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

OR

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