COMP7004 - Systems Scripting
Lecture 2: Bash Basics
Dr. Vincent Emeakaroha
vincent.emeakaroha@mtu.ie
Semester 2, 2026
Shell Scripting: Intro
• Shell: An interface to the Operating System.
• If we work with Mac/Linux, the shell offers an interface (a
communication protocol) to the UNIX System.
• Equivalent to the Windows cmd.exe (but much more powerful
→ UNIX allows users to do many more things than cmd.exe).
• Shell interface: How do we communicate with the OS?
• Command prompt, through which we can run our commands,
programs, or shell scripts.
• To open a command prompt in Linux, type: Ctrl + Alt + T
• To run a command on the open prompt, just type the
command, e.g.,: pwd
• To close the command prompt, type: exit
Shell Scripting: Intro
• UNIX: Two major types of shells (or interfaces to the OS)
• Bourne Shell (provides a command prompt with $ character).
• C Shell (provides a command prompt with % character).
• Within Bourne Shell, different variants:
• Bourne Shell, Korn Shell, Bourne Again Shell, POSIX Shell.
• Popularly known as BASH.
Bash Scripting
• Scripts
• Collection of commands stored in a file.
• Shell reads and acts on commands like normal.
• Shell provides programming features to make scripts powerful.
• Importance
• Scripts are easy to write.
• No additional setup or tools are required for developing or testing
scripts.
• Task automation.
• Unleashing compute power especially on Linux machines.
Limitation of Shell Scripts
• Every line in Shell script creates a new process in the
operating system.
• Shell scripts are slower as compared to compiled
programs.
• There may be problem with cross-platform portability.
• It is not suitable in situations where:
• Extensive file operations are required.
• Data structure such as linked lists or trees are required.
• Direct access to system hardware is required.
• Port or socket I/O is required.
Bash Command Structure
• Command <Options> <Argument s>
• Multiple commands separated by “;”
• Will be executed one after the other
• Commands can be executed directly on a Terminal
window or used in a script.
Basic Linux Commands
Commands
Descriptions
ls
This command is used to check the content of a
directory.
pwd
This command is used to check the present working
directory.
mkdir
work
This command creates a directory called “work” inside
the current working directory.
cd
work
This command will change our current working
directory to the newly created directory “work”.
touch
hello.sh
This command creates an empty file called “
hello.sh”
in the current directory.
cp
hello.sh bye.sh
This command does copying. It will copy the “
hello.sh
”
as “
bye.sh”.
mv
bye.sh welcome.sh
This command is used to rename or move files. It will
rename the file “
bye.sh” into “welcome.sh”.
Help Facilities for Commands
• To understand the working of a command and its possible options use
• man commandName
• Use the GNU Info System (info, info command)
• Listing a Description of a Program (whatis command)
• Many tools have a long−style option, `−−help', that outputs usage
information about the tool, including the options and arguments the
tool takes.
• Example: id --help
Files and Directories
• Regular file
• A collection of data items stored on disk
• Directories
• List of files and inodes
• “.” reference to the current directory
• “..” reference to the parent directory
• Root (/) – “.” and “..” are the same
Viewing File Attributes
• Command “ls –l” (Long listing)
• Displays file properties in a directory
• Example:
-rw-r--r-- 1 bill its2 1028 25 Sep fileOut
-rw-r--r--
Permission
String
1
Link
Count
25 Sep
Date
fileOut
File
Name
bill
User
Name
its2
Group
Name
1028
File
Size
File Type Attribute
• Shows the type of file in a listing
File Type Meaning
-
Regular File
d
Directory
l
Symbolic Link
b
Block Device
c
Character Device
p
Named Pipe
s
Domain Socket
-rw-r--r-- 1 bill its2 1028 25 Sep fileOut
drw-r--r-- 2 bill its2 4096 20 Oct movie
Permissions
• -rw-r-xr--
• “-” indicates the type of file
• “r” means read
• “w” means write
• “x” means execute
• 3 levels of access – user, groups and others
Operation File Directory
Read Read file List files
Write Delete/Modify file Create/Delete file
Execute Run program Access file
Manipulating Permission/User/Group
• Linux commands
• “chmod” for changing permission
• “chown” for changing owner/user
• “chgrp” for changing user group
• To add permission
• chmod u+x filename (grant owner executable right)
• To remove permission
• chmod g – r filename (remove read from group right)
• To change more than one permission
• chmod u + rx filename (grant owner read and executable rights)
Numeric Permissions
• Example: chmod 750 temp 7(rwx), 5(r-x), 0(---)
Octal Binary Symbolic
English Translations
0 000
--- No permissions.
1 001
--x Execute only
2 010
-w- Write only
3 011
-wx Write and execute
4 100
r-- Read only
5 101
r-x Read and execute
6 110
rw- Read and write
7 111
rwx Read, write and
execute
Types of Bash Commands
• Typically three types of commands
• Built-in commands
• Aliases
• Programs
• Written scripts
• Most of them are part of the OS in /bin
All behave the same way
Build-in Commands
• Built-in commands are internal to the Bash and do not create
a separate process. Commands are built-in when:
• They are intrinsic to the language (exit)
• They produce no side effects on the current process (cd)
• They perform faster
• No fork/exec
• Special built-in command examples
• : . break continue eval exec export exit
readonly return set shift trap unset read
Special Built-in Commands
exec : replaces shell with program
cd : change working directory
shift : rearrange positional parameters
set : set positional parameters
wait : wait for background proc. to exit
umask : change default file permissions
exit : quit the shell
eval : parse and execute string
time : run command and print times
export : put variable into environment
trap : set signal handlers
“export” Command
• The export command puts a variable into the environment so it will be accessible to child
processes. For instance:
x=“hello”
bash # Run a child shell.
echo $x # Nothing in x.
exit # Return to parent.
export x
bash
echo $x
hello # It's there.
• If the child modifies x, it will not modify the parent’s original value. Verify this by changing
x in the following way:
x=“ciao”
exit
echo $x
hello
Command History
• Most shells maintain a history of previously entered
commands
• To view the list of commands
• Input “history”
• To rerun a command
• Input “!#”
• Where # = command number
1993 gv ctpserver-trustlabel.eps
1994 cd output/
1995 ls
1996 cd ../
1997 ls
1998 rm output.tar
1999 mv ctpserver.eps output/
2000 exit
2001 history
user@local:~$
Aliases
• Many times you might want to create new commands from
existing commands. Sometimes, existing commands have
complex options to remember.
• We can create new commands as follows:
• alias ll=‘ls –l’
• alias copy=‘cp –rf’
• To list all declared aliases, use command:
• alias
• To remove an alias, use command:
• unalias commandName
Programs: Writing Bash Script
• Involves 3 key steps
1) Write the script using editor of choice
2) Assign permission to execute
3) Put it where shell can find it
Writing Bash Script: File Naming
• In Linux, filenames in lowercase and uppercase are
different. For example:
• The filenames “Hello” and “hello” are two distinct files
• As far as possible avoid using spaces in filenames or
directory names such as:
• Wrong file name – “Hello World.sh”
• Correct file name – “Hello_World.sh” or “HelloWorld.sh”
• This is a serious pitfall and should be taken very
seriously.
Writing Bash Script
• First script example (myScript.sh)
#!/bin/bash
# My first script
echo "Hello World!"
Line 2: Comment - Everything that appears after a "#" symbol is ignored by bash.
Line 1: A special clue given to the shell indicating what program is
used to interpret the script. In this case it is /bin/bash. This line is called “shebang”.
It must be the first line of every script.
Line 3: The echo command - simply prints what it is given on the screen (stdout)
Assigning Permission and Executing
Script
Technique one:
• Assigning permission
• chmod 755 myScript.sh or chmod u+x myScript.sh
• Running script
• ./myScript.sh
• The ./ tells shell to execute the present script otherwise shell will look for it in the
PATH environment variable.
• One can add script to environment variable and then remove “./” when running it.
Technique two:
• Using bash command
• bash myScript.sh
• Assuming you saved the script in a file named myScript.sh
• Not recommended
Filename Expansion
• Special characters representing multiple filenames
• Also referred to as globbing
Character Matches
* 0 or more characters
? 1 character
[ ] Matches any 1 character in [ ]
(including ranges)
[^ ] Matches any 1 character not in [ ]
(including ranges)
Using Script to Run a Task
• We wants to run operations that copies all files into a directory, and then deletes the
directory along with its contents. This can be done with the following commands:
mkdir trash
cp * trash
rm -rf trash
• Instead of having to type all that interactively on the shell, write a shell script to execute
the task:
s
F
F
G
#!/bin/bash
# this script deletes some files
mkdir trash
cp * trash
rm -rf trash
echo “Deleted all files!”
Interactive Script: “read” Command
• The read command allows you to prompt for input and store it
in a variable.
• Part of the built-in commands
• Syntax
• read [options] NAME1 NAME2…...NAMEN
• One line is read from the standard input (keyboard) at a time.
• First word stored in NAME1, second in NAME2, and so forth.
• Example:
• read.sh
read.sh
1: #!/bin/bash
2: echo -n "Enter name of file to delete: "
3: read file
4: echo "Type 'y' to remove it, 'n' to change your mind ... "
5: rm -i $file
6: echo "That was YOUR decision!"
• Line 2 prompts for a string that is read in line 3. Line 4 uses the
interactive remove (rm -i) to ask the user for confirmation.
Thank You!
