How To Compare Strings In Bash

Before we dive in to string comparison in Bash. We should first look at the operators that are available for comparison in Bash.

s1 = s2 --> s1 matches s2

s1 != s2 --> s1 does not match s2

s1 < s2 --> s1 less than s2 (sort by alphabetical order and then compare)

s1 > s2 --> s1 greater than s2 (sort by alphabetical order and then compare)

-n s1 --> s1 is not null

-z s1 --> s1 is null

How To Compare Strings Using Equality Operator =

lets start with s1 = s2 operator.

lets declare two string variables.

s1="string1"
s2="string2"

if [ $s1 = $s2 ];then
    echo "equal"
else
    echo "not equal"
fi
not equal

if [[ $s1 = $s2 ]];then
    echo "equal"
else
    echo "not equal"
fi
not equal

Note, bash allows both [ and [[ for comparison. [[ is improvement over [ in bash.

For the rest of the post, I would use [[.

s1="this is string1"
s2="this is string1"

if [[ $s1 = $s2 ]];then
    echo "equal"
else
    echo "not equal"
fi
equal

Note, we don't need double quotes when we are using double square brackets [[. Lets do above comparison using single square bracket [

if [ $s1 = $s2 ];then
    echo "equal"
else
    echo "not equal"
fi
bash: [: too many arguments
not equal

As we see, we got the error bash: [: too many arguments, to fix this we need to use double quotes around the variable.

if [ "$s1" = "$s2" ];then
    echo "equal"
else
    echo "not equal"
fi
equal

Lets use the inequality operator for the same s1 and s2 string comparison.

s1="this is string1"
s2="this is string1"
if [[ $s1 != $s2 ]];then
    echo "notequal"
else
    echo "equal"
fi
equal

s1="string1"
s2="string2"

if [[ $s1 =~ [a-z]+ ]];then
    echo "partially matched"
else
    echo "equal"
fi
partially matched

s1="john"
s2="chloe"

if [[ $s1 < $s2 ]];then
    echo "s2 greater than s1"
else
    echo "s2 less than s1"
fi
s2 less than s1

Lets change s2 to zuban

s1="john"
s2="zuban"

if [[ $s1 < $s2 ]];then
    echo "s2 greater than s1"
else
    echo "s2 less than s1"
fi
s2 greater than s1

Last but not least lets check if string is null or not.

Lets create a null string.:

x=""

if [[ -z $x ]];then
    echo "String is null"
fi
String is null

x="test"

if [[ -z $x ]];then
    echo "String is null"
else
    echo "string is not null"
fi
string is not null

Lets try -n switch for checking if variable is null or not.

if [[ -n $x ]];then
    echo "String is null"
else
    echo "string is not null"
fi
String is null

I hope you would find above post useful.