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.