Unix command that immediately returns a particular return code?

Question

Is there a standard Unix command that does something similar to my example below

$ <cmd here> 56
$ echo Return code was $?
Return code was 56
$

<cmd here> should be something that can be fork-execed and leaves 56 as the exit code when the process exits. The exit and return shell builtins are unsuitable for what I'm looking for because they affect the invoking shell itself by exiting out of it. <some cmd> should be something that I can execute in non-shell contexts - e.g., invoking from a Python script with subprocess.

E.g., /usr/bin/false always exits immediately with return code 1, but I'd like to control exactly what that return code is. I could achieve the same results by writing my own wrapper script

$ cat my-wrapper-script.sh # i.e., <some cmd> = ./my-wrapper-script.sh
#!/usr/bin/bash
exit $1
$ ./my-wrapper-script.sh 56
$ echo $?
56

but I'm hoping there happens to exist a standard Unix command that can do this for me.

Answer 1

1. A return based function would work, and avoids the need to open and close another shell, (as per Tim Kennedy's comment):

   freturn() { return "$1" ; } 
   freturn 56 ; echo $?
   

   Output:

   56
   

2. using exit in a subshell:

   (exit 56)
   

   With shells other than ksh93, that implies forking an extra process so is less efficient than the above.

3. bash/zsh/ksh93 only trick:

   . <( echo return 56 )
   

   (that also implies an extra process (and IPC with a pipe)).

4. zsh's lambda functions:

   (){return 56}
   

Answer 2

There is no standard UNIX command for just returning a specific value. The GNU Core Utilies provide true and false only.

However, you can easily implement this yourself as ret:

#include <stdlib.h>
int main(int argc, char *argv[]) {
  return argc > 1 ? atoi(argv[1]) : 0;
}

Compile:

cc ret.c -o ret

And run:

./ret 56 ; echo $?

Prints:

56

---

If you need this to work everywhere (where bash is available, that is) without installing any extra tools, you probably need to resort to the following command as @TimKennedy suggested in comments:

bash -c 'exit 56'

---

Note that the valid range of return values is 0..255 inclusive.

Answer 3

If you need the exit status to be set by an executed commands. There is no dedicated command for that¹, but you can use the interpreter of any language that has the capability to exit with an arbitrary exit status. sh is the most obvious one:

sh -c 'exit 56'

With most sh implementations, that's limited to exit codes 0 to 255 (sh will accept greater values but may truncate it or even cause a signal to be sent to the process executing sh like with ksh93 for codes 257 to 320).

An exit code can be any integer (int) value but note that you need to retrieve it with the waitid() interface so the value is not truncated to 8 bits (on Linux, it's still truncated with waitid() as well though). Hence why it's rare (and not a good idea) to use exit codes above 255 (use 0-123 for normal operation).

Alternatively:

awk 'BEGIN{exit 56}'
perl -e 'exit 56'
python -c 'exit(56)'
expect -c 'exit 56'

(those don't truncate the exit code to 8 bits).

With NetBSD find, you can do:

find / -exit 56

_{¹exit is the standard command to do that but being a shell special builtin, there's no requirement that there also be a command by that name in the file system like for regular built-ins, and most systems won't include one}

Answer 4

/* Public domain, http://creativecommons.org/publicdomain/zero/1.0/ */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc,char **argv) {
    if(!strcasecmp(argv[0],"true")) return 0;
    else if (!strcasecmp(argv[0],"false")) return 1;
    else if(argc<2) {fputs("One argument required\n",stderr);return 1;}
    else if(!strcasecmp(argv[argc-1],"true")) return 0;
    else if(!strcasecmp(argv[argc-1],"false")) return 1;
    else return atoi(argv[argc-1]);
}

Save this in a file named returncode.c and gcc -o returncode returncode.c

Answer 5

I wasn't exactly sure if your only problem with the exit command was it exiting out of the current shell, but if so, a subshell might work.

username@host$ $(exit 42)
username@host$ echo $?
42

I tested this on Cygwin Bash just now and it works for me.

Edit: Sorry I missed the part of running it outside a shell context. In that case thsi wouldn't help without wrapping it within a .sh script and executing that from your environment.