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Step 2 of 8 · theory · 4 min

The start, the end and a live log

Nobody reads a log of thousands of lines in full: you usually need the start, the end, or whatever is being written right now.

cat prints a whole file, which suits a short config. With a log of thousands of lines, the start scrolls off the screen. A long file is read in less from the lesson "Working comfortably in the terminal", or in parts:

bash

head -n 3 service.log   # first 3 lines
tail -n 5 service.log   # last 5 lines
tail -f service.log     # the end, then new lines as they arrive

Without -n, both commands show 10 lines. The last lines are last by position in the file. A log appends new records at the end, so that is usually where the fresh events are.

tail -f (f for follow) shows the end of the file and does not exit: it waits for lines to be appended and prints them at once. If no new records come, the terminal looks frozen: tail is waiting. Ctrl+C gets you out.

Check yourself 1 question

The service log has 50,000 lines. A colleague is about to restart the service, and you need to see what it writes. What do you run?

cat prints all 50,000 lines, and you have to dig the restart records out of the end of the output. cat cannot wait for new lines: it prints the file and exits.
It shows the last lines as of now, and the restart records appear later. tail -f prints lines as they are written.
head shows the start of the file, while a log appends new records at the end. Compare head -n 2 service.log with tail -n 2 service.log.

tail -f shows the end of the log and prints the lines the service appends later. Once you have seen what you need, leave with Ctrl+C.

Key points

head and tail with -n show the start and the end of a file; without -n, 10 lines each.
tail -f prints new lines as they are written and waits until you press Ctrl+C.

Found an inaccuracy? Report an error in the lesson

Ubuntu 24.04

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