System administration¶

Recipes and writeups of solutions from problems on different *nix operating systems.

Table of Contents¶

Users and groups
Useful commands
Package management
Path alternatives
Versions
1. Debian
2. Debian minor versioning
Modifying keymaps with xmodmap
Modifying GNOME

Users and groups¶

Listing all known users and groups

cat /etc/group
cat /etc/passwd

Creating new users¶

Creating a new user, managing startup shell and directory

sudo useradd -d /home/[homedir] [username]
# -u for custom user id

sudo passwd [username]
# to change the password

sudo chsh -s /bin/bash [username]
# set startup shell

NB: The whole user creation process is also streamlined with the

sudo adduser [username]

interactive program.

For managing primary groups

sudo usermod -g [groupname] [username]

For managing secondary groups

sudo usermod -a -G [group1],[group2],[...] [username]

Removing a user from a group

sudo gpasswd -d user group

Deleting users

sudo userdel -r [username]
# -r removes home directory aswell

Configuring shells¶

You can discover what shell your terminal is currently running by examining the $SHELL environment variable.

To see the available shells on your machine, use

cat /etc/shells

You can then change the default shell using

chsh

ran as the user you wish to change the shell for, or, alternatively

sudo chsh -s /bin/zsh someUser

to change the shell of someUser to zsh.

`bindkey`¶

Bindkey controls how keyboard shortcuts on the terminal are mapped. This can be set in the relevant shell ~/.*rc file. Common mappings are

bindkey -v

for vi-like map, and

bindkey -e

for emacs mapping.

NB: new and alternative shells may be installed via the relevant package managers.

Execute as user¶

We can run commands as another user or with another group ID using su. For example, to run a command as another user

su someuser -c whoami
# someuser

Sometimes a user will not have a login shell defined, in which case su can spawn a specific shell as needed

su someuser -s /bin/bash -c "echo $SHELL"
# /bin/bash

The command can also be used to switch user. For example, to become root

su -

Useful commands¶

In this section I will document useful commands, which, for brevity, don’t merit a full chapter of their own.

`STOP` and `CONT` a process¶

As an example, consider you wanted to use Wireshark to capture packets of a specific program, however other programs were being very chatty, and working out exactly what Wireshark filter to craft is proving tedious. A quick and dirty solution to this is just to halt the execution of the chatty program

find the pid: We can find the process ID of any program using

ps aux | grep [name]

send a STOP signal Interrupt and halt the program using

kill -STOP [pid]

resume with a CONT signal Using a very similar command, we run

kill -CONT [pid]

SSL with `curl`¶

https://stackoverflow.com/questions/10079707/https-connection-using-curl-from-command-line

`curl` proxies¶

You can either set the environment variables

export http_proxy="http://uname:pw@addr:port"
export https_proxy="https://uname:pw@addr:port"

which curl automatically uses, or, pass in the flag -x http://uname:pw@addr:port.

Package management¶

With dpkg, you can install with

dpkg -i [package].deb

list installations with

dpkg -l | grep [package_name]

Uninstall

dpkg -r [package_name]

and purge with -P instead of -r. Purge will also delete all configuration files.

Path alternatives¶

You can adjust the priority of conflicting program versions, commonly python3 vs python2 using the update-alternatives command. The program linked with the highest priority will become the default

update-alternatives --install /usr/bin/python python /usr/bin/python3.8 2

update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1

You can check the configuration with

update-alternatives --config python

Versions¶

All sorts of valuable version information can be obtained with different commands, most of which are listed on linuxconfig.

Debian¶

lsb_release -cs
# buster

cat /etc/issue
# Debian GNU/Linux 10 \n \l

cat /etc/os-release
# PRETTY_NAME="Debian GNU/Linux 10 (buster)"
# NAME="Debian GNU/Linux"
# VERSION_ID="10"
# VERSION="10 (buster)"
# VERSION_CODENAME=buster
# ID=debian
# HOME_URL="https://www.debian.org/"
# SUPPORT_URL="https://www.debian.org/support"
# BUG_REPORT_URL="https://bugs.debian.org/"

Useful tools

neofetch for graphical system overview

The default tool for printing system information is uname. The most common use is probably for printing the machine hardware name (similar to arch)

uname -m
# x86_64

Another flag to remember is -a, which prints all information.

Debian minor versioning¶

Although minor versions are a convenience measure (and thus, not included in e.g. uname or /etc/os-release), it can still be useful to know. The full debian version can be found with

cat /etc/debian_version

Modifying keymaps with `xmodmap`¶

I don’t like specific default dead keys, such as the backtick symbol. To modify the behaviour of keys, we use xmodmap.

For my case, first identify the keycode using

xmodmap -pke | grep grave

and look for the output like

keycode  21 = dead_circumflex dead_grave equal plus dead_tilde dead_ogonek dead_cedilla dead_ogonek

This will commonly be 21 or 49, depending on the nationality of your Keyboard.

We then copy the line, remove the dead_ prefix from grave, and configure the mapping with

xmodmap -e 'keycode  21 = dead_circumflex grave equal plus dead_tilde dead_ogonek dead_cedilla dead_ogonek'

To make this change permanent, we create a ~/.Xmodmap dotfile with our modifications. Alternatively, to save an entire configuration, use

xmodmap -pke >> ~/.Xmodmap

We load the changes with

xmodmap ~/.Xmodmap

or to ensure the changes are ran when the X server inits, use

echo 'xmodmap ~/.Xmodmap' >> ~/.xinitrc

To undo a keyboard mapping, use

setxkbmap -option

Modifying GNOME¶

Changing the overlay toggle key (default: super):

gsettings set org.gnome.mutter overlay-key 'XF86LaunchB'

notes

System administration

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