How many lines in a file: wc -l

On Linux, you rarely solve "open the file and count by eye." More often you ask the system, and it answers with a number. How many users are in a list, how many errors in a log, how many lines in an export: there's one small command for all of this, wc. This is the first lesson of the track, so let's start with it.

Where to practice

The examples below work in a ubuntu:24.04 container. You can start the same one and follow along step by step:

docker run --rm -it ubuntu:24.04 bash

--rm removes the container after exit. On Linux and in WSL2 the commands work right in your own terminal too. On macOS as well, but the output looks slightly different, more on that below.

Create the file we'll practice on:

printf 'root:x:0\nadmin:x:1\nweb:x:2\nbot:x:3\n' > /tmp/users.lst

\n here means a newline. The file has four lines, one user per line.

What wc prints

wc (word count) with no flags prints three numbers and the file name:

wc /tmp/users.lst

 4  4 35 /tmp/users.lst

That's lines, words, and bytes. Usually you need just one number, and you pick it with a flag: -l for lines, -w for words, -c for bytes.

wc -l /tmp/users.lst

4 /tmp/users.lst

How to get a single number

If you pass the file as an argument, wc tacks its name onto the number. That's handy on screen. But in a script or in a file holding the answer, the name just gets in the way: you need 4, not 4 /tmp/users.lst.

To drop the name, feed the file through input redirection <:

wc -l < /tmp/users.lst

4

The difference is who opens the file. With an argument, wc opens it itself and knows the name. With <, the shell opens the file and feeds its content to the command's input, that is, to stdin. wc sees only a stream of lines with no name attached, so there is no name to print next to the number. cat /tmp/users.lst | wc -l gives the same result.

How to save the answer to a file

Output redirection > sends whatever the command would have printed to the screen into a file instead. If the file doesn't exist, it's created; if it does, it's overwritten.

wc -l < /tmp/users.lst > /tmp/answer
cat /tmp/answer

4

You can use both < and > in one command: the first says where to read from, the second where to write to.

If you need to append to the file instead of overwriting it, use >>:

echo 'guest:x:4' >> /tmp/users.lst
wc -l < /tmp/users.lst

5

What exactly -l counts

wc -l doesn't count "lines you can see", it counts newline characters \n. Usually that's the same thing, because every line of a text file ends with \n. But an empty line is also a \n, and it gets counted:

printf 'a\nb\nc\n\n' | wc -l

4

There are three visible lines, and wc counted four. Keep this in mind when a number looks off: look at the file itself, for example with cat -A, which marks the end of every line with $.

Common mistakes

• wc -l /tmp/users.lst > /tmp/answer: the file ends up with 4 /tmp/users.lst, not a number. You need < before the file name.
• wc -l > /tmp/users.lst: the arrow points the wrong way. The shell immediately clears the file so it can write output into it, and wc with no file waits for input from the keyboard, so the terminal "hangs." Interrupt with Ctrl+C, and the list is already empty. Input is <, output is >.
• A typo in the path: wc: /tmp/user.lst: No such file or directory. Check the name with ls /tmp.
• macOS: the built-in BSD wc pads the number with spaces. wc -l < /tmp/users.lst prints 4, not 4. That doesn't bother a human, but a string comparison in a script may not match. Strip the spaces with tr -d ' '.

What you'll need for the tasks

• Count lines with wc -l and get a single number with <.
• Save command output to a file with > and check the result with cat.
• Check whether the number matches what's actually in the file, using cat -A.