Shell, watch file content as it populates

By Flavio Copes

A guide to the tail UNIX command, used to display the last lines of a file with -n, or to watch a file for new content live with the -f option.

One very useful command line tool I use is tail. It’s a little Unix utility command that is found on almost any *nix system, including macOS and of course Linux.

Here’s the man page for tail. It says the command displays the last part of a file.

The command can be used to display the last x lines of a file by using it with the -n option. For example this command shows the last 2 lines of the file specified:

tail -n 2 <filename>

I almost never used tail like this though. What I used it the most was to “watch” a file for new content appended to it, by using the -f option:

tail -f <filename>

This starts the command and it just waits until there’s something new appended to the file.

For example I have a script that stays active for a while and fetches remote data, then prints some results to a text file. I just watch the results.txt file for incoming data using

tail -f results.txt

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