JIYIK CN >

We can compare strings using various comparison operators and check if a string contains a substring using regular expressions.

String comparison in Bash

String comparison means checking whether the given strings are identical or not. Two or more strings are identical if they are of the same length and contain the same sequence of characters.

We use various string comparison operators which return true or false depending on the condition. Some of the widely used string comparison operators can be listed below.

String1="Hello World!!"
String2="Hello World!!"

String3="Delft Stack"

if [ "$String1" = "$String2" ]; then
    echo "String1 and String2 are equal."
else
    echo "String1 and String2 are not equal."
fi

if [[ "$String1" == "$String2" ]]; then
    echo "String1 and String2 are equal."
else
    echo "String1 and String2 are not equal."
fi

if [[ "$String1" != "$String3" ]]; then
    echo "String1 and String3 are not equal."
else
    echo "String1 and String3 are equal."
fi

Output:

String1 and String2 are equal.
String1 and String2 are equal.
String1 and String3 are not equal.

Here, if we first compare and using =the operator . Since and both have the same length, have the same sequence of characters, the comparison operator returns , so we get as the output of the first block in the program.String1String2String1String2trueString1 and String2 are equal.if-else

Similarly, in the second program, we used ==the operator to compare String1and String2. In this case, we need to use [[for the comparison.

Finally, we compare and using !=the operator .String1String3

Lexical comparison in Bash

Lexical comparison is comparing strings based on alphabetical order. For lexical comparison, we use >and <operators.

name1="Kamal"
name2="Abinash"

if [[ "$name1" > "$name2" ]]; then
    echo "${name1} is greater then ${name2}."
elif [[ "$name1" < "$name2" ]]; then
    echo "${name2} is greater than ${name1}."
else
    echo "Both the namees are equal"
fi

Output:

Kamal is greater then Abinash.

In this program, name1and name2are lexically compared. Since Kis alphabetically after Aand Khas a higher value than A, it "$name1" > "$name2"is returned trueand we get Kamal is greater then Abinash.as output.

Check if a string is empty

We use the "-n" and "-z" operators to check if the string is empty.

String=""
if [[ -z $String ]]; then
  echo "The variable String is an empty string."
fi

Output:

The variable String is an empty string.

In this program, stringis an empty variable. Since -zthe operator returns true, if stringthe length of is 0, hence we get The variable String is an empty string.as the output of the given program.

String="Test"
if [[ -n $String ]]; then
  echo "The variable String is not an empty string."
fi

Output:

The variable String is not an empty string.

In this program, Stringis a non-null variable. Since -nthe operator returns true, if stringthe length of is not 0, therefore we get The variable String is not an empty string.as the output of the given program.

Check if a string contains a substring

To check if a string contains a substring, we can use the =~( Regex) operator.

The operator returns if the string matches the expanded regexexpression . We must craft a suitable expression to do the comparison.regextrueregex

String='My name is Delft.'
if [[ $String =~ .*Delft.* ]]; then
  echo "The given string has Delft on it."
fi

Output:

The given string has Delft on it.

Here, .*Delft.*is the expression to match regex, which means match Delft.any string before and after, 0 or more characters. It checks if there is a substring in the string Delft.

Since Delftexists in the given string, the given condition is satisfied and we get The given string has Delft on it.as the output.