Looking at Files: ls -l, less, and Friends

Every column of ls -l

ls alone lists names. ls -l, the long format, tells you nearly everything about each one ¹:

me@linuxbox:~$ ls -l
total 12
drwxr-xr-x 2 me me 4096 Oct  4 21:20 photos
-rw-r--r-- 1 me me 2140 Oct  4 21:15 notes.txt
lrwxrwxrwx 1 me me    9 Oct  4 21:30 latest -> notes.txt

Reading -rw-r--r-- 1 me me 2140 Oct 4 21:15 notes.txt left to right ¹:

1. Type, the first character: - an ordinary file, d a directory, l a symbolic link.
2. Permissions, the next nine: read, write, execute for the owner, then the group, then everyone else. Module 3 is all about these.
3. Hard links: how many names this file has (next lesson).
4. Owner: the user it belongs to.
5. Group: the group it belongs to.
6. Size in bytes.
7. Last modified, date and time. Files older than about six months show the year instead of the time.
8. Name. For a link, -> and where it points.

total 12 at the top is the space the listing uses on disk, in kilobyte blocks.

The ls options that matter

Most of these work with or without -l, and stack ¹:

ls -lhtr is a classic: long listing, readable sizes, oldest first so the newest lands right above your prompt. And you can list several places at once: ls ~ /etc /tmp.

What kind of file is it?

On Linux, a name says nothing reliable about the contents. file looks at the contents and tells you ¹:

me@linuxbox:~$ file notes.txt photo.jpg /usr/bin/ls /etc
notes.txt:   ASCII text
photo.jpg:   JPEG image data, JFIF standard 1.01
/usr/bin/ls: ELF 64-bit LSB pie executable, x86-64, ...
/etc:        directory

Run file before opening something unfamiliar. “ASCII text” or “UTF-8 Unicode text” means you can read it with the tools below. Anything else is binary, and dumping it to the terminal just prints gibberish (if your terminal gets scrambled, type reset).

Plain text is just characters stored as numbers, one byte each for ordinary letters, with nothing else: no fonts, no formatting. It isn’t a word processor document ¹. Linux runs on text: nearly every setting in /etc and every shell script is a text file you can read ¹.

cat and less

cat prints whole files to the terminal, one after another ². It’s for short files:

me@linuxbox:~$ cat /etc/hostname
linuxbox

For anything longer, use less, a pager you can scroll ¹:

me@linuxbox:~$ less /etc/passwd

It uses the same keys you met in man pages: Space and b to page, /word to search and n for the next match, g and G for top and bottom, q to quit ¹. Add -N for line numbers and -S to stop long lines from wrapping.

head and tail

head shows the first lines of a file, tail the last, ten by default; -n picks how many ^{2 1}:

me@linuxbox:~$ head -n 3 /etc/passwd
me@linuxbox:~$ tail -n 20 /var/log/dpkg.log

tail -f (“follow”) keeps the file open and prints new lines as they’re added, until you press Ctrl-C ¹. It’s how you watch a log while something happens.

Counting and measuring

me@linuxbox:~$ wc /etc/passwd
  34   57 1912 /etc/passwd
me@linuxbox:~$ du -sh ~/photos
3.4G    /home/me/photos
me@linuxbox:~$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc       1007G   12G  944G   2% /

• wc, word count: lines, words, and bytes. wc -l for just lines ¹.
• du -sh, disk usage: the total size of a folder and everything in it (-s summarize, -h readable). du -sh * lists each item in the current folder.
• df -h, disk free: how full each mounted file system is.
• stat file prints everything the system knows about one file: size, owner, permissions, and three timestamps.

Your turn

So

ls -l tells you a file’s type, permissions, owner, size, and age at a glance. file tells you what it really is. cat, less, head, and tail let you read text without opening an editor, and wc, du, and df count lines and measure space.