[SOLVED] Using grep, how to match beginning of line with pattern from stdin

Issue

I have a one-liner that prints out a series a numbers:

124
132
186

I am then piping this output into grep to match these numbers to the beginning of lines in another file but sometimes the second number in the line matches one of the patterns and I get an incorrect match like so:

$ get_id_command | grep -f - users.list
124 => 3456, Charles Charmichael, ccharmichael
132 => 2498, Sarah Walker, swalker
186 => 8934, John Casey, jcasey
240 => 1245, Morgan Grimes, mgrimes

What options do I need for grep to only match patterns at the beginning of the line? I would really like to keep this as a one-linter.

Solution

Prepend a circumflex to each line of your file and it will work. Circumflex does indicate the line start within the pattern. So modify your users.list as described, e.g.

sed -Ei 's|(.*)|^\1|' users.list

After that you should get the desired result by your command

$ get_id_command | grep -f - users.list

Answered By - L. Monty