How to check if a string contains a substring in B

Question

I have a string in Bash:

string="My string"

How can I test if it contains another string?

if [ $string ?? 'foo' ]; then
  echo "It's there!"
fi

Where ?? is my unknown operator. Do I use echo and grep?

if echo "$string" | grep 'foo'; then
  echo "It's there!"
fi

That looks a bit clumsy.

Answer 1

[[ $string == *foo* ]] && echo "It's there" || echo "Couldn't find"

Answer 2

Compatible answer

As there is 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, 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 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*}" ]; }

That's all folks!

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 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 switche:

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

Answer 3

This also works:

if printf -- '%s' "$haystack" | egrep -q -- "$needle"
then
  printf "Found needle in haystack"
fi

And the negative test is:

if ! printf -- '%s' "$haystack" | egrep -q -- "$needle"
then
  echo "Did not find needle in haystack"
fi

I suppose this style is a bit more classic -- less dependent upon features of Bash shell.

The -- argument is pure POSIX paranoia, used to protected against input strings similar to options, such as --abc or -a.

Note: In a tight loop this code will be much slower than using internal Bash shell features, as one (or two) separate processes will be created and connected via pipes.

Answer 4

This Stack Overflow answer was the only one to trap space and dash chars:

# For null cmd arguments checking   
to_check=' -t'
space_n_dash_chars=' -'
[[ $to_check == *"$space_n_dash_chars"* ]] && echo found

Answer 5

How about this:

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

Answer 6

As Paul mentioned in his performance comparison:

if echo "abcdefg" | grep -q "bcdef"; then
    echo "String contains is true."
else
    echo "String contains is not true."
fi

This is POSIX compliant like the 'case "$string" in' answer provided by Marcus, but is slightly easier to read than the case statement answer. Also note that this will be much much slower than using a case statement, as Paul pointed out, don't use it in a loop.