Find Line Number Matching Given Expression in Shell

Say you want to find the line number that matches a given search string. For instance,
$ cat some_file.txt
this is some file line 1
it was written in Vim line 2
let's learn unix commands line 3

To find an expression, say unix, we can use grep
$ grep unix some_file
let's learn unix commands line 3

To show the line number of the expression, use -n option
$ grep -n unix some_file
3:let's learn unix commands line 3

To output the line number only, pipe with cut command
$ grep -n unix some_file | cut -d : -f 1
3

Here, -d option specifies delimiter, and -f option specifies the position separated by the delimiter.

If there are multiple lines matching the given expression, you can always use -m option of grep to print just first N of them
$ grep -n -m 2 unix some_file | cut -d : -f 1
1
2

Happy hacking!