[SOLVED] Can I make bad [ syntax in a bash if statement cause the shell to exit with an error?

Issue

Consider the following bash code:

if [ "$a" = "foo"]
then
  echo "TRUE"
fi
echo $?

we get:

bash: [: missing `]'
0

So basically, the test in the if fails due to a typo (no space between "foo" and ]), but the exit code of the whole if is still 0 aka. success.

This answer explains why the exit code is 0 (we get the exit code of if which is 0 because nothing was true).

So far so good, but this behavior is extremely dangerous as it can lead to silent errors. Is there a way to rewrite the condition, or change any bash settings, such that typos like these trigger a non-zero exit of the script / ìf?

Solution

Yes, there is a way to rewrite the condition: Don't use [.

#!/bin/bash -e
if [[ "$a" = "foo"]]
then
  echo "TRUE"
fi
echo "REACHED"

properly results in:

yourscript: line 1: syntax error in conditional expression

You can see this tested at https://ideone.com/ZH1Znb

---

Alternately, you can check the exit status:

#!/bin/bash -e

# using legacy ksh syntax because some versions of bash are picky
# about function names with POSIX-y syntax; in new bash, though,
# this also works with `[() {` as the first line.
function [ {
  builtin [ "$@" || {
    rc=$?
    case $rc in
      0|1) return $?;;
      *) echo "ERROR: Bad test syntax" >&2; exit "$rc";;
    esac
  }
}

if [ "$a" = foo]
then
  echo "TRUE"
fi
echo "REACHED"

but this is obviously a horrid abomination.

---

Note that while the above examples use set -e, this feature has serious pitfalls, and I strongly advise against its use.

Answered By - Charles Duffy

Answer Checked By - Dawn Plyler (WPSolving Volunteer)