What Is /Usr/Bin/[ and How Do I Use It?
I Was Looking at Coreutils and Found This as One of the Files Included as Part of Coreutils: /Usr/Bin/[. What Is [ and What Does It Do? It Is an Executable. I...
I was looking at coreutils and found this as one of the files included as part of coreutils: /usr/bin/[. What is [ and what does it do?
It is an executable. I just don't know what it does or how to use it.
$ file /usr/bin/[
/usr/bin/[: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.15, stripped
When I try to run it, I think it is defaulting to the bash built in line expansion. Instead of actually running the file.
$ "/usr/bin/["
/usr/bin/[: missing ‘]’
$ /usr/bin/\[
/usr/bin/[: missing ‘]’
2 Answers
It's an equivalent of the command test. (See info test.) Generally you use it in scripts in conditional expressions like:
if [ -n "$1" ]; then
echo $1
fi
The closing bracket is required to enclose the conditional. (Well, it looks like its required just to look nicer in the code. Does anybody know any other practical reason for it?)
It is equivalent to the test command.
Instead of
if /usr/bin/test -z "$VAR"
then
echo VAR not set
fi
You can use:
if /usr/bin/[ -z "$VAR" ]
then
echo VAR not set
fi
It can be used in loops too:
i=0
while [ $i -lt 10 ]
do
echo $i
((i++))
done
You can also use them in one-liners like this:
[ -z "$VAR" ] && echo VAR not set && exit
[ -f foo.txt ] && cat foo.txt
Sarah Jenkins is a veteran tech journalist with over 12 years of experience covering artificial intelligence, mobile innovations, and digital ethics. Her insights have appeared in leading technology publications worldwide.
