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Linux - Shell: Fundamental

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Linux - Shell: Fundamental

What is the Shell

shell
- a command-line interpreter that provides a user interface to interact with the Linux kernel.
- It processes commands entered by the user and executes them, often by calling programs or performing built-in operations.

Types of Shells

Bash (Bourne Again Shell)
- Default shell in Red Hat-based systems (like RHEL).
sh (Bourne shell): Basic and simple.
zsh: Advanced features, customizable.
csh and tcsh: C-like syntax.
ksh (KornShell): Powerful scripting capabilities.

Find Shell Path

List available shell

cat /etc/shells
# /bin/sh
# /bin/bash
# /usr/bin/sh
# /usr/bin/bash

Ways to find current shell

echo $SHELL
# /bin/bash

echo $0
# -bash

Shell of a user

cat /etc/passwd
# rheladmin:x:1000:1000:rheladmin:/home/rheladmin:/bin/bash

Shell Prompt

Format: [user_name@instance_name current_path] $
#:
- pound sign,
- current user is a root
$:
- dollar sign
- current user is a normal user
~:
- tilde sign
- home directory
- e.g.,
  - ~jason: /home/jason
  - ~root: /root
  - ~ftp: /var/ftp, the home dir of ftp service account

Shell prompt customization

Use an environment variable to customize.
Bash, ksh, and sh use $PS1.
Csh, tcsh, and zsh use $prompt.
Customizing the Prompt with PS1

Sysmbol	Desc
`\h`	Short hostname, up to the first period
`\H`	Full hostname
`\n`	Newline
`\d`	Date in “Weekday Month Date” format “Tue May 26”
`\t`	Current time in 24-hour `HH:MM:SS` format
`\A`	Current time in 24-hour `HH:MM` format
`\T`	Current time in 12-hour `HH:MM:SS` format
`\@`	Current time in 12-hour `am/pm` format
`\u`	Username of the current user
`\w`	Current working directory
`\W`	Basename of the current working directory
`\$`	if the effective UID is `0`, a `#`, otherwise a `$`

Example

PS1="[\A \u@\h \W] \$ "

Persist PS1 Changes

echo 'export PS1="[\A \u@\h \W]\$ "' >> ~/.bash_profile

Environmental Variables

Environmental Variables
- Storage location that has name-value pairs
- 这些是由操作系统或用户设置的特殊变量，用于配置 Shell 的行为和影响其执行环境。
- Typically uppercase
- Access the contents by executing:
  - echo $VAR_NAME
types of variables
- local variable:
  - value of a local variable is only available in the current shell.
- environment variable
  - the value stored in an environment variable is accessible to the program, as well as any subprograms that it spawns during its lifecycle.

Environment Variables

Environment Variable
- a storage location that has a name and a value.
  - They often effect the way programs behave.
  - used to enhance and to standardize your shell environment on Linux systems.
- Name/Value pairs
  - NAME=value
- Default: upper case
Value:
- do not use space sign
vs program/process
- When a process is started it inherits the exported environment variables of the process that spawned it.
- A variable that is set or changed only effects the current running process unless it is exported.
- The variables that are not exported are called local variables.
- The export command allows variables to be used by subsequently executed commands

Persisting Environment Variables

echo 'export TZ="US/Pacific"' >> ~/.bash_profile

Common Env Var

Environment Variables	Variable Description
`EDITOR`	The program to run to perform edits.
`HOME`	The Home directory of the user.
`LOGNAME`	The login name of the user.
`MAIL`	The location of the user’s local inbox.
`OLDPWD`	The previous working directory.
`PATH`	A colon separated list of directories to search for commands.
`PAGER`	This program may be called to view a file.
`PS1`	The primary prompt string.
`PWD`	The present working directory.
`USER`	The username of the user.

Env Var Management

Command	Desc
`env`	list all env var
`printenv`	print all environment
`printenv \\| less`	print all environment
`printenv ENV_VAR1 ENV_VAR2`	print environment var, without `$`
`echo $ENV_VAR1 $ENV_VAR2`	print environment var
`export ENV_VAR="value"`	Create/Update an environment var
`unset ENV_VAR`	Removing an env var

Lab: Env

echo $PATH
# /root/.local/bin:/root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin

printenv PATH
# /root/.local/bin:/root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin

echo $SHELL
# /bin/bash

printenv SHELL
# /bin/bash

Lab: Create and export Local variable; Unset the variable

# define local
VR1=RHEL9
# display
echo $VR1
# RHEL9

# create a sub-shell
bash
# try to display local var\
# note: nothing returns
echo $VR1
#

# exit the subshell
exit
# make a var an env var
echo $VR1
# RHEL9
export VR1

# create sub-shell
bash
# display env
echo $VR1
# RHEL9

Unset

unset VR1
# try to display
# note: nothing returns
echo $VR1

Customize the primary shell prompt

$PS1: store primary command prompt

# display the default prompt
echo $PS1
# [\u@\h \W]\$

# modify temporarily
export PS1="<$LOGNAME on $(hostname) in \$PWD>"
# new prompt
# <root on ServerB in /var/log>

# permanently
# Edit the .profile file
vi /root/.bash_profile
# export PS1="<$LOGNAME on $(hostname) in $PWD>"

# logout and login
exit
su -
# <root on ServerB in /root>

Executing Commands

Defaut:
- $PATH determines command search path.
can execute command not in $PATH.
- can specify a command with a full path.
  - /path/to/command_name
- Execute command in this dir
  - ./command

Privileged Execution

root:
- prompt: #
- can do any operation that Linux system can do.
- restricted to system administrators.
- be required to install, start, or stop an application.
Normal user:
- prompt: $
- can only do a subset of the things root can do.
sudo: execute a command as another user

Command Path

Path:
- An environment variable
- Contains a list of directories separated by colons.
- Controls the command search path
  - when a command is typed in the prompt, it will be searched for in the directories that are listed in the path env variable.
- echo $PATH
which:
- shows the full path of (shell) commands.

Lab: which, whereis, type

which cd
# /usr/bin/cd

type cd
# cd is a shell builtin

whereis cd
# cd: /usr/bin/cd /usr/share/man/man1/cd.1.gz /usr/share/man/man1p/cd.1p.gz

`man`

Navigating Man Pages

Shortcut	Desc
`Enter`	Move down one line.
`Space`	Move down one page.
`g`	Move to the top of the page.
`G`	Move to the bottom of the page.
`q`	Quit.

Search the short descriptions and manual page names for the keyword

man -k key_word

Aliases

Aliases
- Shortcuts
- Use for long commands
- Use for commands you type often
  - Fix Typos
  - Make Linux behave like another OS

Commands

Command	Desc
`alias`	List all aliases
`alias name='value'`	Create an alias
`unalias name`	Remove an alias
`unalias -a`	Remove all aliases

Example

alias cls='clear'

Persisting Aliases

Add aliases to your personal initialization files.
- .bash_profile

echo 'alias cls='clear'' >> ~/.bash_profile

Shell History

Shell history
- Executed commands are added to the history.
- can be displayed and recalled.
- stored in memory and on disk
  - redhat: ~/.bash_history
  - ~/.history
  - ~/.histfile

`history`

three variables—HISTFILE, HISTSIZE, and HISTFILESIZE—that control the location and history storage.
.bash_history: in the user’s home directory
HISTSIZE: Controls the number of commands to retain in history
- default: 1000

echo $HISTFILE
# /root/.bash_history

echo $HISTSIZE
# 1000

echo $HISTFILESIZE
# 1000

Searching history:
- Ctrl-r: reverse shell history search
- Arrows: Change the command
- Enter: execute the command
- Ctrl-g: cancel shell history search

CMD	DESC
`history`	Displays the shell history

history
# 1  clear
# 2  cat /etc/hostname
# 3  clear
# 4  vi /etc/hosts
# 5  clear
# 6  useradd xanadu
# ...

history 5
# 362  echo $HISTSIZE
# 363  echo $HISTFILESIZE
# 364  history
# 365  history --help
# 366  history 5

# To re-execute
# note: #3=clear
!3

# re-execute the most recent occurrence starting with "ch"
# note: chage
!ch

# To remove entry 24 from history:
history -d 24

disable the shell’s history expansion feature

set +o history
# reenble
set -o history

`!`syntax

Syntax	DESC
`!N`	Repeat command line number N
`!!`	Repeat the previous command line
`!string`	Repeat the most recent command line starting with “string”

!:N <Event> <Separator> <Word>
- !: the most recent command line
- !=!!
- :N: represens a word on the command line.
  - 0 = command
  - 1 = first argument
!^: the 1st argument
!$: the last argument
Example

head file1 file2 hamlet.txt
# output

!!
# repeat the last command and output

vi !:2
# vi file2
# ! is to repeat the most recent command, which is !!, which is head command
# :N represents the 2nd argument, which is file2.
# so the this command is to open the file2 in vim editor.

head file1 file2 hamlet.txt
vi !$

Grep

CMD	DESC
`grep root /etc/passwd`	Search for pattern
`grep ^root /etc/passwd`	Search for pattern at the beginning of the line
`grep bash$ /etc/passwd`	Search for pattern at the end of the line
`grep 'aliases and functions' ~/.bashrc`	Search for pattern
`grep -n 'nologin' /etc/passwd`	show the line numbers
`grep -v root /etc/passwd`	Search exclusively
`grep -v $^ ~/.bashrc`	exclude all the empty lines
`grep -i path /etc/bashrc`	case-insensitive search
`grep -w acce.. /etc/lvm/lvm.conf`	begin with letters “acce” followed by exactly two characters
`ls -l /etc \\| grep -E 'cron\\|ly'`	Using Regex
`grep -ve ^& -ve ^# /etc/ssh/sshd_config`	Multiple patterns

Note_Tech

Linux - Shell: Fundamental

What is the Shell

Types of Shells

Find Shell Path

Shell Prompt

Shell prompt customization

Persist PS1 Changes

Environmental Variables

Environment Variables

Persisting Environment Variables

Common Env Var

Env Var Management

Lab: Env

Lab: Create and export Local variable; Unset the variable

Customize the primary shell prompt

Executing Commands

Privileged Execution

Command Path

Lab: which, whereis, type

man

Aliases

Commands

Persisting Aliases

Shell History

history

!syntax

Grep

`man`

`history`

`!`syntax