If errexit is on, how do I run a command that might fail and get its exit code?

Question

All of my scripts have errexit turned on; that is, I run set -o errexit. However, sometimes I want to run commands like grep, but want to continue execution of my script even if the command fails.

How do I do this? That is, how can I get the exit code of a command into a variable without killing my whole script?

I could turn errexit off, but I'd prefer not to.

Answer 1

Your errexit will only cause the script to terminate if the command that fails is "untested". Per man sh on FreeBSD:

         Exit immediately if any untested command fails in non-interactive
         mode.  The exit status of a command is considered to be explic-
         itly tested if the command is part of the list used to control an
         if, elif, while, or until; if the command is the left hand oper-
         and of an ``&&'' or ``||'' operator; or if the command is a pipe-
         line preceded by the ! keyword.

So .. if you were thinking of using a construct like this:

grep -q something /path/to/somefile
retval=$?
if [ $retval -eq 0 ]; then
  do_something  # found
else
  do_something_else  # not found
fi

you should instead use a construct like this:

if grep -q something /path/to/somefile; then
  do_something  # found
else
  do_something_else  # not found
fi

The existence of the if keyword makes the grep command tested, thus unaffected by errexit. And this way takes less typing.

Of course, if you REALLY need the exit value in a variable, there's nothing stopping you from using $?:

if grep -q something /path/to/somefile; then
  do_something  # found
else
  unnecessary=$?
  do_something $unnecessary  # not found
fi

Answer 2

Here's a way to achieve this: you can "turn off" set -o errexit for some lines of code, and then turn it on again when you decide:

set +e  #disables set -o errexit
grep -i something file.txt
rc="$?" #capturing the return code for last command
set -e #reenables set -o errexit

Another option would be the following:

grep -i something file.txt || rc="$?"

That would allow you to capture the return code on the variable rc, without interrupting your script. You could even extend this last option to capture and process the return code on the same line without risking to trigger an exit:

grep -i something file.txt || rc="$?" && echo rc="$?" > somefile.txt && command || :

The last bit ||: will guarantee that the line above always returns a return code = 0 (true).