Master Linux Shell: Essential Commands, Prompt Customization, and Session Management

1 What is a shell

Shell is the user interface of Linux, a command interpreter that receives user commands and passes them to the kernel for execution.

<code>echo ${SHELL}
# /bin/bash</code>

To view all shells installed on the system:

<code>cat /etc/shells
# /bin/sh
# /bin/bash
# /usr/bin/sh
# /usr/bin/bash
# /bin/tcsh
# /bin/csh</code>

2 Setting the hostname

Temporary change:

<code>hostname localhost</code>

Permanent change (CentOS 7 / Ubuntu 18.04+):

<code>hostnamectl set-hostname localhost</code>

3 Command prompt

Prompt format placeholders:

\e – escape character (\033)

\u – current user

\h – short hostname

\H – full hostname

\w – current working directory

\W – basename of current directory

\t – 24‑hour time

\T – 12‑hour time

! – command history number

Example to view current PS1:

<code>cat $PS1</code>

Persist prompt on CentOS:

<code>echo 'PS1="\[\e[1;32m\][\t \[\e[1;33m\]\u\[\e[35m\]@\h\[\e[1;31m\] \W\[\e[1;32m\]]\[\e[0m\]\\$"' > /etc/profile.d/env.sh</code>

Persist prompt on Ubuntu:

<code>echo "PS1='\[\e[1;35m\][\u@\h \W]\\$\[\e[0m\]'" >> .bashrc</code>

4 Internal vs external commands

Internal commands are built into the shell and reside in memory; external commands are executable files on disk loaded only when invoked.

To list internal commands:

<code>help</code>

Enable or disable an internal command:

<code>enable command      # enable
enable -n command   # disable
enable -n           # list disabled internal commands</code>

To locate an external command:

<code>which -a --skip-alias ${command}</code>

5 Command aliases

Define short aliases for frequently used commands:

<code>alias                # list all aliases
alias name='command' # create alias
unalias name         # remove alias
unalias -a           # remove all aliases</code>

Aliases defined in the current shell are temporary; to make them permanent, add them to

~/.bashrc

(user) or

/etc/bashrc

(system) and reload with:

<code>source ~/.bashrc
source /etc/bashrc</code>

6 Common commands

Examples of frequently used system queries:

<code>lscpu
cat /proc/cpuinfo
free
cat /proc/meminfo
lsblk
cat /proc/partitions
arch
uname -r
cat /etc/os-release
lsb_release -a
halt
poweroff
reboot
shutdown -h now
whoami
who
w</code>

7 Session management

Screen:

<code>screen -ls               # list sessions
screen -S name           # create session
Ctrl+a d                 # detach
screen -x name          # attach
exit                     # close session
screen -r name          # resume session</code>

Tmux (more powerful terminal multiplexer):

<code>tmux new -s session_name
tmux detach
Ctrl+b d
tmux attach -t <session-name>
tmux kill-session -t <session-name>
tmux switch -t <session-name>
tmux split-window
tmux split-window -h
tmux list-keys
tmux list-commands</code>

Key bindings for pane management (e.g., Ctrl+b % for vertical split, Ctrl+b " for horizontal split, Ctrl+b arrow keys to move between panes, Ctrl+b x to close a pane, etc.) are also listed.

8 Echo output

Syntax:

<code>echo [-neE] [string]</code>

Options:

-E (default) – disable backslash interpretation

-n – omit trailing newline

-e – enable backslash interpretation

Common escape sequences when -e is used include \a (bell), \b (backspace), \c (suppress newline), \e (escape), \n (newline), \r (carriage return), \t (tab), \\ (backslash), \0nnn (octal), \xHH (hex).

ANSI color codes example:

<code>"\033[background;colormString\033[0m"</code>

Foreground colors 30‑37, background colors 40‑47, with typical mappings (30/40 black, 31/41 red, 32/42 green, 33/43 yellow, 34/44 blue, 35/45 magenta, 36/46 cyan, 37/47 white). Additional control codes such as \033[0m (reset), \033[1m (bold), \033[4m (underline), cursor movement, clear screen, hide/show cursor, etc., are also described.

9 Bash shortcuts

Common key combinations for editing and navigating the command line are listed, including Ctrl+L (clear screen), Ctrl+C (abort), Ctrl+U (delete to start), Alt+f (forward word), Alt+b (backward word), Ctrl+R (reverse search), and many others.

10 File globbing

Pattern examples:

<code>*      # matches any number of characters except leading .
?      # matches any single character
~      # home directory
[0-9]  # digit range
[!a-z] # characters not in range</code>

POSIX character classes such as [:digit:], [:lower:], [:upper:], [:alpha:], [:alnum:], [:blank:], [:space:], [:punct:], [:print:], [:cntrl:], [:graph:], [:xdigit:] are also shown.

11 Secure file deletion

Use

shred

instead of

rm

for secure deletion. Common options:

-z – add a final overwrite with zeros

-v – verbose output

-u – truncate and delete after overwriting

-n N – overwrite N times (default 3)