Bash Gotchas [[ ... ]] v/s [ ... ]: Part-2

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The $? Variable and Exit Statuses

In our previous discussion, we explored the subtleties between [[ ... ]] and [ ... ] in Bash scripting and when to use each. However, a common mistake is always using [[ ]] or [ ] with if conditions, even when it's not appropriate. Remember, [[ ]] and [ ] are used for evaluating conditional expressions such as string comparisons, pattern matching, or file tests - not for checking the exit status of commands or functions.

Common Mistake: Misusing [[...]] or [...] to Check Command Exit Status

Let's consider an example to illustrate this point:

function myfunc() {
    echo "myfunc ran"
    [[ $# -eq 0 ]] && return 1 || return 0
}

if [[ myfunc ]]; then
    echo "return success"
else
    echo "return failure"
fi

What Is the Output?

return success

Explanation

In this example, myfunc never runs in the if condition. This is because [[ myfunc ]] is evaluating the conditional expression with the string "myfunc", not executing the function. In Bash, any non-empty string inside [[ ]] evaluates to true, so the then branch is always executed, regardless of the function's existence or its return value.

Attempting to Fix with Command Substitution

Now, let's try using command substitution:

function myfunc() {
    echo "myfunc ran"
    [[ $# -eq 0 ]] && return 1 || return 0
}

if [[ $(myfunc) ]]; then
    echo "return success"
else
    echo "return failure"
fi

What Is the Output?

return success

Explanation

Here, $(myfunc) executes myfunc and captures its output, not its exit status. The if [[ ... ]] condition now evaluates the output of myfunc, which is the string "myfunc ran". Since this is a non-empty string, the condition evaluates to true, and "return success" is printed.

However, this still doesn't correctly check the function's exit status.

The Correct Way to Check Function Exit Status

To properly check whether myfunc succeeds or fails based on its exit status, you should call the function directly in the if condition without [[ ]] or [ ]. In Bash, if statements can directly evaluate the exit status of a command or function.

Corrected Script

function myfunc() {
    echo "myfunc ran"
    [[ $# -eq 0 ]] && return 1 || return 0
}

if myfunc; then
    echo "return success"
else
    echo "return failure"
fi

if myfunc "arg"; then
    echo "return success"
else
    echo "return failure"
fi

Output:

myfunc ran
return failure
myfunc ran
return success

Explanation

• First if myfunc; then block:

  • myfunc is called without arguments, so $# -eq 0 is true.

  • The function executes return 1, indicating failure.

  • The if condition evaluates the exit status 1 (non-zero), which is considered false.

  • "return failure" is printed.

• Second if myfunc "arg"; then block:

  • myfunc is called with one argument, so $# -eq 0 is false.

  • The function executes return 0, indicating success.

  • The if condition evaluates the exit status 0, which is considered true.

  • "return success" is printed.

Key Takeaways

• Use if myfunc; then to check a function's exit status.

  • The if statement directly evaluates the exit status of the command or function.

  • An exit status of 0 is true; any non-zero exit status is false.

• Do not use [[ ]] or [ ] when you intend to check the exit status of a command or function.

  • [[ ]] and [ ] are for evaluating conditional expressions, not for executing commands.

  • Placing a command inside [[ ]] or [ ] does not execute it; it treats it as a string or expression.

• Command substitution $(...) captures the output of a command, not its exit status.

  • Using $(myfunc) inside [[ ]] captures the output of myfunc.

  • If the output is non-empty, the condition will be true, regardless of the function's exit status.

• What about using [ ] (the test command)?

  • Similar to [[ ]], using if [ myfunc ]; then checks if the string myfunc is non-empty.

  • It does not execute the myfunc function.

• Using the $? Variable:

  • Alternatively, you can capture the exit status using $? immediately after the function call. This method works but is less concise than calling the function directly in the if condition.

myfunc
if [ $? -eq 0 ]; then
  echo "return success"
else
  echo "return failure"
fi

Conclusion

Bash has many such gotchas due it’s subtle nature. These gotchas can be significantly time consuming to debug if you don’t know them. Have you encountered other "gotchas" in Bash scripting?
Share them with us—we’d love ❤ to discuss more!