Intro To '[' Command In Linux

2023-08-09 - By Robert Elder

     I use the 'opening left square bracket' command as a syntactically convenient substitute for the 'test' command:

[ 0 ]

echo $?

0

The '[' Can Be An Executable

     I can write a single-line if statement using the square bracket command like this:

if [ 5 -gt 3 ]; then echo "a"; else echo "b"; fi

a

     or like this:

if /usr/bin/[ 5 -gt 3 ]; then echo "a"; else echo "b"; fi

a

     or I can use the 'test' command like this:

if /usr/bin/test 5 -gt 3; then echo "a"; else echo "b"; fi

a

     In both scenarios, the expression to evaluate and the closing square bracket are simply passed like regular arguments to the test command executable.

Executable Program Or Shell Built-In?

     In practice, the '[' command can be confusing due to the fact that the 'test' and the square bracket commands can both exist as built-in shell features, or as stand-alone executables:

type -a [

[ is a shell builtin
[ is /usr/bin/[
[ is /bin/[

type -a test

test is a shell builtin
test is /usr/bin/test
test is /bin/test

which [

/usr/bin/[

which test

/usr/bin/test

     In the GNU Coreutils implementation of the 'test' command, you can see that the 'test' and the '[' commands are implemented by the same piece of source code.

     Furthermore, you can see that the man pages of these two commands are completely identical:

vim -d <(man [) <(man test)

     And that's why the '[' command is my favourite Linux command.