How do you tell if a string contains another string in POSIX sh?

Question

I want to write a Unix shell script that will do various logic if there is a string inside of another string. For example, if I am in a certain folder, branch off. Could someone please tell me how to accomplish this? If possible I would like to make this not shell specific (i.e. not bash only) but if there's no other way I can make do with that.

#!/usr/bin/env sh

if [ "$PWD" contains "String1" ]
then
    echo "String1 present"
elif [ "$PWD" contains "String2" ]
then
    echo "String2 present"
else
    echo "Else"
fi

Answer 1

Here's yet another solution. This uses POSIX substring parameter expansion, so it works in Bash, Dash, KornShell (ksh), Z shell (zsh), etc. It also supports special characters in strings.

test "${string#*"$word"}" != "$string" && echo "$word found in $string"

A functionalized version with some tests:

# contains(string, substring)
#
# Returns 0 if the specified string contains the specified substring,
# otherwise returns 1.
contains() {
    string="$1"
    substring="$2"
    if [ "${string#*"$substring"}" != "$string" ]; then
        return 0    # $substring is in $string
    else
        return 1    # $substring is not in $string
    fi
}

testcontains() {
    testnum="$1"
    expected="$2"
    string="$3"
    substring="$4"
    contains "$string" "$substring"
    result=$?
    if [ $result -eq $expected ]; then
        echo "test $testnum passed"
    else
        echo "test $testnum FAILED: string=<$string> substring=<$substring> result=<$result> expected=<$expected>"
    fi
}

testcontains  1 1 'abcd' 'e'
testcontains  2 0 'abcd' 'ab'
testcontains  3 0 'abcd' 'bc'
testcontains  4 0 'abcd' 'cd'
testcontains  5 0 'abcd' 'abcd'
testcontains  6 1 '' 'a'
testcontains  7 0 'abcd efgh' 'cd ef'
testcontains  8 0 'abcd efgh' ' '
testcontains  9 1 'abcdefgh' ' '
testcontains 10 0 'abcd [efg] hij' '[efg]'
testcontains 11 1 'abcd [efg] hij' '[effg]'
testcontains 12 0 'abcd *efg* hij' '*efg*'
testcontains 13 0 'abcd *efg* hij' 'd *efg* h'
testcontains 14 1 'abcd *efg* hij' '*effg*'
testcontains 15 1 'abcd *efg* hij' '\effg\'
testcontains 16 0 'a\b' '\'
testcontains 17 0 '\' '\'
testcontains 18 1 '[' '\'
testcontains 19 1 '\' '['
testcontains 20 0 '-n' 'n'
testcontains 21 1 'n' '-n'
testcontains 22 0 '*\`[]' '\`'