Linux – How to use grep to search only on the first line of files for a specific string

Two more alternatives :

With awk

awk '{if ($0~"pattern") print $0; nextfile;}' mydir/*

or if your awk version doesn't support nextfile (thanks to Stéphane Chazelas for the suggestion) :

awk 'FNR==1{if ($0~"pattern") print $0;}' mydir/*

will read only the first line before switching to next file, and print it only if it matches "pattern".

Advantages are that one can fine-tune both the field on which to search the pattern for (using e.g. $2 to search on the second field only) and the output (e.g. $3 to print the third field, or FILENAME, or even mix).

Note that with the FNR ("current input record number", i.e. line number) version you can fine-tune further the line(s) on which you want to grep : FNR==3 for the third line, FNR<10 for the 10 first lines, etc. (I guess in this case, if you are dealing with very large files and your awk version supports it you may want to mix FNR with nextfile to improve performances.)

With head, keeping filenames

head -n1 -v mydir/files*|grep -B1 pattern

-v option of head will print filenames, and option -B1 of grep will print the previous line of matching lines — that is, the filenames. If you only need the filenames you can pipe it further to grep :

head -n1 -v mydir/*|grep -B1 pattern|grep ==>

As noticed by don_crissti in comments, beware of filenames matching the pattern themselves, though…