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