Work in the shell

• An alias is a short name for a longer command, like ll for ls -l --color=auto.
• A built-in is part of Bash itself, like cd, echo or history.
• A program (external command) is a file on the disk, found through the folders in $PATH.

$type ll
ll is aliased to `ls -l --color=auto'
$type cd
cd is a shell builtin
$type passwd
passwd is /usr/bin/passwd
$which passwd
/usr/bin/passwd
$echo $PATH
/home/student/.local/bin:/home/student/bin:/usr/local/bin:/usr/bin

When you type a name, Bash looks in this order: alias → built-in → the folders of $PATH (left to right). The current folder is not in the path, so start your own script with ./:

$./backup.sh

$ls -l -a /etc
$ls -la /etc
$ls --all --format=long /etc

• Short options have one dash and one letter (-l). You can join them: -la.
• Long options have two dashes and a word (--all). They are easier to read in scripts.
• Arguments are names of files, folders, users …

Join commands on one line:

$ps aux | grep sshd
$ls -l /etc | less
$cat /var/log/messages | grep -i error | wc -l
$dnf list installed | tee installed.txt | wc -l

tee writes the data to a file and passes it on, so you can save a step in the middle of a pipe.

$history
$history 10
$!42
$!!
$!dnf
$sudo !!

The variable HISTSIZE sets how many commands Bash keeps in memory, HISTFILESIZE how many are kept in the file.

$systemctl sta<Tab>
$systemctl status ssh<Tab>
$nmcli con mod enp1s0 ipv4.<Tab><Tab>

Completion for options and sub-commands comes from the package bash-completion. It is installed on most systems.

$MYDIR=/var/www/html
$echo $MYDIR
$export MYDIR
$env | grep MYDIR
$unset MYDIR

• NAME=value makes a shell variable (no spaces around =).
• export NAME makes it an environment variable: programs started from this shell see it.
• env (or printenv) shows the environment variables; set shows all variables and functions.

Important variables: PATH (where to find programs), HOME (your home folder), USER, LANG (language), PS1 (the prompt), EDITOR (the editor that other tools open).

1. Put your own settings in ~/.bashrc, for example an alias.

   $echo "alias ll='ls -l --color=auto'" >> ~/.bashrc

2. Load the file in the current shell without logging out.

   $source ~/.bashrc

For all users, do not change /etc/profile: add a small file in /etc/profile.d/ instead. Updates of the system do not overwrite it.

1. Quick help:

   $useradd --help

2. The full manual. Search with /word, n for the next match, q to quit.

   $man useradd
   $man 5 passwd

3. Do not know the command? Search the short descriptions of all manuals:

   $man -k partition
   $apropos partition | grep 8

4. If man -k says "nothing appropriate", build the search database:

   #mandb

5. More documentation, often with examples:

   $ls /usr/share/doc/
   $info coreutils

#dnf install command-line-assistant
$c "how do I make a user an administrator"

It needs a registered system and an internet connection. Do not type secrets or personal data in a question.

• A command has three parts: the command, its options (-l, --all) and its arguments (files, users …).
• The shell finds commands as aliases, built-ins and programs in $PATH; type NAME shows which one runs.
• Redirect output with > (overwrite), >> (add), errors with 2>, both with &>; read input with <.
• A pipe | sends the output of one command into the next command.
• history, !number, !! and Ctrl+R repeat old commands; Tab completes names.
• vim has modes: Esc for command mode, i to type, :wq to save and quit, :q! to quit without saving.
• Variables hold values; export makes them visible to child programs. Login settings live in /etc/profile, ~/.bash_profile and ~/.bashrc.
• Help comes from man, man -k (apropos), --help, info and /usr/share/doc.