VulnTech Making Decisions: if and test | Learn Linux

if: run something only when a condition holds

if runs a block only when a test succeeds. The shape is always the same, and fi (if backwards) closes it:

n=7
if [ "$n" -gt 5 ]; then
  echo "$n is big"
fi

In the terminal

student@vulntech:~$ n=7
student@vulntech:~$ if [ "$n" -gt 5 ]; then
>   echo "$n is big"
> fi
7 is big

The [ ... ] is the test command. It checks one thing and succeeds or fails, and if acts on the result. The spaces inside the brackets are required.

else and elif

hour=14
if [ "$hour" -lt 12 ]; then
  echo "Good morning"
elif [ "$hour" -lt 18 ]; then
  echo "Good afternoon"
else
  echo "Good evening"
fi

In the terminal

student@vulntech:~$ hour=14
student@vulntech:~$ if [ "$hour" -lt 12 ]; then
>   echo "Good morning"
> elif [ "$hour" -lt 18 ]; then
>   echo "Good afternoon"
> else
>   echo "Good evening"
> fi
Good afternoon

Testing numbers

Test	True when
`[ "$a" -eq "$b" ]`	a equals b
`[ "$a" -ne "$b" ]`	a is not equal to b
`[ "$a" -lt "$b" ]`	a is less than b
`[ "$a" -le "$b" ]`	a is less than or equal to b
`[ "$a" -gt "$b" ]`	a is greater than b
`[ "$a" -ge "$b" ]`	a is greater than or equal to b

Testing files

These are the checks you'll use most in real scripts, deciding whether a file or folder is there before touching it:

Test	True when
`[ -e path ]`	It exists (file or folder)
`[ -f path ]`	It's a regular file
`[ -d path ]`	It's a folder
`[ -r path ]` / `-w` / `-x`	You can read / write / run it
`[ -s path ]`	It exists and isn't empty

if [ -d /etc ] && [ ! -f /etc/nope ]; then
  echo "/etc is a folder and /etc/nope does not exist"
fi

In the terminal

student@vulntech:~$ if [ -d /etc ] && [ ! -f /etc/nope ]; then
>   echo "/etc is a folder and /etc/nope does not exist"
> fi
/etc is a folder and /etc/nope does not exist

&& joins two tests (and), || means or, and a ! inside the brackets flips a test to mean not.

Testing strings

Test	True when
`[ "$a" = "$b" ]`	The strings are equal
`[ "$a" != "$b" ]`	They're different
`[ -z "$a" ]`	a is empty
`[ -n "$a" ]`	a is not empty

word=""
if [ -z "$word" ]; then
  echo "nothing was set"
fi

In the terminal

student@vulntech:~$ word=""
student@vulntech:~$ if [ -z "$word" ]; then
>   echo "nothing was set"
> fi
nothing was set

Common mistake: Always quote the variables in a test. If $a is empty, [ $a = yes ] becomes [ = yes ] and errors; [ "$a" = yes ] is fine.

Remember: if [ test ]; then ... elif [ test ]; then ... else ... fi. Numbers use -gt, -eq; files use -f, -d; and always quote your variables.