.rst
+++ b/doc/usage/cmd/wget.rst
@@ -184,8 +184,3 @@ CONFIG_WGET_CACERT=y (for the wget cacert command).
 TCP Selective Acknowledgments in the legacy network stack can be enabled via
 CONFIG_PROT_TCP_SACK=y. This will improve the download speed. Selective
 Acknowledgments are enabled by default with lwIP.
-
-Return value
-------------
-
-The return value $? is 0 (true) on success and 1 (false) otherwise.
diff --git a/doc/usage/cmdline.rst b/doc/usage/cmdline.rst
index 58240c5279c..79b9baf7bfe 100644
--- a/doc/usage/cmdline.rst
+++ b/doc/usage/cmdline.rst
@@ -55,6 +55,12 @@ General rules
    command will cause "run" to terminate, i. e. the remaining
    variables are not executed.
 
+#. The variable ``$?`` will be set as the return value of any command. The
+   possible values are 0 on success or 1 on any error e. g. invalid syntax or
+   failure of the command. Any exceptions to this are documented by the
+   specific command, e.g. the :doc:`for command <cmd/for>` sets ``$?`` based on
+   the last command run within the loop.
+
 Representing numbers
 --------------------