Why do I need parenthesis In bash `set -e` and negated return code

Question

I have a shell script which checks for windows line endings.

set -e
(! git ls-files | xargs grep -I $'\r')

I am using the ! character to negate the return code of the command. Grep will return code 0 when a file with carriage return is found, and ! negates the value to the return code is then 1 and the script exits. When used with grep (no xargs) this works without parentheses. When xargs is used the negation takes place according to $?, as echo $? will print 1, however the script does not exit! After adding parentheses around the whole command, it works as expected. Why are the parentheses needed?

Answer 1

Your problem has nothing to do with xargs.

The -e option of bash is a little tricky.

-e Exit immediately if a pipeline (which may consist of a single simple command), a list, or a compound command , exits with a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test following the if or elif reserved words, part of any command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command's return value is being inverted with !.

Let's look at a much simpler example:

$ cat exit_on_error_test.sh 
#!/bin/bash

trap 'echo Interrupted because of an error' ERR
set -e

! true
echo Exit status: $?

$ ./exit_on_error_test.sh 
Exit status: 1
$

So, even though the exit status of "! true" was non-zero, the script was allowed to run to the end and output the value of the exit status. That's because we didn't have any failing command - the non-zero exit code was due to deliberate negation.

However, if we enclose "! true" in parentheses we introduce a failing (compound) command.

$ cat exit_on_error_test.sh 
#!/bin/bash

trap 'echo Interrupted because of an error' ERR
set -e

(! true) # This as a whole is now a failing (compound) command
echo Exit status: $?

$ ./exit_on_error_test.sh 
Interrupted because of an error
$