What is an exit code
Every script, command, or binary exits with a return code. You can see this value in the special variable $?. Return codes are numeric and are limited to being between 0-255 because an unsigned 8-bit integer is used. If you use a value of -1, it will return 255. Each execution terminates with an exit code, whether successful or not, with an error message or silently. For example:
$ date ; echo $? Sat Jan 18 08:06:07 IST 2020 0
As you can see, the exit code is 0 because the command was executed without issues. Now, let’s try this:
$ wrngcmd ; echo $? -bash: wrngcmd: command not found 127
It is a command not found as we just typed a meaningless bunch of characters.
Reserved Bash Exit Codes
So similar to the exit code “0” which denotes success of the command, bash has some reserved exit codes for different situations. So ideally if you are using exit command in a script and need to specify an exit code, do not use these reserved exit codes as they may create conflicting results.
| Exit Code Number | Meaning | Example | Comments |
|---|---|---|---|
| 1 | Catchall for general errors | let “var1 = 1/0” | Miscellaneous errors, such as “divide by zero” and other impermissible operations |
| 2 | Misuse of shell builtins | empty_function() {} | Missing keyword or command |
| 126 | Command invoked cannot execute | /dev/null | Permission problem or command is not an executable |
| 127 | “command not found” | illegal_command | Possible problem with $PATH or a typo |
| 128 | Invalid argument to exit | exit 3.14159 | exit takes only integer args in the range 0 – 255 (see first footnote) |
| 128 +n | Fatal error signal “n” | kill -9 $PPID of script | $? returns 137 (128 + 9) |
| 130 | Script terminated by Control-C | Ctrl-C | Control-C is fatal error signal 2, (130 = 128 + 2, see above) |
| 255* | Exit status out of range | exit -1 | exit takes only integer args in the range 0 – 255 |
Lets understand the exit code “128 +n” with an example. Run the never-ending loop as shown below:
#!/bin/bash
while true; do
echo ${$}
done
If you run this script it will print the same PID indefinitely untill you kill it or do a “CTRL+C”. Let try doing a “CTRL+C” and see what is the exit code.
# sh test.sh .... 2582 2582 2582 2582 2582 2582 ^C geeklab$ echo $? 130
As you can see the exit code is “128+2” i.e. 130. Similarly if we kill the script using “kill -9” the exit code should be “128+9” i.e. 137. Lets see an example of that too:
# sh test.sh .... 2582 2582 2582 ...
# kill -9 [pid_of_script]
.... 2602 2602 Killed: 9 geeklab$ echo $? 137