Position of a string within a string using Linux shell script?

With bash

a="The cat sat on the mat"
b=cat
strindex() { 
  x="${1%%"$2"*}"
  [[ "$x" = "$1" ]] && echo -1 || echo "${#x}"
}
strindex "$a" "$b"   # prints 4
strindex "$a" foo    # prints -1
strindex "$a" "ca*"  # prints -1

You can use grep to get the byte-offset of the matching part of a string:

echo $str | grep -b -o str

As per your example:

[user@host ~]$ echo "The cat sat on the mat" | grep -b -o cat
4:cat

you can pipe that to awk if you just want the first part

echo $str | grep -b -o str | awk 'BEGIN {FS=":"}{print $1}'

I used awk for this

a="The cat sat on the mat"
test="cat"
awk -v a="$a" -v b="$test" 'BEGIN{print index(a,b)}'

echo $a | grep -bo cat | sed 's/:.*$//'

This is just a version of the glenn jackman's answer with escaping, the complimentary reverse function strrpos and python-style startswith and endswith function based on the same principle.

Edit: updating escaping per @bruno's excellent suggestion.

strpos() { 
  haystack=$1
  needle=$2
  x="${haystack%%"$needle"*}"
  [[ "$x" = "$haystack" ]] && { echo -1; return 1; } || echo "${#x}"
}

strrpos() { 
  haystack=$1
  needle=$2
  x="${haystack%"$needle"*}"
  [[ "$x" = "$haystack" ]] && { echo -1; return 1 ;} || echo "${#x}"
}

startswith() { 
  haystack=$1
  needle=$2
  x="${haystack#"$needle"}"
  [[ "$x" = "$haystack" ]] && return 1 || return 0
}

endswith() { 
  haystack=$1
  needle=$2
  x="${haystack%"$needle"}"
  [[ "$x" = "$haystack" ]] && return 1 || return 0
}

This can be accomplished using ripgrep (aka rg).

❯ a="The cat sat on the mat"
❯ echo $a | rg --no-config --column 'cat'
1:5:The cat sat on the mat
❯ echo $a | rg --no-config --column 'cat' | cut -d: -f2
5

If you wanted to make it a function you can do:

function strindex() {
    local str=$1
    local substr=$2
    echo -n $str | rg --no-config --column $substr | cut -d: -f2
}

...and use it as such: strindex <STRING> <SUBSTRING>

strindex "The cat sat on the mat" "cat"
5

You can install ripgrep on MacOS with: brew install --formula ripgrep.

A variation (bash) on @Orwellophile 's answer, done out the long way. However, it also does the comparison with string lengths instead of comparing strings. You never know how long a string might be! :-) Hopefully, while clearly longer, this answer will be clearer.

function strpos ()
{
    local -r needle="${1:?}"    ## Prevents empty strings
    local -r haystack="${2:?}"  ## Prevents empty strings

    ## From a copy, attempts to remove characters from the end of a string, greedily.
    local -r remainingHaystack="${haystack%%"$needle"*}"
    local -ir remainingHaystackLength="${#remainingHaystack}"

    ## When the needle is not found in haystack, these values will be equal.
    if (( $remainingHaystackLength == ${#haystack} )); then
        echo -n -1
        return 1
    fi
    
    echo -n $remainingHaystackLength
}

If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the value of parameter with the shortest matching pattern (the ‘%’ case) or the longest matching pattern (the ‘%%’ case) deleted.

Example:

If parameter = "/usr/bin/foo/bin", and word = "/bin"

 ${parameter%word}    ## /usr/bin/foo/bin  --> /usr/bin/foo (non-greedy)
 ${parameter%%word}   ## /usr/bin/foo/bin  --> /usr         (greedy)

If parameter is ‘@’ or ‘*’, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

Bash Reference Manual: Shell Expansion

Most simple is - expr index "The cat sat on the mat" cat

it will return 5