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

Question

If I have the text in a shell variable, say $a:
a="The cat sat on the mat"
How can I search for "cat" and return 4 using a Linux shell script, or -1 if not found?

Answer 1

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

Answer 2

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}'

Answer 3

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)}'

Answer 4

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

Answer 5

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.

Answer 6

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
}

Answer 7

Most simple is -
expr index "The cat sat on the mat" cat
it will return 5