How to check if a string contains a substring in Bash

Question

I have a string in Bash:

string=\"My string\"

How can I test if it contains another string?

if [ $string ?? \'foo\' ]; then         


        

Answer 1

If you prefer the regex approach:

string='My string';

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

Answer 2

stringContain variants (compatible or case independent)

As these Stack Overflow answers tell mostly about Bash, I've posted a case independent Bash function at the very bottom of this post...

Anyway, there is my

Compatible answer

As there are already a lot of answers using Bash-specific features, there is a way working under poorer-featured shells, like BusyBox:

[ -z "${string##*$reqsubstr*}" ]

In practice, this could give:

string='echo "My string"'
for reqsubstr in 'o "M' 'alt' 'str';do
  if [ -z "${string##*$reqsubstr*}" ] ;then
      echo "String '$string' contain substring: '$reqsubstr'."
    else
      echo "String '$string' don't contain substring: '$reqsubstr'."
    fi
  done

This was tested under Bash, Dash, KornShell (ksh) and ash (BusyBox), and the result is always:

String 'echo "My string"' contain substring: 'o "M'.
String 'echo "My string"' don't contain substring: 'alt'.
String 'echo "My string"' contain substring: 'str'.

Into one function

As asked by @EeroAaltonen here is a version of the same demo, tested under the same shells:

myfunc() {
    reqsubstr="$1"
    shift
    string="$@"
    if [ -z "${string##*$reqsubstr*}" ] ;then
        echo "String '$string' contain substring: '$reqsubstr'.";
      else
        echo "String '$string' don't contain substring: '$reqsubstr'."
    fi
}

Then:

$ myfunc 'o "M' 'echo "My String"'
String 'echo "My String"' contain substring 'o "M'.

$ myfunc 'alt' 'echo "My String"'
String 'echo "My String"' don't contain substring 'alt'.

Notice: you have to escape or double enclose quotes and/or double quotes:

$ myfunc 'o "M' echo "My String"
String 'echo My String' don't contain substring: 'o "M'.

$ myfunc 'o "M' echo \"My String\"
String 'echo "My String"' contain substring: 'o "M'.

Simple function

This was tested under BusyBox, Dash, and, of course Bash:

stringContain() { [ -z "${2##*$1*}" ]; }

Then now:

$ if stringContain 'o "M3' 'echo "My String"';then echo yes;else echo no;fi
no
$ if stringContain 'o "M' 'echo "My String"';then echo yes;else echo no;fi
yes

... Or if the submitted string could be empty, as pointed out by @Sjlver, the function would become:

stringContain() { [ -z "${2##*$1*}" ] && [ -z "$1" -o -n "$2" ]; }

or as suggested by Adrian Günter's comment, avoiding -o switches:

stringContain() { [ -z "${2##*$1*}" ] && { [ -z "$1" ] || [ -n "$2" ];};}

Final (simple) function:

And inverting the tests to make them potentially quicker:

stringContain() { [ -z "$1" ] || { [ -z "${2##*$1*}" ] && [ -n "$2" ];};}

With empty strings:

$ if stringContain '' ''; then echo yes; else echo no; fi
yes
$ if stringContain 'o "M' ''; then echo yes; else echo no; fi
no

Case independent (Bash only!)

For testing strings without care of case, simply convert each string to lower case:

stringContain() {
    local _lc=${2,,}
    [ -z "$1" ] || { [ -z "${_lc##*${1,,}*}" ] && [ -n "$2" ] ;} ;}

Check:

stringContain 'o "M3' 'echo "my string"' && echo yes || echo no
no
stringContain 'o "My' 'echo "my string"' && echo yes || echo no
yes
if stringContain '' ''; then echo yes; else echo no; fi
yes
if stringContain 'o "M' ''; then echo yes; else echo no; fi
no