5 Quick Bash Techniques to Test If a String Contains a Substring

When writing Bash scripts, you often need to verify whether a string contains a given substring. Below are several common techniques.

1. Using grep

str1="abcdefgh"
str2="def"
result=$(echo $str1 | grep "${str2}")
if [[ "$result" != "" ]]
then
    echo "包含"
else
    echo "不包含"
fi

This method pipes the long string to grep and checks if the result is non‑empty.

2. String operator =~

str1="abcdefgh"
str2="def"
if [[ $str1 =~ $str2 ]]
then
    echo "包含"
else
    echo "不包含"
fi

The =~ operator directly tests for a match.

3. Using wildcards

str1="abcdefgh"
str2="def"
if [[ $str1 == *$str2* ]]
then
    echo "包含"
else
    echo "不包含"
fi

The asterisk (*) acts as a wildcard for any characters surrounding the substring.

4. Using a case statement

str1="abcdefgh"
str2="def"
case $str1 in
    *"$str2"*) echo Enemy Spot ;;
    *) echo nope ;;
esac

The pattern *"$str2"* matches when the substring is present.

5. Using parameter replacement

str1="abcdefgh"
str2="def"
if [[ ${str1/${str2}//} == $str1 ]]
then
   echo "不包含"
else
   echo "包含"
fi

This technique removes the substring from the original string and compares the result.

Checking if a file contains a given string

Method 1: grep -c

# grep -c returns the number of matching lines
FIND_FILE="/home/linduo/test/Test.txt"
FIND_STR="Hello Weijishu"
if [ `grep -c "$FIND_STR" $FIND_FILE` -ne '0' ]; then
    echo "The File Has Hello Weijishu!"
    exit 0
fi

Method 2: cat with while read

FIND_FILE="/home/weijishu/test/Test.txt"
FIND_STR="Hello Weijishu"
cat $FIND_FILE | while read line
do
    if [[ $line =~ $FIND_STR ]]; then
        echo "The File Has Hello Weijishu!"
    fi
done

Note the difference between using [[ ... ]] and [ ... ] in conditional expressions.