Command substitution in an if condition masking return values

[yuya-takeyama]

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

This is a script hiding return value:

#!/bin/bash

set -eu
set -o pipefail

if [ -z "$(ls ./tmp)" ]; then
  echo "./tmp is empty"
else
  echo "./tmp is NOT empty"
fi

echo "script completed"

It doesn't stop the execution even if ./tmp doesn't exist. (unexpected)

$ bash before.sh
ls: ./tmp: No such file or directory
./tmp is empty
script completed

Here's a fixed version:

--- before.sh	2019-03-08 00:28:48.000000000 +0900
+++ after.sh	2019-03-08 00:28:04.000000000 +0900
@@ -3,7 +3,8 @@
 set -eu
 set -o pipefail
 
-if [ -z "$(ls ./tmp)" ]; then
+ls_result=$(ls ./tmp)
+if [ -z "$ls_result" ]; then
   echo "./tmp is empty"
 else
   echo "./tmp is NOT empty"

#!/bin/bash

set -eu
set -o pipefail

ls_result=$(ls ./tmp)
if [ -z "$ls_result" ]; then
  echo "./tmp is empty"
else
  echo "./tmp is NOT empty"
fi

echo "script completed"

It stops at the failure of ls. (expected)

$ bash after.sh
ls: ./tmp: No such file or directory

Here's what shellcheck currently says:

No issues detected!

Here's what I wanted or expected to see:

To be honest, I don't understand the exact reasons that the former script completes running the whole script.
But I think it's helpful if it could detect such a problem.

[dseynhae]

I stumble with the same problem on my Windows 10 PC/WSL:
The minimal code to reproduce it:

#!/bin/bash
if ! _exe=$(command -v dummy); then
  echo "_exe ($_exe) does not exist"
fi

• I have installed shellcheck 0.6.0 from chocolatey. In my local environment (no remote session to WSL), there are no issues reported.

• However, if I remote into my WSL (and I made sure that there is no shellchecker installed in my Ubuntu18.04LTS WSL console), the Windows shellchecker complains:

/!\ Syntax error: expected "word" somewhere in the file [xx,yy]

• I see the same problem with the shellchecker on WSL:

sudo apt install shellchecker from the Bionic repository installs an older version (with this error).
sudo apt install ./shellcheck_0.5.0-3_amd64.deb also shows the error.

It looks like the shellchecker parser is expecting a simple command with local variable assignments (and complains that the actual word corresponding to the command is missing), and doesn't understand that an assignment with a command substitution is an expression in itself, returning an exit code.

Except, it works online, and on Windows 10 with 6.0.0 (when NOT using a remote session with WSL).

[junkblocker]

Just to make it not sounds like it's a WSL specific problem (which it couldn't be), this throws a warning on *nix (not just on WSL)

if ! target="$(/usr/bin/readlink "$HOME/.cache")" || [[ -z "$target" ]]; then
    :
fi

with

Syntax error: expected "word" somewhere in the file

[DoxasticFox]

@yuya-takeyama

An optional check was added in #2303 to detect this.

To be honest, I don't understand the exact reasons that the former script completes running the whole script.

This wiki explains set -e's quirks.