Configuring the system by command¶

Exercise configuring-the-system-by-command-annex.pdf · PDF

A message on your phone says sent 11:20. Your friend is in New York. What does her phone show? 05:20. Same instant, different label. Somebody had to tell each phone where it is.

That is configuration: the facts a machine cannot find out alone — its name, its clock, its language, and where to look for programs. In Monitoring performance you looked at a machine. Today you change it, with the keyboard only. And you will finally understand the most common error message of your career: command not found.

The sheet on the kitchen wall¶

Back to the kitchen of Unit 1. The operating system is the kitchen manager, and the manager keeps a sheet on the wall: the name of the kitchen, the wall clock, the language of the labels, and the list of drawers where the tools are. Today you read and edit that sheet.

The hostname¶

The hostname is the name a machine gives itself.

You see it in the prompt (marta@deb-lab), in every log line and on the network.

$ hostnamectl
 Static hostname: deb-lab
Operating System: Debian GNU/Linux 13 (trixie)
    Architecture: x86-64

(Other lines appear too.) The name is stored in the file /etc/hostname. hostnamectl set-hostname writes that file and tells the running kernel, so no reboot is needed:

sudo hostnamectl set-hostname deb-yoursurname   # small letters, no spaces
sudo nano /etc/hosts                            # change the old name on the 127.0.1.1 line

The terminal that is already open keeps the old name in its prompt: open a new one. If you forget /etc/hosts, every sudo complains:

sudo: unable to resolve host deb-surname: Name or service not known

It is only a warning — sudo still works — but the fix is the line 127.0.1.1 deb-surname in /etc/hosts. At the end of the exercise, put the name deb-lab back in both places: the rest of the course uses it.

The clock: UTC, time zones and NTP¶

Three ideas:

• The kernel counts time in UTC (Coordinated Universal Time): one clock for the whole planet, no summer time.
• The time zone is only a rule to show UTC as local time. Zones are named Region/City: Europe/Madrid.
• NTP (Network Time Protocol) keeps the clock right by asking time servers on the Internet.

$ timedatectl
               Local time: Mon 2026-10-05 22:52:24 CEST
           Universal time: Mon 2026-10-05 20:52:24 UTC
                Time zone: Europe/Madrid (CEST, +0200)
System clock synchronized: yes
              NTP service: active

Try another zone, look, and put Madrid back:

timedatectl list-timezones                      # about 500 lines; q leaves the pager
sudo timedatectl set-timezone America/New_York
date                                            # 6 hours less than Madrid
sudo timedatectl set-timezone Europe/Madrid

Look at the Universal time line while you do it: it does not move. Only the way the time is shown changes.

Why does a server care about ten minutes of error? Think of the logs from Logs and journald: with wrong clocks, the events of two machines cannot be put in order.

The locale¶

A locale is a set of conventions for showing things: the language of messages, the date format, the decimal comma, the currency, the alphabetical order.

locale                          # shows LANG=... and the LC_* variables

The same question, two locales:

$ LC_ALL=es_ES.UTF-8 locale currency_symbol decimal_point
€
,
$ LC_ALL=en_GB.UTF-8 locale currency_symbol decimal_point
£
.

A locale must be generated before you can use it. On Debian, the list lives in /etc/locale.gen:

sudo nano /etc/locale.gen       # delete the "# " before ca_ES.UTF-8 UTF-8 and es_ES.UTF-8 UTF-8
sudo locale-gen
locale -a                       # now lists ca_ES.utf8 and es_ES.utf8
LC_TIME=ca_ES.UTF-8 date        # dilluns, 5 d’octubre de 2026, ...

Careful to uncomment the UTF-8 lines, not their older neighbours.

Environment variables¶

An environment variable is a named note that a process carries and hands to the programs it starts: HOME, USER, LANG, PATH...

Picture the bash process with a small sheet of notes. When it starts a child program, the child receives a copy of the sheet.

flowchart LR
    subgraph BASH ["bash (your terminal)"]
        V1["COURSE=SI<br><i>shell variable only</i>"]
        V2["export COURSE<br><b>now on the sheet</b>"]
    end
    BASH -->|"starts, hands a COPY<br>of the exported notes"| CHILD["<b>child process</b><br>bash -c, python3, a script"]
    CHILD -.->|"can never change<br>the parent's sheet"| BASH

• NAME=value makes a shell variable: only this shell sees it.
• export NAME puts it on the sheet that children receive.
• A child can never change the parent's sheet. Closing the terminal throws the sheet away.

Test it — the single quotes matter, so that the child shell, not yours, reads the variable:

echo $USER $HOME $SHELL
COURSE=SI
bash -c 'echo $COURSE'     # empty line: the child did not get it
export COURSE
bash -c 'echo $COURSE'     # SI

Two classic mistakes: spaces around = (COURSE = SI runs a command called COURSE), and double quotes in the test (then your shell fills in the value and the test proves nothing).

PATH: how the shell finds a command¶

You type ls. There is no file called ls in your folder. How does bash find it?

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

PATH is a list of directories separated by :. The shell looks in them in order and runs the first match.

In the kitchen: the chef asks for "the whisk"; the manager has a list of drawers and opens them in order. If the whisk is in a drawer that is not on the list, the answer is "there is no whisk" — command not found — even though it exists.

echo $PATH | tr ':' '\n'   # one directory per line: 5 for a normal user on Debian 13
type ls                    # ls is aliased to `ls --color=auto'
type cd                    # cd is a shell builtin: not a file at all
which python3              # the directory where the python3 file lives

Your own command¶

mkdir ~/bin
nano ~/bin/hello

#!/bin/bash
echo "Hello from $(hostname), it is $(date +%H:%M)"

chmod +x ~/bin/hello
hello                            # bash: hello: command not found
~/bin/hello                      # works: you gave the full path
export PATH="$HOME/bin:$PATH"    # add your drawer at the front of the list
hello                            # works now, in this terminal

So "command not found" almost never means the program is missing. It means: it is not in any drawer of my list. Your first question is always: where is the file, and is that directory in PATH?

Making it permanent: .bashrc and aliases¶

Open a new terminal and type hello. Not found again. Everything you did lived in one shell and died with it. To keep it, write it in a file that bash reads when it starts:

So your hello will work on its own after the next login: ~/bin exists now.

An alias is a short name for a long command: alias ll='ls -la'.

nano ~/.bashrc        # no sudo! add at the end:
#   alias ll='ls -la'
#   export EDITOR=nano
source ~/.bashrc      # re-read it in this terminal
type ll               # ll is aliased to `ls -la'
echo $EDITOR          # nano

The same on Windows¶

hostname
Get-TimeZone
Get-Culture
$env:Path -split ';'
$env:COURSE = "SI"
notepad $PROFILE

Three differences: environment variables are written $env:NAME; the PATH separator is ; and not :; the start-up file is the one whose path is stored in $PROFILE (it may not exist yet).

Key takeaways¶

• The hostname lives in /etc/hostname; hostnamectl set-hostname changes it live, and /etc/hosts must follow.
• The kernel keeps UTC; the time zone (Europe/Madrid, from the tz database) only changes how it is shown; NTP keeps it right.
• A locale is a set of conventions (dates, decimals, currency, language), generated with locale-gen.
• An environment variable is a note handed to child processes; export is what puts it on the sheet.
• PATH is the ordered list of directories where the shell looks for commands: "command not found" usually means "not in PATH".
• Changes in a terminal die with it; ~/.bashrc (every terminal) and ~/.profile (login) make them permanent. An alias is a short name for a command.

Quick check¶

Next: put the unit so far on one page you can trust under pressure — Sysadmin cheat sheet.