Bash

Shell and Environment Variables

Current shell

Where a program is executed.

Sub-shell (child shell)

created within a shell to run a program.
two types of variables: local (or shell) and environment. local variable
- Private to the shell in which it is created.
- Only used by programs that are started in that shell. environment variable
- Inherited from the current shell to the sub-shell during the execution of a program
- Value stored in an environment variable is accessible to the program, as well as any sub-programs that it spawns during its lifecycle.
- Any environment variable set in a sub-shell is lost when the sub-shell terminates.
- env or the printenv command to view predefined environment variables.
- Common predefined environment variables:
  - DISPLAY
    - Stores the hostname or IP address for graphical terminal sessions
  - HISTFILE
    - Defines the file for storing the history of executed commands
  - HISTSIZE
    - Defines the maximum size for the HISTFILE
  - HOME
    - Sets the home directory path LOGNAME Retains the login name
  - MAIL
    - Contains the path to the user mail directory
  - PATH
    - Directories to be searched when executing a command. Eliminates the need to specify the absolute path of a command to run it.
  - PPID
    - Holds the identifier number for the parent program
  - PS1
    - Defines the primary command prompt PS2 Defines the secondary command prompt
  - PWD
    - Stores the current directory location
  - SHELL
    - Holds the absolute path to the primary primary shell file
  - TERM
    - Holds the terminal type value
  - UID
    - Holds the logged-in user’s UID
  - USER
    - Retains the name of the logged-in user

Setting and unsetting variables

export, unset, and echo to define and undefine environment variables
Use uppercase for variables

echo command

Restricted to showing the value of a specific variable

env command

Displays the environment variables only.

[root@localhost ~]# env
SHELL=/bin/bash
HISTCONTROL=ignoredups
HISTSIZE=1000
HOSTNAME=localhost
PWD=/root
LOGNAME=root
XDG_SESSION_TYPE=tty
MOTD_SHOWN=pam
HOME=/root
LANG=en_US.UTF-8
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=01;37;41:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.webp=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=01;36:*.au=01;36:*.flac=01;36:*.m4a=01;36:*.mid=01;36:*.midi=01;36:*.mka=01;36:*.mp3=01;36:*.mpc=01;36:*.ogg=01;36:*.ra=01;36:*.wav=01;36:*.oga=01;36:*.opus=01;36:*.spx=01;36:*.xspf=01;36:
SSH_CONNECTION=192.168.0.233 56990 192.168.0.169 22
XDG_SESSION_CLASS=user
SELINUX_ROLE_REQUESTED=
TERM=xterm-256color
LESSOPEN=||/usr/bin/lesspipe.sh %s
USER=root
SELINUX_USE_CURRENT_RANGE=
SHLVL=1
XDG_SESSION_ID=1
XDG_RUNTIME_DIR=/run/user/0
SSH_CLIENT=192.168.0.233 56990 22
which_declare=declare -f
PATH=/root/.local/bin:/root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
SELINUX_LEVEL_REQUESTED=
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/0/bus
MAIL=/var/spool/mail/root
SSH_TTY=/dev/pts/0
BASH_FUNC_which%%=() {  ( alias;
 eval ${which_declare} ) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot $@
}
_=/usr/bin/env
OLDPWD=/dev/vg200

printenv command

Displays the environment variables only.

set command

View both local and environment variables.

Command and Variable substitutions

PS1 Environment variable sets what the prompt looks like.
Default value is \u@\h \W\$
- \u
  - logged-in user name
- \h
  - System hostname
- \W
  - Working directory
- \$
  - End of command prompt
Command whose output you want assigned to a variable must be encapsulated within either backticks hostname or parentheses $(hostname).

Input, Output, and Error Redirections

Input, output, and error character streams:
- standard input (or stdin)
  - Input redirection
  - <
- standard output (or stdout)
  - - Append instead of overwrite
  - &>
    - Redirect standard error and output
- standard error (or stderr)
  - 2>
File descriptors:
- 0, 1, and 2
  - 1
    - Represents standard output location
- Can use these to represent character streams instead of <, and >
noclobber feature
- prevent overwriting of the output file
- set -o noclobber
  - activates the feature
- set +o noclobber
  - deactivate the feature

[root@localhost ~]# vim test.txt
[root@localhost ~]# set -o noclobber
[root@localhost ~]# echo "Hello" > test.txt
-bash: test.txt: cannot overwrite existing file
[root@localhost ~]# set +o noclobber
[root@localhost ~]# echo "Hello" > test.txt
[root@localhost ~]# cat test.txt
Hello

History Substitution

Command history or history expansion.
Can disable and re-enable if required.
Values may be altered for individual users by editing .bashrc or .bash_profile in the user’s home directory.
Three variables
- HISTFILE
  - Defines the name and location of the history file to be used to store command history,
  - Default is .bash_history in the user’s home directory.
- HISTSIZE
  - Size of the history buffer for the current shell.
- HISTFILESIZE
  - Sets the maximum number of commands allowed for storage in the history file at the beginning of the current session and are written to the HISTFILE from memory at the end of the current terminal session.
  - Usually, HISTSIZE and HISTFILESIZE are set to a common value.

`history` command

Displays or reruns previously executed commands.
Gets the history data from the system memory as well as from the .bash_history file.
Shows all entries by default.
set +o history
- disable history expansion
set -o history
- re-enable history expansion

[root@localhost ~]# set +o history

[root@localhost ~]# history | tail 
  126  ls
  127  vim test.txt
  128  set -o noclobber
  129  echo "Hello" > test.txt
  130  set +o noclobber
  131  echo "Hello" > test.txt
  132  cat test.txt
  133  history | tail 
  134  set +0 history
  135  set +o history
  
[root@localhost ~]# vim test2.txt

[root@localhost ~]# history | tail 
  126  ls
  127  vim test.txt
  128  set -o noclobber
  129  echo "Hello" > test.txt
  130  set +o noclobber
  131  echo "Hello" > test.txt
  132  cat test.txt
  133  history | tail 
  134  set +0 history
  135  set +o history
  
[root@localhost ~]# set -o history

[root@localhost ~]# vim test2.txt

[root@localhost ~]# history | tail 
  128  set -o noclobber
  129  echo "Hello" > test.txt
  130  set +o noclobber
  131  echo "Hello" > test.txt
  132  cat test.txt
  133  history | tail 
  134  set +0 history
  135  set +o history
  136  vim test2.txt
  137  history | tail

Add timestamps to history output system wide: echo "export HISTTIMEFORMAT='%F %T '" >> /etc/profile && source /etc/profile

Editing at the Command line

Common key combinations:

Ctrl+a / Home
- Moves the cursor to the beginning of the command line
Ctrl+e / End
- Moves the cursor to the end of the command line
Ctrl+u
- Erase everything at and before cursor
Ctrl+k
- Erase Everything at cursor and after
Alt+f
- Moves the cursor to the right one word at a time
Alt+b
- Moves the cursor to the left one word at a time
Ctrl+f / Right arrow
- Moves the cursor to the right one character at a time
Ctrl+b / Left arrow
- Moves the cursor to the left one character at a time

Tab completion

Hitting the Tab key twice automatically completes the entire name.
In case of multiple possibilities matching the entered characters, it completes up to the point they have in common and prints the rest of the possibilities on the screen.

Tilde Substitution (tilde expansion)

Performed on words that begin with the tilde character (~). ~
- refers to user’s home directory.

~+ - refers to current directory

~- - Refers to previous working directory.

~USER - Refers to specific user’s home directory.

Alias Substitution (command aliasing or alias)

Define shortcuts for commands.
Bash shell includes several predefined aliases that are set during user login.
Shell gives precedent to an alias if an alias matches a command or program.
- can still run the command without using the alias but preceding command with a backslash. \

alias command

Set an alias.
Internal shell command.

[root@localhost ~]# alias
alias cp='cp -i'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias mv='mv -i'
alias rm='rm -i'
alias xzegrep='xzegrep --color=auto'
alias xzfgrep='xzfgrep --color=auto'
alias xzgrep='xzgrep --color=auto'
alias zegrep='zegrep --color=auto'
alias zfgrep='zfgrep --color=auto'
alias zgrep='zgrep --color=auto'

[root@localhost ~]# alias frog='pwd'
[root@localhost ~]# frog
/root

unalias command

Unset an alias.
Internal shell command.

[root@localhost ~]# unalias frog
[root@localhost ~]# frog
-bash: frog: command not found

Metacharacters and Wildcard

Metacharacters
- Special characters that possess special meaning to the shell.
- Used in pattern matching (a.k.a. filename expansion or file globbing) and regular expressions. dollar sign ($)
mark the end of a line
Used in regular expressions

caret (^)

mark the beginning of a line
Used in regular expressions

period (.)

Match a single position
Used in regular expressions

asterisk (*)

used in pattern matching
wildcard character
Matches zero to an unlimited number of characters except for the leading period (.) in a hidden filename.
Used in regular expressions

question mark (?)

Used in pattern matching
Wildcard character
Matches exactly one character except for the leading period in a hidden filename.
Used in regular expressions

pipe (|)

Send the output of one command as input to the next.
Also used to define alternations in regular expressions.
Can use as many times in a command as you need. (pipeline) - Can be used as an OR operator (alternation)
- this|that|other

angle brackets (< >)

Redirections

curly brackets ({})

Used in regular expressions
Match an element a specific number of times

square brackets ([])

Used in pattern matching
Wildcard character
Match either a set of characters or a range of characters for a single character position.
Order in which they are listed has no importance.
Range of characters must be specified in a proper sequence such as [a-z] or [0-9].
Used in regular expressions

parentheses (())

Create a sub shell

plus (+)

Match a character one or more time

exclamation mark (!)

inverse matches

semicolon (;)

Run a second command after the ;

Quoting Mechanisms

Disable special meaning of metacharacters

Backslash (\)

Escape character
Cancel out a special character’s meaning.

single quotation (‘’)

Mask the meaning of all encapsulated special characters.

double quotation (“”)

Mask the meaning of all but the backslash (), dollar sign ($), and single quotes (‘’).

Regular expressions (regexp or regex)

Text pattern or an expression that is matched against a string of characters in a file or supplied input in a search operation.
Pattern may include a single character, multiple random characters, a range of characters, word, phrase, or an entire sentence.
Any pattern containing one or more white spaces must be surrounded by quotation marks.

`grep` command

Searches the contents of one or more text files or input supplied for a match.

Flags -i

case insensitive search

-n

number the lines

-v

exclude these lines

-w

find an exact match for a word

-E

match one or the other

-e

-Use patterns for matching

Jobs

job
- Process that is started in the background and controlled by the terminal where it is spawned.
- Assigned a PID (process identifier) by the kernel and, additionally, a job ID by the shell.
- Does not hold the terminal window where it is initiated.
- Can run multiple job simultaneously.
- Can be brought to foreground, returned to the background, suspended, or stopped.
job control
- management of multiple jobs within a shell environment

commands and control sequences for administering the jobs.

jobs
- Shell built-in command
- display jobs.
bg
- Shell built-in command
- Move a job to the background or restart a job in the background that was suspended with Ctrl+z.
fg
- Shell built-in command
- Move a job to the foreground
Ctrl+z
- Suspends a foreground job and allows the terminal window to be used for other purposes

jobs command

output plus sign (+) - indicates the current background job minus sign (-) - signifies the previous job. Stopped - currently suspended - can be signaled to continue their execution with bg or fg

Shell Startup Files

Sourced by the shell following user authentication at the time of logging in and before the command prompt appears.
Aliases, functions, and scripts can be added to these files as well.
two types of startup files:
- system-wide
  - Set the general environment for all users at the time of their login to the system.
  - Located in the /etc directory
  - Maintained by the Linux admin.
  - System-wide startup files for bash shell users:
    - /etc/bashrc
      - Defines functions and aliases, sets umask for user accounts with a non-login shell, establishes the command prompt, etc.
      - May include settings from the shell scripts located in the /etc/profile.d directory.
    - /etc/profile
      - Sets common environment variables such as PATH, USER, LOGNAME, MAIL, HOSTNAME, HISTSIZE, and HISTCONTROL for all users, establishes umask for user accounts with a login shell, processes the shell scripts located in the /etc/profile.d directory, and so on.
    - /etc/profile.d
      - Contains scripts for bash shell users that are executed by the /etc/profile file.
      - Files can be edited and updated.
- per-user
  - Override or modify system default definitions set by the system-wide startup files.
  - By default, two files, in addition to the .bash_logout file, are located in the skeleton directory /etc/skel and are copied into user home directories at the time of user creation.
  - .bashrc
    - Defines functions and aliases. This file sources global definitions from the /etc/bashrc file.
  - .bash_profile
    - Sets environment variables and sources the .bashrc file to set functions and aliases.
  - .gnome2/
    - Directory that holds environment settings when GNOME desktop is started. Only available if GNOME is installed.
  - .bash_logout
    - Executed when the user leaves the shell or logs off.
    - May be customized
Startup file order:
- /etc/profile > .bash_profile > .bashrc > /etc/bashrc
Per user settings must be added to the appropriate file for persistence.

Bash Labs

Lab: View environment variables

env
printenv

Lab: Viewing Environment variable values

View the value for the PATH variable

echo $PATH

View the value for the HOME variable

echo $HOME

View the value for the SHELL variable

echo $SHELL

View the value for the TERM variable

echo $TERM

View the value for the PPID variable

echo $PPID

View the value for the PS1 variable

echo $PS1

View the value for the USER variable

echo $USER

Lab: Setting and Unsetting Variables

Define a local variable called VR1:

VR1=RHEL9

View the value of VR1:

echo $VR1

Type bash at the command prompt to enter a sub-shell and then run echo $VR1 to check whether the variable is visible in the sub-shell.

echo $VR1

Exit out of the subshell:

exit

Turn VR1 into an environment variable:

export VR1

Type bash at the command prompt to enter a sub-shell and then run echo $VR1 to check whether the variable is visible in the sub-shell.

echo $VR1

Undefine this variable and erase it from the shell environment:

unset VR1

Define a local variable that contains a value with one or more white spaces:

VR2="I love RHEL 9"

Define and make the variable an environment variable at the same time:

export VR3="I love RHEL 9"

View local and environment variables:

set

Lab: Modify Primary Command Prompt

Change the value of the variable PS1 to reflect the desired information:

export PS1="< $LOGNAME on $HOSTNAME in \$PWD > "

Edit the .bash_profile file for user1 and define the value exactly as it was run in Step 1.

vim .bash_profile

Test by logging off as user1 and logging back in. The new command prompt will be displayed.

Lab: Redirecting Standard Input

Have the cat command read the /etc/redhat-release file and display its content on the standard output (terminal screen):

cat < /etc/redhat-release

Lab: Redirecting Standard Output

Direct the ls command output to a file called ls.out:

ls > ls.out

Do the same thing but using file descriptors:

ls 1> ls.out

Activate the noclobber feature then try the redirect feature again:

set -o noclobber
ls > ls.out

Deactivate noclobber

set +o noclobber

Direct the ls command to append the output to the ls.out file instead of overwriting it:

ls >> ls.out
or
ls 1>> ls.out

Lab: Redirecting Standard Error

Direct the find command issued as a normal user to search for all occurrences of files by the name core in the entire directory tree and sends any error messages produced to /dev/null

find / -name core -print 2> /dev/null

Redirect both standard output and error:

ls /usr /cdr &> outerr.out
or
ls /usr /cdr 1> outerr.out 2>&1
\# means to redirect file descriptor 1 to file outerr.out as well as to file descriptor 2.

Same as above but append to file:

ls /usr/cdr &>> outerr.out

Lab: Show History Variables

View HISTFILE Variable:

echo $HISTFILE

View HISTSIZE variable:

echo $HISTSIZE

View HISTFILESIZE variable:

echo $HISTFILESIZE

Lab: History command

Rune history without any options:

history

Display 10 entries:

history 10

Run the 15th command in history:

!15

re-execute the most recent occurrence of a command that started with a particular letter or series of letters (ch for example):

!ch

Issue the most recent command that contained “grep”:

!?grep?

Remove entry 24 from history:

history -d 24

Repeat the last command executed:

!!

Lab: Tilde expansion

Display user’s home directory:

echo ~

Display the current directory:

echo ~+

Display the previous working directory:

echo ~-

Display user1’s home directory:

echo ~user1

cd into the home directory of user1 and confirm:

cd ~user1
pwd

cd into a subdirectory:

cd ~/Documents/

View directory information of the root user’s desktop:

ls -ld ~root/Desktop

Lab: Command Aliasing

shows all aliases that are currently set for user1:

su - user1
alias

Run alias as root:

alias

Create an alias “search” to abbreviate the find command with several switches and arguments. Enclose the entire command within single quotation marks (‘’) to ensure white spaces are taken care of. Do not leave any spaces before and after the equal sign (=).

alias search='find / -name core -exec ls -l {} \;'

Search with the new alias:

search

Create and alias by the same name as rm command but adding the -i flag:

alias rm='rm -i'

Run rm without using the alias:

rm file1

Remove the two aliases we just created:

unalias search rm

Lab: Wildcards and Metacharacters

List all files in the /etc directory that begin with letters “ma” and followed by any characters:

ls /etc/ma*

List all hidden files and directories in /home/user1:

ls -d .*

List all files in the /var/log directory that end in “.log”:

ls /var/log/*.log

List all files and directories under /var/log with exactly four characters in their names:

ls -d /var/log/????

Include all files and directories that begin with either of the two characters and followed by any number of characters.

ls /usr/bin/[yw]*

Match all directory names that begin with any letter between “m” and “o” in the /etc/systemd/system directory:

ls -d /etc/systemd/system/[m-o]*

Inverse results of the previous:

ls -d /etc/systemd/system/[!m-o]*

Lab: Piping

Pipe the output to the less command in order to view the directory listing one screenful at a time:

ls -l /etc | less

Run the last command and pipe the output to nl to number each line:

last | nl

Send the output of ls to grep for the lines that do not contain the pattern “root”. Piped again for a case-insensitive selection of all lines that exclude the pattern “dec”. Number the output, and show the last four lines on the display:

ls -l /proc | grep -v root | grep -iv dec | nl | tail -4

Lab: Quoting Mechanisms

Remove a file called * without deleting everything in the directory:

rm \*

Display $LOGNAME without expanding the LOGNAME variable:

echo '$LOGNAME'

Run the following with double quotes and without:

echo "$SHELL"
echo "\$PWD"
echo "'\'"

Lab: grep and regex

Search for the pattern “operator” in the /etc/passwd file:

grep operator /etc/passwd

Search for the space-separated pattern “aliases and functions” in the $HOME/.bashrc file:

grep 'aliases and functions' .bashrc

Search for the pattern “nologin” in the passwd file and exclude (-v) the lines in the output that contain this pattern. Add the -n switch to show the line numbers associated with the matched lines.

grep -nv nologin /etc/passwd

Find any duplicate entries for the root user in the passwd file. Prepend the caret sign (^) to the pattern “root” to mark the beginning of a line.

grep ^root /etc/passwd

Identify all users in the passwd file with bash as their primary shell.

grep bash$ /etc/passwd

Show the entire login.defs file but exclude all the empty lines:

grep -v ^$ /etc/login.defs

Perform a case-insensitive search (-i) for all the lines in the /etc/bashrc file that match the pattern “path.”

grep -i path /etc/bashrc

Print all the lines from the /etc/lvm/lvm.conf file that contain an exact match for a word (-w) “acce” followed by exactly two characters:

grep -w acce.. /etc/lvm/lvm.conf

Print all the lines from the ls command output that include either (-E) the pattern “cron” or “ly”.

ls -l /etc | grep -E 'cron|ly'

Show all the lines from the /etc/ssh/sshd_config file but exclude (-v) the empty lines and commented lines. Use the -e flag multiple times instead of | for either or.

sudo grep -ve ^$ -ve ^# /etc/ssh/sshd_config

Learn more about regex:

man 7 regex

Consult the grep man pages:

man grep

Lab: Managing jobs

Issue the jobs command with the -l switch to view all the jobs running in the background:

jobs -l

bring job ID 1 to the foreground and start running it:

fg %1

Suspend job 1 with ctrl+z and then let it run in the background:

bg %1

Terminate job ID 1, supply its PID (31726) to the kill command:

kill 31726

Lab: Shell Startup Files

View the first 10 lines of /etc/bashrc:

head /etc/bashrc

View the first 10 lines of /etc/profile:

head /etc/profile

View the directory /etc/profile.d

ls -l /etc/profile.d/

View .bashrc

cat ~/.bashrc

View .bash_profile

cat ~/.bash_profile

Lab: Customize the Command Prompt (user1)

Permanently customize the primary shell prompt to display “<user1@server1 in /etc >:” when this user switches into the /etc directory. The prompt should always reflect the current directory directory path.

vim ~/.bash_profile
export PS1='$USERNAME $PWD'

Lab 7-2: Redirect the Standard Input, Output, and Error (user1)

Run the ls command on /etc, /dvd, and /var. Have the output printed on the screen and the errors forwarded to file /tmp/ioerror.

ls /etc /dvd /var 2> /tmp/ioerror

Check the file after the command execution and analyze the results:

cat /tmp/ioerror