calling-commands

Checking Return Values

Always check return values and give informative return values.

For unpiped commands, use $? or check directly via an if statement to keep it simple.

Example:

if ! mv "${file_list[@]}" "${dest_dir}/"; then
  echo "Unable to move ${file_list[*]} to ${dest_dir}" >&2
  exit 1
fi

# Or
mv "${file_list[@]}" "${dest_dir}/"
if (( $? != 0 )); then
  echo "Unable to move ${file_list[*]} to ${dest_dir}" >&2
  exit 1
fi

Bash also has the PIPESTATUS variable that allows checking of the return code from all parts of a pipe. If it's only necessary to check success or failure of the whole pipe, then the following is acceptable:

tar -cf - ./* | ( cd "${dir}" && tar -xf - )
if (( PIPESTATUS[0] != 0 || PIPESTATUS[1] != 0 )); then
  echo "Unable to tar files to ${dir}" >&2
fi

However, as PIPESTATUS will be overwritten as soon as you do any other command, if you need to act differently on errors based on where it happened in the pipe, you'll need to assign PIPESTATUS to another variable immediately after running the command (don't forget that [ is a command and will wipe out PIPESTATUS).

tar -cf - ./* | ( cd "${DIR}" && tar -xf - )
return_codes=( "${PIPESTATUS[@]}" )
if (( return_codes[0] != 0 )); then
  do_something
fi
if (( return_codes[1] != 0 )); then
  do_something_else
fi

Builtin Commands vs. External Commands

Given the choice between invoking a shell builtin and invoking a separate process, choose the builtin.

We prefer the use of builtins such as the Parameter Expansion functions in bash(1) as it's more robust and portable (especially when compared to things like sed).

Examples:

# Prefer this:
addition=$(( X + Y ))
substitution="${string/#foo/bar}"

# Instead of this:
addition="$(expr "${X}" + "${Y}")"
substitution="$(echo "${string}" | sed -e 's/^foo/bar/')"