SC2243

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

Optional - avoid-nullary-conditions

This is an optional rule, which means that it has a special "long name" and is not enabled by default. See the optional page for more details. In short, you have to enable it with the long name instead of the "SC" code like you would with a normal rule:

.shellcheckrc

enable=avoid-nullary-conditions # SC2243

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.

Related resources:

• Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!