File: m4.info,  Node: Sysval,  Next: Mkstemp,  Prev: Esyscmd,  Up: Shell commands

13.4 Exit status
================

To see whether a shell command succeeded, use ‘sysval’:

 -- Builtin: sysval
     Expands to the exit status of the last shell command run with
     ‘syscmd’ or ‘esyscmd’.  Expands to 0 if no command has been run
     yet.

     sysval
     ⇒0
     syscmd(`false')
     ⇒
     ifelse(sysval, `0', `zero', `non-zero')
     ⇒non-zero
     syscmd(`exit 2')
     ⇒
     sysval
     ⇒2
     syscmd(`true')
     ⇒
     sysval
     ⇒0
     esyscmd(`false')
     ⇒
     ifelse(sysval, `0', `zero', `non-zero')
     ⇒non-zero
     esyscmd(`echo dnl && exit 127')
     ⇒
     sysval
     ⇒127
     esyscmd(`true')
     ⇒
     sysval
     ⇒0

   ‘sysval’ results in 127 if there was a problem executing the command,
for example, if the system-imposed argument length is exceeded, or if
there were not enough resources to fork.  It is not possible to
distinguish between failed execution and successful execution that had
an exit status of 127, unless there was output from the child process.

   On UNIX platforms, where it is possible to detect when command
execution is terminated by a signal, rather than a normal exit, the
result is the signal number shifted left by eight bits.

     dnl This test assumes kill is a shell builtin, and that signals are
     dnl recognizable.
     ifdef(`__unix__', ,
           `errprint(` skipping: syscmd does not have unix semantics
     ')m4exit(`77')')dnl
     changequote(`[', `]')
     ⇒
     syscmd([{ /bin/sh -c 'kill -9 $$'; } 2>/dev/null; st=$?;
     test $st = 137 || test $st = 265])
     ⇒
     ifelse(sysval, [0], , [errprint([ skipping: shell does not send signal 9
     ])m4exit([77])])dnl
     syscmd([kill -9 $$])
     ⇒
     sysval
     ⇒2304
     syscmd()
     ⇒
     sysval
     ⇒0
     esyscmd([kill -9 $$])
     ⇒
     sysval
     ⇒2304