How to check if a string contains a substring in Bash

You can use Marcus's answer (* wildcards) outside a case statement, too, if you use double brackets:

string='My long string'
if [[ $string == *"My long"* ]]; then
  echo "It's there!"
fi

Note that spaces in the needle string need to be placed between double quotes, and the * wildcards should be outside. Also note that a simple comparison operator is used (i.e. ==), not the regex operator =~.

If you prefer the regex approach:

string='My string';

if [[ $string =~ "My" ]]; then
   echo "It's there!"
fi

I am not sure about using an if statement, but you can get a similar effect with a case statement:

case "$string" in 
  *foo*)
    # Do stuff
    ;;
esac

You should remember that shell scripting is less of a language and more of a collection of commands. Instinctively you think that this "language" requires you to follow an if with a [ or a [[. Both of those are just commands that return an exit status indicating success or failure (just like every other command). For that reason I'd use grep, and not the [ command.

Just do:

if grep -q foo <<<"$string"; then
    echo "It's there"
fi

Now that you are thinking of if as testing the exit status of the command that follows it (complete with semi-colon), why not reconsider the source of the string you are testing?

## Instead of this
filetype="$(file -b "$1")"
if grep -q "tar archive" <<<"$filetype"; then
#...

## Simply do this
if file -b "$1" | grep -q "tar archive"; then
#...

The -q option makes grep not output anything, as we only want the return code. <<< makes the shell expand the next word and use it as the input to the command, a one-line version of the << here document (I'm not sure whether this is standard or a Bashism).

The accepted answer is best, but since there's more than one way to do it, here's another solution:

if [ "$string" != "${string/foo/}" ]; then
    echo "It's there!"
fi

${var/search/replace} is $var with the first instance of search replaced by replace, if it is found (it doesn't change $var). If you try to replace foo by nothing, and the string has changed, then obviously foo was found.

Bash 4+ examples. Note: not using quotes will cause issues when words contain spaces, etc. Always quote in Bash, IMO.

Here are some examples Bash 4+:

Example 1, check for 'yes' in string (case insensitive):

    if [[ "${str,,}" == *"yes"* ]] ;then

Example 2, check for 'yes' in string (case insensitive):

    if [[ "$(echo "$str" | tr '[:upper:]' '[:lower:]')" == *"yes"* ]] ;then

Example 3, check for 'yes' in string (case sensitive):

     if [[ "${str}" == *"yes"* ]] ;then

Example 4, check for 'yes' in string (case sensitive):

     if [[ "${str}" =~ "yes" ]] ;then

Example 5, exact match (case sensitive):

     if [[ "${str}" == "yes" ]] ;then

Example 6, exact match (case insensitive):

     if [[ "${str,,}" == "yes" ]] ;then

Example 7, exact match:

     if [ "$a" = "$b" ] ;then

Example 8, wildcard match .ext (case insensitive):

     if echo "$a" | egrep -iq "\.(mp[3-4]|txt|css|jpg|png)" ; then

Example 9, use grep on a string case sensitive:

     if echo "SomeString" | grep -q "String"; then

Example 10, use grep on a string case insensitive:

     if echo "SomeString" | grep -iq "string"; then

Example 11, use grep on a string case insensitive w/ wildcard:

     if echo "SomeString" | grep -iq "Some.*ing"; then

Enjoy.

How about this:

text="   <tag>bmnmn</tag>  "
if [[ "$text" =~ "<tag>" ]]; then
   echo "matched"
else
   echo "not matched"
fi

Accepted answer is correct but it is hard to read and understand.
For problems related to searching you should always use the $needle in a $haystack idiom.
Since its suggested edit queue is full, I post this:

haystack='There are needles here.'
if [[ "$haystack" == *"needle"* ]]; then
    echo "It's there!"
fi

Since the POSIX/BusyBox question is closed without providing the right answer (IMHO), I'll post an answer here.

The shortest possible answer is:

[ ${_string_##*$_substring_*} ] || echo Substring found!

or

[ "${_string_##*$_substring_*}" ] || echo 'Substring found!'

Note that the double hash is obligatory with some shells (ash). Above will evaluate [ stringvalue ] when the substring is not found. It returns no error. When the substring is found the result is empty and it evaluates [ ]. This will throw error code 1 since the string is completely substituted (due to *).

The shortest more common syntax:

[ -z "${_string_##*$_substring_*}" ] && echo 'Substring found!'

or

[ -n "${_string_##*$_substring_*}" ] || echo 'Substring found!'

Another one:

[ "${_string_##$_substring_}" != "$_string_" ] && echo 'Substring found!'

or

[ "${_string_##$_substring_}" = "$_string_" ] || echo 'Substring found!'

Note the single equal sign!

Extension of the question answered here How do you tell if a string contains another string in POSIX sh?:

This solution works with special characters:

# contains(string, substring)
#
# Returns 0 if the specified string contains the specified substring,
# otherwise returns 1.
contains() {
    string="$1"
    substring="$2"

    if echo "$string" | $(type -p ggrep grep | head -1) -F -- "$substring" >/dev/null; then
        return 0    # $substring is in $string
    else
        return 1    # $substring is not in $string
    fi
}

contains "abcd" "e" || echo "abcd does not contain e"
contains "abcd" "ab" && echo "abcd contains ab"
contains "abcd" "bc" && echo "abcd contains bc"
contains "abcd" "cd" && echo "abcd contains cd"
contains "abcd" "abcd" && echo "abcd contains abcd"
contains "" "" && echo "empty string contains empty string"
contains "a" "" && echo "a contains empty string"
contains "" "a" || echo "empty string does not contain a"
contains "abcd efgh" "cd ef" && echo "abcd efgh contains cd ef"
contains "abcd efgh" " " && echo "abcd efgh contains a space"

contains "abcd [efg] hij" "[efg]" && echo "abcd [efg] hij contains [efg]"
contains "abcd [efg] hij" "[effg]" || echo "abcd [efg] hij does not contain [effg]"

contains "abcd *efg* hij" "*efg*" && echo "abcd *efg* hij contains *efg*"
contains "abcd *efg* hij" "d *efg* h" && echo "abcd *efg* hij contains d *efg* h"
contains "abcd *efg* hij" "*effg*" || echo "abcd *efg* hij does not contain *effg*"

grep -q is useful for this purpose.

The same using awk:

string="unix-bash 2389"
character="@"
printf '%s' "$string" | awk -vc="$character" '{ if (gsub(c, "")) { print "Found" } else { print "Not Found" } }'

Output:

Not Found

string="unix-bash 2389"
character="-"
printf '%s' "$string" | awk -vc="$character" '{ if (gsub(c, "")) { print "Found" } else { print "Not Found" } }'

Output:

Found

Original source: http://unstableme.blogspot.com/2008/06/bash-search-letter-in-string-awk.html

The generic needle haystack example is following with variables

#!/bin/bash

needle="a_needle"
haystack="a_needle another_needle a_third_needle"
if [[ $haystack == *"$needle"* ]]; then
    echo "needle found"
else
    echo "needle NOT found"
fi

case $string in (*foo*)
  # Do stuff
esac

This is the same answer as https://stackoverflow.com/a/229585/11267590. But simple style and also POSIX Compliant.

You can use a logic && to be more compact

#!/bin/bash

# NO MATCH EXAMPLE
string="test"
[[ "$string" == *"foo"* ]] && {
        echo "YES"
}

# MATCH EXAMPLE
string="tefoost"
[[ "$string" == *"foo"* ]] && {
        echo "YES"
}

msg="message"

function check {
    echo $msg | egrep [abc] 1> /dev/null

    if [ $? -ne 1 ];
    then 
        echo "found" 
    else 
        echo "not found" 
    fi
}

check

This will find any occurance of a or b or c

With jq:

string='My long string'
echo $string | jq -Rr 'select(contains("long"))|"It is there"'

The hardest thing in jq is to print the single quote:

echo $string | jq --arg quote "'" -Rr 'select(contains("long"))|"It\($quote)s there"'

Using jq just to check the condition:

if jq -Re 'select(contains("long"))|halt' <<< $string; then
    echo "It's there!"
fi