COMP7004 - Systems Scripting
Lecture 3: Shell Variables & Commands
Dr. Vincent Emeakaroha
vincent.emeakaroha@mtu.ie
Semester 2, 2026
Variables Types
• There are two types of variables:
• Local variables
• Environment variables
• Local variables
• Set and initialized by programmers in a script
• Environment variables are set by the operating system
and can usually be viewed using the env command.
• Environment variables hold special values. They are also
known as Shell variables
• Important for writing shell scripts
Local Variables
• Bash uses variables as in any other programming languages. Their values are always
stored as strings, but there are mathematical operators in the Bash language that will
convert variables to numbers for calculations.
• We have no need to declare a variable, just assigning a value to its reference will
create it.
• Example
#!/bin/bash
STR=“Hello World!”
echo $STR
• Line 2 creates a variable called STR and assigns the string "Hello World!" to it.
• Line 3 retrieves the value of this variable by putting ‘$’ sign at its beginning
and then output the value to the terminal.
Variables: Warning
• Bash scripting does not check variable types. This means that a
variable can hold either number or character data at a time.
• Example:
count=0
count=“Money”
• Changing the variable value TYPE can lead to confusion for the
programmer or someone trying to modify a script, so it is
recommended to use a variable for only a single TYPE of data
value in a script.
• \ is the bash escape character and it preserves the literal value
of the next character that follows it.
Environment Variables
Name Meaning
$HOME Absolute path to your home directory
$PATH A list of directories to search for commands
$SHELL Absolute path to login shell
$LOGNAME/$USER Your user/login name
$PS1 Primary prompt
$PS2 Secondary prompt
$MAIL Absolute path to mailbox
$TERM Type of your terminal
Additional Shell Features
• Tilde file expansion
~ $HOME
~username home directory of user
~+ $PWD
~- $OLDPWD
Shell Parameters
• Positional parameters are assigned from the shell’s argument when it is
invoked. Positional parameter “N” may be referenced as “${N}”, or as
“$N” when “N” consists of a single digit.
• Positional (special) parameters
• $# is the number of parameters passed
• $0 returns the name of the shell script running as well as
its location in the file system
• $* gives a single word (string) containing all the
parameters passed to the script
• $@ gives an array of words containing all the parameters
passed to the script
• Example
• sparameter.sh
sparameter.sh
#!/bin/bash
echo "$#; $0; $1; $2; $*; $@"
• Calling it as:
./sparameters.sh arg1 arg2
• Output:
2; ./sparameters.sh; arg1; arg2; arg1 arg2; arg1 arg2
Setting PATH Variable
• Usually, we run custom commands like a script file in the
following way:
./command
• By setting PATH=$PATH:. Our working directory is included in
the search path for commands, and we simply type:
command
• If we type in
mkdir ~/bin
and we include the following lines in the ~/.bash_profile or ~/.bashrc:
PATH=$PATH:$HOME/bin
export PATH
• We obtain that the directory /home/userid/bin is permanently
included in the search path for commands.
Overview of Useful Commands
10
File and Directory Management
• cd Change the current directory. With no arguments "cd"
changes to the users home directory. (cd <directory path>)
• chmod Change the file permissions.
• Example: chmod 751 myfile : change the file
permissions to rwx for owner, rx for group and x for others
• Example: chmod go+r myfile : Add read permission for
the group and others (character meanings u-user, g-group, o-
other, + add permission,-remove,r read,w-write,x-execute)
• Example: chmod +s myfile - Setuid bit on the file which
allows the program to run with user or group privileges of the
setted user ID.
File and Directory Management
• chown Change owner.
• Example: chown <owner1> <filename> : Change ownership
of a file to owner1.
• chgrp Change group.
• Example: chgrp <group1> <filename> : Change group of a file to
group1.
• cp Copy a file from one location to another.
• Example: cp file1 file2 : Copy file1 to file2
• Example: cp –r dir1 dir2 : Copy dir1 to dir2
• md5sum Prints the MD5 Checksum
File and Directory Management
• ls List contents of a directory.
• Example: ls, ls –l , ls –al, ls –ld, ls –R
• (-rwxrwxr-x 1 juan juan 0 Sep 26 12:25 foo )
• |more will list page wise
• mkdir Make a directory.
• Example: mkdir <directory name> : Makes a directory
• Example mkdir –p /www/chache/var/log will create all the
directories starting from www.
• mv Move or rename a file or directory.
• Example: mv <source> <destination>
File and Directory Management
• pwd Print or list the present working directory with full
path.
• rm Delete files (Remove files). (rm –rf <directory/file>)
• rmdir Remove a directory. The directory must be empty.
(rmdir <directory>)
• touch Change file timestamps to the current time. Make
the file if it doesn't exist. (touch <filename>)
• whereis Locate the binary and man page files for a
command. (whereis <program/command>)
• which Show full path of commands where given
commands reside. (which <command>)
File Viewing and Editing
• pico Simple text editor.
• nano Command-line text editor.
• vi Editor with a command mode and text mode. Starts in command
mode.
• gedit GUI Text Editor
• tail Look at the last 10 lines of a file (Default behaviour).
• Example: tail –f <filename> , Wait for input at file end
• Example: tail –n100 <filename>, Display last 100 entries
• head Look at the first 10 lines of a file (Default behaviour).
• Example: head <filename>
File Manipulation and Viewing
• cat or more View / output the content of a file.
• Example: cat <filename>
• cmp Compare two files.
• Example: cmp <file1> <file2>, Compare byte by byte
file1 and file2
• cut Remove sections from each line of files.
• diff Show the differences between files.
• Example: diff <file1> <file2>, Find differences between file1 &
file2.
• echo Display a line of text.
File Manipulation and Viewing
• sleep Delay for a specified amount of time.
• Example: sleep 5, Pause for 5 seconds
• sort Sort a file alphabetically.
• Example: sort <filename>, Sort the content of file
alphabetically and output result to the screen but original file
remain unchanged.
• uniq Remove duplicate lines from a sorted file.
• wc Count lines, words, characters in a file.
• Example: wc –c/w/l <filename>, Count
character/word/lines in the file.
File Compression and Restoring
• gzip - zip a file to a gz file.
• Example: gzip <filename>
• gunzip - unzip a gz file.
• Example: gunzip <gz filename>
• tar Archives files and directories. Can store files and
directories on tapes.
• Example: tar -zcvf <destination>
<files/directories> - Archive copy groups of files.
• Example: tar –zxvf <compressed file> to uncompress
• zip – Compresses a file to a .zip file.
• unzip – Uncompresses a file with .zip extension.
Pipes
• An important early development in Unix was the invention
of "pipes," a way to pass the output of one command to the
input of another.
eg. $ who | wc −l
By combining these two commands, giving the wc
command the output of who, you can build a new
command to list the number of users currently logged in on
the system.
Input/Output (I/O) Redirection
• All I/O are handled by the kernel via a mechanism called file
descriptor
• A file descriptor is an index into a file-descriptor table
maintained and used by kernel to reference open files and I/O
streams.
• The first three file descriptors are 0, 1 and 2.
• 0 is standard input (stdin)
• 1 is standard output (stdout)
• 2 is standard error (stderr)
• When a file descriptor is assigned to something else, it is called
I/O redirection.
Basic I/O Redirection Commands
< sample.txt
The command will take input from
sample.txt
> sample.txt
The output result will be stored in
sample.txt
>> sample.txt
The successive outputs will be appended to sample.txt
• Examples:
• ls > log.txt # Redirects the listing output into log.txt
• wc < log.txt # Counts the number of words, characters or lines from log.txt
• echo “Hello” >> content.txt # Appends “Hello” to the file content.txt
Regular Expressions
• A "regular expression" is a pattern that describes a set of
strings.
• Regular expressions are used when you want to search for
specific lines of text containing a particular pattern.
• An important means of specifying search options for grep
command.
What is grep?
• grep stands for
• "general regular expression parser“
• A powerful search command for UNIX
• Used to search for text strings and regular expressions
within one or more files
• Full details with the command:
• man grep
Common grep Command Options
grep [options] pattern [files]
• -b Display the block number at the beginning of each line.
• -c Display the number of matched lines.
• -h Display the matched lines, but do not display the filenames.
• -i Ignore case sensitivity.
• -l Display the filenames, but do not display the matched lines.
• -n Display the matched lines and their line numbers.
• -s Silent mode.
• -v Display all lines that do NOT match.
• -w Match whole word.
grep -c jane my_file.txt
Example Usage
• Search file for a user
• grep bill /etc/passwd
• Search file ignoring word case
• grep -i bill /etc/passwd
• Search recursively all files and directories under given
directory
• grep -r bill /etc/
Example Usage
• Search for a specific word in file
• grep -w jane $HOME/course.txt
• Search for 2 different words in file
• grep -w ‘jane\|victor' $HOME/course.txt
• Count lines that matched in file
• grep -c 'word' $HOME/course.txt
Example Usage
• Display lines that did not match a pattern
• grep -v ‘mark’ $HOME/course.txt
• Number of lines that contain matched pattern
• grep -n 'word' $HOME/course.txt
• Display filenames that matched pattern, but not lines
from the files
• grep -l ‘word' *.txt
Using Wildcards in grep
• Dot ( . ) matches 1 character
• Asterisks ( * ) matches multiple characters
• Examples:
• grep b.g myfile ➔ finds the words “big”, “bag”
• grep b.*k myfile ➔ finds the word “back”,
“buck”, “book”
Regular Expression and grep
• ^ (Caret) = match expression at the start of a line.
• Example: ^A simple man -> matches this line.
• $ (Dollar Sign) = match expression at the end of a line.
• Example: Sample$ -> matches this line.
• \ (Back Slash) = turn off the special meaning of the next character.
• Example: \^A simple man -> does not recognise this as regular expression.
• [ ] (Brackets) = match any one of the enclosed characters.
• Example: [aeiou] -> matches any of the characters.
• Use Hyphen "-" for a range. E.g., [0-9].
• [^ ] = match any one character except those enclosed in [ ].
• Example: [^0-9] -> ignores the range of 0 to 9.
Regular Expression and grep
• . (Period) = match a single character of any value,
except end of line.
• * (Asterisk) = match zero or more of the preceding
character or expression.
• \{x,y\} = match x to y occurrences of the preceding.
• \{x\} = match exactly x occurrences of the preceding.
• \{x,\} = match x or more occurrences of the preceding.
Regular Expression Usage Examples
• grep bob files {search files for lines with ‘bob'}
• grep '^bob' files {‘bob' at the start of a line}
• grep ‘bob$' files {‘bob' at the end of a line}
• grep '^bob$' files {lines containing only ‘bob'}
• grep '\^b' files {lines starting with '^b', "\" escapes
the ^}
• grep '[Bb]mug' files {search for starting with ‘B' or ‘b'}
• grep 'B[oO][bB]' files {search for BOB, Bob, BOb or BoB }
• grep '^$' files {search for empty lines}
• grep '[0-9][0-9]' files {search for pairs of numeric digits}
Thank You!
