SC1014

Use if cmd; then .. to check exit code, or if [ "$(cmd)" = .. ] to check output.

Problematic code:

# WRONG
if [ grep -q pattern file ]
then
  echo "Found a match"
fi

# WRONG
if ! [[ logname == $LOGNAME ]]
then
    echo "Possible `su` shell"
fi

Correct code:

if grep -q pattern file
then
  echo "Found a match"
fi

if ! [[ $(logname) == $LOGNAME ]]
then
    echo "Possible `su` shell"
fi

Rationale:

[ ... ] as shell syntax is a simple command that tests for whether certain conditions are true or false, such as whether the value assigned to a variable has a non-zero length ([ -n "${foo}" ]) or whether a file system object is a directory ([ -d "${dir}" ]). If-then-(elif-then)-else-fi statements are logical constructs which themselves contain lists of commands which can include simple commands.

[ is just regular command, like whoami or grep, but with a funny name (see ls -l /bin/[). It's a shorthand for test. [[ is similar to both [ and test, but [[ offers some additional unary operators, such as '=~' the regular expression comparison operator. It allows one to use extglobs such as @(foo|bar) (a "bashism"), among some other less commonly used features.

[[, [ and test are often used within if...fi constructs in the conditional commands position: which is between the 'if' and the 'then.'

There are certain shell syntaxes which can be wrapped directly around simple commands, in particular:

• (1) { ...;}, group commands,
• (2) $( ... ), command substitutions,
• (3) <( ... ) and >( ... ), process substitutions,
• (4) ( ... ), subshells, and
• (5) $(( ... )) and (( ... )), arithmetic evaluations.

Some examples include:

• (1) { echo {a..z}; echo {0..9};} > ~/f,
• (2) [[ $(logname) == $LOGNAME ]],
• (3) readarray -t files < <( find ...),
• (4) (cd /foo || exit 1; tar ...), and
• (5) dd bs=$((2**12)) count=1 if=/dev/zero of=/tmp/zeroed-block, respectively.

Note how in example (2) logname is enclosed directly within a command substitution, which is itself enclosed within a [[ reserved word / conditional expression / compound command.

If you want to check the exit status of a certain command, use that command directly as demonstrated in the correct code, above.

If you want to check the output of a command, use "$(..)" to get its output, and then use test/[ or [[ to do a string comparison:

# Check output of `whoami` against the string `root`
if [ "$(whoami)" = "root" ]
then
  echo "Running as root"
fi

Exceptions:

None.

Related Resources:

For more information, see this problem in the Bash Pitfall list, or generally Tests and Conditionals in the wooledge.org BashGuide

• How do I use a file grep comparison inside a bash if/else statement?
• Bash Pitfalls: if [grep foo myfile]