Bash Conditions: if and case

Conditions let a script choose what to do, for example only creating a folder if it does not exist yet.

The if Statement

Example

bash

marks=72

if [ "$marks" -ge 50 ]; then
    echo "Pass"
else
    echo "Fail"
fi

Output

Pass

The condition goes inside [ ], with a space after [ and before ].
then starts the code to run, and fi (if backwards) ends the block.

elif

Example

bash

temp=38

if [ "$temp" -gt 40 ]; then
    echo "Extreme heat warning"
elif [ "$temp" -gt 30 ]; then
    echo "Hot day, drink water"
else
    echo "Pleasant weather"
fi

Output

Hot day, drink water

Comparing Numbers

Bash uses letters, not symbols, to compare numbers inside [ ]:

Operator	Meaning
`-eq`	equal
`-ne`	not equal
`-gt`	greater than
`-lt`	less than
`-ge`	greater than or equal
`-le`	less than or equal

You can also use (( )) for number tests with normal symbols: if (( marks >= 50 )); then.

Comparing Strings

Test	True when
`[ "$a" = "$b" ]`	the strings are the same
`[ "$a" != "$b" ]`	the strings are different
`[ -z "$a" ]`	the string is empty
`[ -n "$a" ]`	the string is not empty

Example

bash

city="Karachi"
answer=""

if [ "$city" = "Karachi" ]; then
    echo "Welcome to the City of Lights"
fi

if [ -z "$answer" ]; then
    echo "You did not type anything"
fi

Output

Welcome to the City of Lights
You did not type anything

Always put variables inside double quotes in tests. If a variable is empty and has no quotes, [ $answer = "yes" ] becomes [ = "yes" ] and Bash shows an error.

Testing Files

Test	True when
`-e path`	the file or folder exists
`-f path`	it is a regular file
`-d path`	it is a folder
`-r path`	you can read it
`-s path`	the file is not empty

Example

bash

folder="backups"

if [ ! -d "$folder" ]; then
    mkdir "$folder"
    echo "Created $folder"
else
    echo "$folder already exists"
fi

The first time you run this it prints Created backups. The next time it prints backups already exists. The ! means "not".

Combining Conditions and [[ ]]

Bash also has [[ ]], a safer and more powerful test. Inside it you can join conditions with && (and) and || (or), and use * patterns:

Example

bash

age=22
city="Lahore"

if [[ $age -ge 18 && $city == "Lahore" ]]; then
    echo "Adult from Lahore"
fi

file="photo.jpg"
if [[ $file == *.jpg ]]; then
    echo "This is a JPG image"
fi

Output

Adult from Lahore
This is a JPG image

case

case compares one value against several patterns. It is cleaner than a long elif chain.

Example

bash

day="Sat"

case "$day" in
    Mon|Tue|Wed|Thu)
        echo "Class day"
        ;;
    Fri)
        echo "Short day"
        ;;
    Sat|Sun)
        echo "Weekend"
        ;;
    *)
        echo "Unknown day"
        ;;
esac

Output

Weekend

Each pattern ends with ), each block ends with ;;, and *) catches anything else. esac (case backwards) closes it.

Exercise

Which test checks whether the number in $n is greater than 10?

[ $n > 10 ][ "$n" -gt 10 ][ "$n" -greater 10 ]( $n gt 10 )