Many times when writing Shell scripts, you may find yourself in a situation where you need to perform an action based on whether a file exists or not.

In Bash, you can use the test command to check whether a file exists and determine the type of the file.
The test command takes one of the following syntax forms:

 test EXPRESSION
 [ EXPRESSION ]
 [[ EXPRESSION ]]

If you want your script to be portable, you should prefer using the old test [ command, which is available on all POSIX shells. The new upgraded version of the test command [[ (double brackets) is supported on most modern systems using Bash, Zsh, and Ksh as a default shell.