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⎕SH, exit codes and stderr

Note that ⎕SH calls /bin/sh; this cannot be altered.

If the command, or command pipe issued using ⎕SH exits with a non-zero exit code, then ⎕SH will terminate with a DOMAIN ERROR, and all output from the command will be lost. To avoid this, add an exit 0 to the end of the command string, and the DOMAIN ERROR will be suppressed. However, this technique does require that some other method is used to determine that the command pipe failed. Example: 

      ⍴⎕SH 'grep no_such_user /etc/passwd'
DOMAIN ERROR
      ⍴⎕SH 'grep no_such_user /etc/passwd'
     ∧

but 

      ⍴⎕SH 'grep no_such_user /etc/passwd ; exit 0'
0

If you are interested in the exit code from the command pipe, rather than any partial output, then, in Version20.0 onwards ⎕DMX.Message has the exit code at the end of the text. Dyalog intends that this value will appear in a more user-friendly form in ⎕DMX at some point.

Example: 

      z←⎕SH 'exit 17'
DOMAIN ERROR
      ⎕DMX.Message
Command interpreter returned failure code 17

⎕SH only captures stdout; unless redirected, any output on stderr will appear in the same terminal window as the session; hitting RD (default Ctrl-L) will force a screen redraw, thereby returning the session to its state before the error output appeared.

⎕SH and starting jobs in background

It is possible to run tasks from within APL using ⎕SH: 

      ⎕sh'myjob'

However, in this case, APL will wait until myjob has completed, and will return the output from myjob (assuming that is that myjob completes with a non-zero exit code). It is possible to start a job that will run in background, without APL waiting for that job to complete, with the job continuing even if APL is terminated:

Example: 

      ⎕sh 'sleep 40000 </dev/null >/dev/null 2>&1 &'

More useful might be to save the stdout and stderr of the command, and pipe the input in from a file; it might also be useful to have the job continue to run even after the user has both quit APL and logs out from the server: 

      ⎕sh 'nohup myjob <my.in >my.out 2>my.err &'