Working with Files

Reading files

cat notes.txt          # print the whole file (fine for small files)
less notes.txt         # page through a big file (arrows/space to move, q to quit)
head notes.txt         # first 10 lines
head -n 3 notes.txt    # first 3 lines
tail notes.txt         # last 10 lines
tail -n 3 notes.txt    # last 3 lines
tail -f /var/log/syslog  # follow a file as it grows (Ctrl+C to stop)

cat on a 10,000-line log file will blast it all past your screen — that's what less is for. tail -f ("follow") is how you watch a log file live while something happens.

wc notes.txt           # word count: lines, words, bytes

Check: On a file containing hello linux, wc prints 1 2 12 — 1 line, 2 words, 12 bytes (the newline counts as a byte).

Editing files

The terminal's classic editor is nano — it shows its commands at the bottom (^X means Ctrl+X). It's simple and always available:

nano notes.txt

Edit, then Ctrl+O to save, Ctrl+X to exit. (You'll meet vim eventually — it has a learning cliff, not a curve. Nano first.)

On a desktop Linux, you can also use the graphical editor. No shame in it.

Redirection: files from commands

These two symbols are worth more than they look:

echo "hello" > greeting.txt     # > writes output INTO a file (overwrites!)
echo "again" >> greeting.txt     # >> APPENDS to a file
cat < greeting.txt               # < feeds a file in as input

Careful: > overwrites silently. echo oops > notes.txt replaces your file with "oops" — no warning, no undo. When in doubt, use >> (append), or back the file up first with cp.

Wildcards

ls *.txt        # every file ending in .txt
ls photo-*.jpg # photo-1.jpg, photo-2.jpg, ...
ls ?.txt       # a.txt, b.txt — ? matches exactly one character

The shell expands wildcards before the command runs — ls never sees the *, it sees the list of matching files. If nothing matches, the pattern is passed through literally, which confuses beginners: ls *.xyz with no matches tries to list a file literally named *.xyz.

Compressing and archiving

tar -czf backup.tar.gz projects/   # pack projects/ into backup.tar.gz
tar -tzf backup.tar.gz              # list what's inside (without unpacking)
tar -xzf backup.tar.gz              # unpack it

tar packs many files into one (originally for tape archives); the z adds gzip compression. You'll mostly unpack other people's tarballs — -tzf to peek first is a good habit, so nothing sprays files across your current directory unexpectedly.

Next: Permissions