SC2243 – ShellCheck Wiki

Prefer explicit `-n` to check for output (or run command without `[`/`[[` to check for success)

Problematic code:

if [ "$(mycommand --myflags)" ]
then
  echo "True"
fi

Correct code:

# Check that the command outputs something on stdout
if [ -n "$(mycommand --myflags)" ]
then
  echo "The command had output on stdout"
fi

# Check instead that the command succeeded (exit code = 0)
if mycommand --myflags
then
  echo "The command reported success"
fi

(if the command instead outputs "0" or "false", see SC2244 for integer and "boolean" comparisons)

Rationale:

[ "$(mycommand)" ] is equivalent to [ -n "$(mycommand)" ] and checks whether the command's output on stdout was non-empty.

Users more familiar with other languages are often surprised to learn that it is nothing like e.g. if (myfunction()), since it does not care about what the command/function returns.

Using an explicit -n helps clarify that this is purely a string operation. And of course, if the intention was to check whether the command ran successfully, now would be a good time to fix it as in the alternate example.

Exceptions:

If you are familiar with the semantics of [, you can ignore this suggestion with no ill effects.

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