Examining Stuff: `test`

read -p "Type Y or N:" user_input

if test $user_input = 'Y'; then
    echo 'You typed Y'
fi

read -p "Type Y or N:" user_input

if [ $user_input = 'Y' ]; then
    echo 'You typed Y'
fi

if a variable value contains whitespaces, should be quoted (unless [[ ]] version is used)

Expressions

Expression	When is `true`
a string	if its length is greater than 0
`-n string`	if its length is greater than 0
`-z string`	if its length equals 0
`string1 = string2`	if both equal
`string1 != string2`	if both are different
`file1 -ef file2`	if both files have the same number of devices and i-nodes
`file1 -nt file2`	if `file1` was modified later than `file2`
`file1 -ot file2`	if `file1` was modified before `file2`
`-b file`	if exists and is special file block
`-c file`	if exists and is special file character
`-d file`	if exists and is a directory
`-e file`	if exists
`-f file`	if exists and is regular file
`-g file`	if exists and has setgid byte set
`-G file`	if exists and is related to a group of a user
`-k file`	if exists and has sticky bit set
`-l file`	if exists and is symbolic link
`-o file`	if exists and a user is its owner
`-p file`	if exists and is a named pipe
`-r file`	if exists and a user can read it
`-s file`	if exists and is not empty (its size is greater than 0 bytes)
`-t file_descriptor`	if file descriptor is open and connected to screen or keyboard (0 - stdin, 1 - stdout, 2 - stderr)
`-u file`	if exists and has setuid byte set
`-w file`	if exists and a user can write in it
-`x file`	if exists and a user can execute it (and find)

== is supported in [[ ]] variant