Grab The First Line Of A File

You can grab the first line of a file with sed using either the p (print) command or the d (delete) command.

First, the print command can be told to print the line matching the line number 1. That combined with the -n flag, which suppresses all lines not explicitly printed, will print just the first line in the file.

$ sed '1 p' README.md
# TIL

Second, the delete command can be told to delete all lines that aren't the first (1) line.

$ sed '1! d' README.md
# TIL

The 1 will match on the first line. By following it with !, that will negate it so that it represents all lines except 1.

See man sed for more details.

Note: there are more efficient ways, not using sed, to get the first line in a file. This is an exercise in using and understanding some sed features.