head, tail, wc and nl — Looking Before You Process

A log file can have two million lines. Opening it in an editor is slow, and printing all of it floods the terminal. Before doing anything clever with a file, Asha takes a quick look: the top, the bottom, how big it is, and where each line sits. Four small tools do this.

The rule

Peek before you process: head, tail, count, number.

head shows the first lines, 10 by default:

tail shows the last lines:

Two tail tricks are worth memorising:

• tail -n +2 means "start from line 2", so it prints everything except the header. That's perfect for CSV files.
• tail -f follows a growing file and prints new lines as they're added. Press Ctrl+C to stop. It's the standard way to watch a log live.

wc (word count) counts lines, words and bytes:

-l is lines, -w is words (runs of text separated by spaces) and -c is bytes. Each CSV row has no spaces, so here every line counts as one word.

nl numbers lines:

By default nl skips numbering blank lines. -ba ("body: all") numbers every line. cat -n does much the same.

All four read stdin when you don't give a file, so they fit neatly into pipes.

The shortcut

Turn the question into "which end, and how many?" The start means head, the end means tail, "everything except the first few" means tail -n +N, and "how many" means wc -l.

The trap is tail -n 2 versus tail -n +2. Without the plus you get the last two lines. With it you get everything from line 2 onward.

Drills

Answer fast — trust the rule.

Remember

• Peek before you process: head for the top, tail for the bottom, wc to count, nl to number.
• Ask "which end, and how many?", and use tail -n +2 to skip a header.
• tail -n 2 is the last two lines, tail -n +2 is everything from line 2, and tail -f follows a live file.