Reading and Editing Files

Almost everything on a Linux system is configured with a text file and diagnosed with a text file. Reading them quickly is a genuine skill, and it starts with knowing which of four tools to reach for.

Pick the right reader

cat dumps a whole file to the screen. Perfect for short files, terrible for long ones — a 5,000-line log will scroll past faster than you can read and leave you at the bottom.

cat /etc/hostname

less opens a file in a pager you can scroll and search. This is the right default for anything you can't see in one screen.

less /var/log/syslog

Inside less: arrow keys or Space to scroll, /word to search forwards, n for the next match, G to jump to the end, g for the start, and q to quit. That last one matters — being stuck in a pager with no idea how to leave is a rite of passage nobody enjoys.

head and tail show you the first or last ten lines:

head -n 20 access.log     # first 20 lines
tail -n 50 access.log     # last 50 lines

tail -f is the one you'll use most in real work. It shows the end of a file and then keeps watching, printing new lines as they're written:

tail -f /var/log/nginx/error.log

Leave that running in one window, reproduce your bug in another, and watch the error appear in real time. Ctrl+C stops it.

Editing without leaving the terminal

Sooner or later you'll need to change a config file on a machine that has no GUI. You have two realistic options.

nano — the one to learn first

nano config.txt

nano behaves the way you expect a text editor to behave. Type to insert, arrow keys to move. The commands are listed along the bottom of the screen, where ^ means Ctrl:

• Ctrl+O — write out (save), then Enter to confirm the filename
• Ctrl+X — exit
• Ctrl+K — cut the current line

If nano is available, use it. There is no prize for suffering.

vim — the one you'll eventually meet anyway

vim is on essentially every Unix machine ever built, including minimal containers and rescue images where nano isn't installed. You don't need to learn vim properly today. You need to not be trapped by it.

The one idea that makes vim make sense: it has modes. When it opens you are in normal mode, where keys are commands, not text. This is why typing "hello" into a fresh vim does something alarming instead of writing "hello".

The survival sequence:

1. Press i to enter insert mode. Now typing works normally.
2. Press Esc to return to normal mode.
3. Type :wq and Enter to write and quit.
4. Or type :q! and Enter to quit and throw away every change.

That's it. Esc, then :wq to save or :q! to bail. Those four keystrokes get you out of any vim session you didn't mean to start.

Next: permissions — the reason things say "Permission denied", and what rwx actually means.