Explain Usage Of The Linux Left Bracket /usr/bin/[ Command

Q. I see [ command in /usr/bin directory. What is the /usr/bin/[ command used for?

A.This is test command and it is used to check file types and compare values.

General syntax is as follows:
[ EXPRESSION ]

For example, find out if /etc/passwd exists or not:
[ -f /etc/passwd ] && echo "Yes" || echo "No"
Regularly you write it as follows:

if [ -f /etc/passwd ] 
then
   echo "Yes"
else
  echo "No"
fi

if test -f /etc/passwd 
then
   echo "Yes"
else
  echo "No"
fi

Read test or [ command man page for all other expressions.
man [ man test

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

1 comment

Doesn’t work as written in all shells (e.g. BASH – not tested in others)

root@localhost:~/root# ls -l
total 0
-rw——-. 1 root root 0 Oct 25 14:19 test

root@localhost:~/root# [ -a test] echo “Y” || echo “N” // <– Fails
bash: [: missing `]'
N

root@localhost:~/root# [ -a test ]; echo "Y" || echo "N" // <– Requires ';' after ']'
Y

In Bash, the semi-colon ';' separates commands on the same line allowing multiple commands on a single line

–PB–