Editing files

Sooner or later you need to change a file on a server: a port number in a config file, a line in a script, a setting that a colleague asked you to turn on. Servers have no desktop and no graphical editor, so you edit the file in the terminal. Two editors cover almost every case. nano is simple and shows its keys on screen. vim is harder to learn, but it is installed on nearly every Linux machine, including the small ones where nano is missing.

By the end you can open a file in nano, change it and save it, do the same in vim, and get out of vim when you are stuck.

Get the editors and a file to practise on

The Docker Ubuntu image leaves out both editors to stay small. Install them; the Installing software topic explains what these commands do.

$ apt update && apt install -y nano vim

On a normal Ubuntu server nano is usually there already, and vi (a smaller relative of vim) is there on almost every distribution.

Create a small config file to practise on. printf prints text, and each \n starts a new line:

$ cd ~
$ printf '# app settings\nport=8080\nlog_level=info\nworkers=2\n' > app.conf
$ cat app.conf
# app settings
port=8080
log_level=info
workers=2

Before you edit a real config file, copy it. If your change breaks something, you can put the old file back in one command:

$ cp app.conf app.conf.bak

nano

Open the file by giving its name. If the file does not exist, nano creates it when you save.

$ nano app.conf

  GNU nano 7.2                        app.conf
# app settings
port=8080
log_level=info
workers=2



                                [ Read 4 lines ]
^G Help      ^O Write Out ^W Where Is  ^K Cut       ^T Execute   ^C Location
^X Exit      ^R Read File ^\ Replace   ^U Paste     ^J Justify   ^/ Go To Line

nano works like a basic text box. Move with the arrow keys, type to insert text, and use Backspace and Delete to remove it. The two lines at the bottom are a reminder of the commands. ^ means Ctrl, so ^X is Ctrl+X. M- means Alt.

Saving takes two steps. Ctrl+O asks for the file name, already filled in, and Enter confirms it:

File Name to Write: app.conf

nano then reports how many lines it wrote:

                               [ Wrote 4 lines ]

If you press Ctrl+X with unsaved changes, nano asks first:

Save modified buffer?
 Y Yes
 N No           ^C Cancel

Y saves (press Enter to confirm the name), N throws your changes away, and Ctrl+C takes you back to the file.

Error messages often name a line number. nano -l app.conf shows line numbers down the left side.

vim and its modes

vim is worth learning for one reason: it is always there. Some commands also open it without asking you, for example when a tool needs you to type a message. If you have never used it, it looks broken, because typing letters does not type letters.

That is because vim has modes:

Open the file:

$ vim app.conf

# app settings
port=8080
log_level=info
workers=2
~
~
"app.conf" 4L, 50B                                            1,1           All

The ~ lines are past the end of the file. The bottom line shows the file's size and, on the right, the cursor's line and column.

You start in normal mode. Move to where you want to type, press i, and the bottom line changes:

-- INSERT --                                                  1,1           All

Now you can type and delete as normal. When you are done, press Esc to go back to normal mode. If you are ever unsure which mode you are in, press Esc. It does no harm in normal mode.

Saving and quitting vim

Save and quit from normal mode by typing a command that starts with :. The command appears at the bottom of the screen and runs when you press Enter.

If you try :q after changing the file, vim refuses:

E37: No write since last change (add ! to override)

You decide: :wq to keep the change, or :q! to drop it.

When you are lost in vim and want out without changing anything, this always works: press Esc twice, type :q! and press Enter.

Moving, searching and changing text in vim

The arrow keys move the cursor in normal mode. These normal-mode keys cover what you need on a server:

Searching puts the cursor on the first letter of the match, which makes cw handy. To change info to debug, type /info, Enter, cw, debug, Esc.

If a search finds nothing, vim tells you at the bottom:

E486: Pattern not found: nothere

You will also see h, j, k and l used to move left, down, up and right. They come from keyboards that had no arrow keys, and many people still use them because their hands stay on the letters.

Editing files you do not own

Most config files live under /etc and belong to root. As an ordinary user you can open them but not save them; nano says Permission denied when you try to write. Open the file as root instead, with sudo nano /etc/... or sudo vim /etc/.... The Users and permissions topic explains owners and sudo.

After editing, compare your file with the copy you made. diff prints only the lines that changed: < lines are from the first file, > lines from the second.

$ diff app.conf.bak app.conf
2c2
< port=8080
---
> port=9090

Tasks

Do these in your Linux shell, with the editors installed and app.conf created as shown above.

Task 1: Change a file with nano

Make a backup of app.conf called app.conf.bak. Then, in nano, change the port from 8080 to 9090 and add a new last line timeout=30. Save, exit, and check the result with cat and diff.

$ cp app.conf app.conf.bak
$ nano app.conf

$ cat app.conf
# app settings
port=9090
log_level=info
workers=2
timeout=30
$ diff app.conf.bak app.conf
2c2
< port=8080
---
> port=9090
4a5
> timeout=30

Task 2: Change a file with vim, then walk away from a change

Open app.conf in vim. Change log_level=info to log_level=debug, delete the workers=2 line, then save and quit. Open the file again, delete the first line, and quit without saving.

$ vim app.conf

$ cat app.conf
# app settings
port=9090
log_level=debug
timeout=30

Quiz

1. You are on a server and run a command that opens a file in an editor you do not recognise. The bottom line is empty, there are ~ lines down the left side, and typing makes strange things happen. How do you get out without changing the file?

2. In vim you type :q and see E37: No write since last change. What does it mean, and what are your two choices?

3. The bottom of nano shows ^O Write Out and M-U Undo. Which keys do you press for each?

4. Why copy a config file before you edit it, and why learn vim when nano is easier?