Bash Guide - A Quick Guide To Learn Bash Scripting

Table of Contents

1. Basic Commands
2. System Information
3. Process Management
4. Network Operations
5. Shell Programming

1. Basic Commands

i. ls

List files and directories.

ls [options]

Options:

• l - long listing format
• a - show all files (including hidden files)

ii. cd

Change current directory (move to another directory)

cd directory-path

iii. cp

Copies a directory or file to destination

cp source destination

iv. mv

Moves a source from one path to another. This command is also used to rename directories and files.

mv path1 path2
mv filename1 filename2

v. rm

Removes a file(s) or directory(s).

rm file1 file2
rm -r directory1

Options:

• f - force deletion without any prompt

vi. touch

Create a file. If file already exist, update dates accessed and modified

touch filename

vii. file

Determine filetype of file(s).

file filename

Options:

• i - output MIME type strings

viii. diff

Compares files line by line and displays the difference.

diff file1 file2

ix. cat

Concatenate FILE(s) to standard output. It can be used to display text files on screen, create, copy, merge files.

cat [option] file
cat file1 file2 
cat file1 file2 > newmergedfile
cat < file1 > file2 #copy file1 to file2

x. more

Displays output one screen at a time. Move to next page by pressing space or press q to quit.

more filename

xi. less

Similar to more but allows both forward and backward movements. You can also use arrow keys to move one line at a time.

less filename

2. System Information

i. uname

Prints kernel information on the screen.

uname [option]
uname -a

ii. ps

Lists your running processes & their status.

ps [option]
ps -a
ps -u user

iii. jobs

Lists the jobs running in the background along with the job number.

jobs

iv. df

Displays file system disk space usage.

df

v. du

Displays the disk usage of files and/or directories.

du [options] filename|dirname
du -sh /sbin/file1

Options:

• h - Human readable format (size is displayed in KB, MB, GB)
• a - display all (shows disk space for directories at every level and also individual files)
• s - Suppress or Summarize (only shows total disk space occupied)

v. top

Displays a dynamic real-time view of a running system with the list of active processes.

top

vi. last

Shows listing of last logged in user(s).

last
last username

vii. quota

Displays the disk quota (limits) and usage.

quota

3. Process Management

i. kill

Sends a signal to a process. Commonly used to terminate a process.

kill [signal] PID

ii. killall

Kill all processes by name.

killall processname

Options:

• e - Require an exact match for long names
• i - Interactively ask for confirmation before killing
• q - Quiet mode

iii. fg

Moves a background job to foreground

fg [job-id]

iv. bg

Runs a job in the background.

bg [job-id]

v. nohup

nohup i.e. No Hangup, runs the given command with hangup signals ignored, so that the command can continue running in the background after you log out.

nohup command [arg...]

4. Network Operations

i. ping

Used to test if a host is reachable. Sends ICMP ECHO_REQUEST to network host(s).

ping [option] host
ping host
ping -t host

ii. dig

Domain information groper, DNS lookup utility.

dig domain [options] [query]

iii. w

Shows who is logged on and what they are doing.

w [option] [user]

5. Shell Programming

Shell is a user program or an environment provided for user interaction. Shell is a command language interpreter that executes commands read from the standard input device (keyboard) or from a file.

Shell is not part of system kernel but uses the system kernel to execute programs, create files etc.

Several shells available with Linux including:

• SH (POSIX shell or Bourne shell)
• BASH ( Bourne-Again SHell )
• CSH (C SHell)
• KSH (Korn SHell)
• TCSH (TENEX/TOPS C shell)

A shell script begins with a #! commonly referred to as shebang, also known as sha-bang, hashbang, pound-bang or hash-pling.

It's syntax follows:

#!interpreter [optional-arg]

#!/bin/sh
#!/bin/bash
#!/bin/csh -f

Executing a shell script

Syntax:

[interpreter] script-name
./script-name

On your terminal type:

bash scriptA
sh scriptA
./scriptA

i. Variables

Variables in shell programming are typeless (no data types). A valid variable name can consist of characters, numbers, hyphens and underscores. There can be no spaces around the "=" assignment sign: that is VAR=value is valid; VAR = value is not.

Variables can be accessed by prepending $ before the VARIABLE_NAME.

MY_MESSAGE="HELLO!"
echo $MY_MESSAGE

ii. Quotations

There are 4 types of quotes with Shell programming:

Quote	Name	Description
"	Double Quote	Evaluates some special charactes ($,`,\$,\',\" and \\) & does command substitutions but not meta-characters
'	Single Quote	No special characters are expanded
`	Back Quote	Execute everything in between back quotes
\	Backslash	Everything immediately following a backslash will not be executed

iii. Arrays

Shell supports Array variable. They hold multiple values just like most programming languages. Following are the ways to set values to ARRAY variable:

ARRAY_NAME[Index]=Value
ARRAY_NAME=([INDEX1]=VALUE1 [INDEX2]=VALUE2)
ARRAY_NAME=(VALUE1 VALUE2)

To get a specific value from the ARRAY:

${ARRAY_NAME[Index]}

To get all the values from the ARRAY:

${ARRAY_NAME[*]}
${ARRAY_NAME[@]}

iv. Functions

Functions are sequence of code that are grouped together to execute some shell commands in order to return/output some values. Functions may change the state of variable(s) and even exit the shell script execution too. They are defined as:

function myFunction {
# Perform some operations
# echo/return/exit
}

v. Loops

Bash provides 3 types of loops: for, while and until.

for Syntax:

for VARIABLE in 1 2 3 4 5 .. N
do
  statement
done

for VARIABLE in file1 file2 file3
do
  statement
done

for OUTPUT in $(command)
do
  command on $OUTPUT
done

OR

for (( EXPRESSION_1; EXPRESSION_2; EXPRESSION_3 ))
  do
    statement
  done

OR with Bash v4.0+

for i in {START..END..INCREMENT}
  do 
    statement
  done

while Syntax:

while condition; do
  statement
done

until Syntax:

until condition; do
  statement
done

vi. Conditions

Like other programming languages, bash also supports conditional statements. A sequence of statements can execute if a condition is true.

Syntax:

if [expression1]; then
  # executed if expression1 is true
  statement
elif [expression2]; then
  # executed if expression1 is false but expression2 is true
  statement
else
  # executed if all the above expressions are false
  statement
fi