A construct
$(command)
is substituted with the output of the shell command command.
Xenv performs substitution by running $SHELL -c
command and replacing the construct with the standard output produced
by the command, with any trailing newlines removed. If the
SHELL variable is not set, ‘/bin/sh’ is assumed.
The command is passed to the shell verbatim. This means, in
particular, that variable references in the command are expanded by
the shell, rather than by xenv and, as a consequence, that the
options -u, -r, and -e don’t affect command
substitutions.
By default, xenv will wait for as long as it takes for the
command to terminate. It is, however, possible to limit command
execution time using the -t n option. If this option
is given, the command is permitted to run no longer than n
seconds. If it doesn’t terminate within that time, it will be
forcibly terminated by sending it the SIGKILL signal, and a
diagnostic message to that effect will be printed on standard error.
Command substitutions can be disabled using the --Wno-command command line option (see feature control).