COMP7004 - Systems Scripting
Lecture 18: Bash Revision
Dr. Vincent Emeakaroha
vincent.emeakaroha@mtu.ie
Semester 2, 2026
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 or execute from the current
working directory
2
Arithmetic Evaluations
• The let built-in command can be used to do mathematical functions:
let X=10+2*7
echo $X
24
let Y=X+2*4
echo $Y
32
• Two methods of evaluating arithmetic expression are by
$[expression] or $((expression))
echo “$((123+20))”
143
VALORE=$[123+20]
echo “$[123*$VALORE]”
17589
3
Control Flow
• Rather than just executing a predetermined sequence
of commands, a script can be made flexible by using
the control flow constructs available:
• if-then-else
• case
• loops
• while
• for
• until
• select
4
“for” Statement
• The for structure is used when you are looping through a
range of command list.
• Syntax structure:
for var in list
do
statements
done
• Statements are executed with “var” set to each value in the
list.
• Example
• for1.sh
5
for1.sh
#!/bin/bash
sum=0
for num in 1 2 3 4 5
do
sum=$(($sum + $num))
done
echo $sum
6
Array Variable
• Unlike scalar variable, array is capable of holding multiple
values at the same time.
• Provides a means of grouping variables
• Same naming convention as for scalar variables
• Array declaration
• array_name[index] = value; or declare –a array_name = (value1,
value2,…...)
• E.g., declare -a array_name = (Stephen, Sean, Bill, John)
• The key word “declare –a” can also be left out
• Accessing array values
• ${array_name[index]}
7
array1.sh
#!/bin/bash
NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"
echo "First Index: ${NAME[0]}"
echo "Second Index: ${NAME[1]}”
echo "First Method: ${NAME[*]}"
echo "Second Method: ${NAME[@]}"
8
Array: Search and Replace
• An element of an array can be replaced with another
element
• Searches array until element is found
• Uses regular expression to match element
• Enables on the fly data manipulations
• Example:
• array4.sh
9
array4.sh
#!/bin/bash
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
echo ${Unix[@]/Ubuntu/Mint}
• Output
Debian Red hat Mint Suse Fedora UTS OpenLinux
• Note: this replacement is not permanent.
• Create a new array to make it permanent.
10
Loading File Content into Array
• A file content can be read line by line into an array.
• Example file content:
• Cat myfile.txt
Hello
How are you
Working
Hard
Big money
11
Loading File Content into Array:
“for” Loop
• array9.sh
#!/bin/bash
IFS=$'\r\n'
filecontent=( `cat "myfile.txt" `)
for t in "${filecontent[@]}"
do
echo $t
done
echo "Finished reading file content!"
Output:
Hello
How are you
Working
Hard
Big money
Finished reading file content!
12
Shell Functions
• A shell function is similar to a shell script
• stores a series of commands for execution later
• shell stores functions in memory
• Functions can be called upon to execute its task when needed.
• shell executes a shell function in the same shell that called it
• Where to define
• In .profile
• In your script
• Or on the command line
• Remove a function
• Use unset built-in
13
Functions: Syntax
• Functions are declared as follows
function_name(){
List_of_commands
}
• The name of the function is “function_name”
• Use to invoke the function anywhere in the script
• Can be named anything
• Function names should be descriptive of its actions
14
function.sh
#!/bin/bash
# Define your function here
Hello () {
echo "Hello Welcome to the World"
}
# Invoke your function
Hello
15
Functions: Passing Parameters
• A Bash function can accept parameters during invocation
• Unlike C-programming
• Function parameters are not declared together with function name
• Invalid: function_name(param1, param2,…) {}
• Function parameters are accessible inside functions as:
• $1, $2, etc.
• $# reflects number of parameters
• $0 contains name of script (not name of function)
• Example
• function2.sh
16
function2.sh
#!/bin/bash
# Define your function here
Hello () {
echo "Hello $1 $2 Welcome to the World"
}
# Invoke your function
Hello Vincent Emeakaroha
17
Functions: Parameter Example
• Write a function that accepts two parameter values
and do an addition.
#!/bin/bash
# Define your function to take two parameter
myAdd () {
echo “$1 + $2 = $(($1+$2))”
}
echo “Please enter first number: “
read num1
echo “Please enter second number: “
read num2
myAdd $num1 $num2
18
Functions: Parameter Example
#Proper Parameter Passing
#!/bin/bash
# Define your function to take two parameter
myAdd () {
echo “$1 + $2 = $(($1+$2))”
}
echo “Please enter first number: “
read num1
echo “Please enter second number: “
read num2
myAdd $num1 $num2
#Use of Global Variable - Incorrect
#!/bin/bash
echo “Please enter first number: “
read num1
echo “Please enter second number: “
read num2
# Define your function to take two parameter
myAdd () {
echo “$num1 + $num2 = $(($num1+$num2))”
}
myAdd $num1 $num2 ##OR myAdd alone
19
Functions: Return Value
• Functions can return value using the syntax
• return value
• Returned value mostly numerical. String (characters) is possible but
complex
• This command terminates the execution of the function
• Returned value is stored in the shell variable “$?” if not customised
• “$?” can hold value between 0 – 255.
• Using the command “exit” in a function
• Terminates the function and the whole script
• Example
• function3.sh
20
function3.sh
#!/bin/bash
# Define your function here
Hello () {
echo "Hey $1 $2 Welcome to the World"
return 20
}
# Invoke your function
Hello Vincent Emeakaroha
# Capture value returned by last command
ret=$?
echo "Return value is $ret"
21
AWK
22
Operation Style and Usages
• awk operation:
• scans a file line by line
• splits each input line into fields
• compares input line/fields to pattern
• performs action(s) on matched lines
• Useful for:
• transforming data files
• producing formatted reports
• Programming constructs:
• format output lines
• arithmetic and string operations
• conditionals and loops
23
Structure of an AWK Script
• An awk script consists of:
• An optional BEGIN segment
• For preparing execution prior to reading input
• pattern - action pairs
• Processing the input data
• For each pattern matched, the corresponding action is
taken
• An optional END segment
• Processing after end of input data
BEGIN {action}
pattern {action}
pattern {action}
.
.
.
pattern { action}
END {action}
24
Record and Fields
25
Local Variable
• awk scripts can define and use variables
BEGIN { sum = 0 }
{ sum ++ }
END { print sum }
• Some variables are predefined
• Examples are the special variables
• NR Number of Records
• FS Field Separator
• Etc.
26
AWK Special Variables
FS Field separator (default=whitespace)
RS Record separator (default=\n)
NF Number of fields in current record
NR Number of the current record
OFS Output field separator (default=space)
ORS Output record separator (default=\n)
FILENAME Current filename
• Similar to Bash environment variables.
27
Basic AWK Command Types
1. Direct command:
• awk [options] ’command‘ file(s)
2. Script file:
• awk [options] –f scriptfile file(s)
• -f indicates usage of script file containing script commands
• Special characters
• $0 is the entire line – a record
• $1 is the first field, $2 is the second field, ….
28
Example: Records and Fields
cat emps
Tom Jones 4424 5/12/66 543354
Mary Adams 5346 11/4/63 28765
Sally Chang 1654 7/22/54 650000
Billy Black 1683 9/23/44 336500
awk '{print NR, $0}' emps
1 Tom Jones 4424 5/12/66 543354
2 Mary Adams 5346 11/4/63 28765
3 Sally Chang 1654 7/22/54 650000
4 Billy Black 1683 9/23/44 336500
29
Example: Space as Field Separator
cat emps
Tom Jones 4424 5/12/66 543354
Mary Adams 5346 11/4/63 28765
Sally Chang 1654 7/22/54 650000
Billy Black 1683 9/23/44 336500
awk '{print NR, $1, $2, $5}' emps
1 Tom Jones 543354
2 Mary Adams 28765
3 Sally Chang 650000
4 Billy Black 336500
30
Example: Colon as Field Separator
cat em2
Tom Jones:4424:5/12/66:543354
Mary Adams:5346:11/4/63:28765
Sally Chang:1654:7/22/54:650000
Billy Black:1683:9/23/44:336500
awk -F: '/Jones/{print $1, $2}' em2
Tom Jones 4424
31
Logical Operators
Operator Meaning Example
&& Logical AND a && b
|| Logical OR a || b
! NOT ! a
Examples:
awk '($2 > 5) && ($2 <= 15) {print $0}'
file
awk '$3 == 100 || $4 > 50 {print $0}' file
32
Invoking Script File
• File: grades.txt
John 85 92 78 94 88
Andrea 89 90 75 90 86
Jasper 84 88 80 92 84
• awk script: average.awk
# average five grades
{ total = $2 + $3 + $4 + $5 + $6
avg = total / 5
print $1, avg }
• Run as:
awk –f average.awk grades.txt
Output
33
Control Statements
• Conditional
• if-else
• Repetition
• for
• with counter
• with array index
• while
• do-while
• also: break, continue
34
if Example 1: Missing Scores
cat student-scores
Bill 2143 78 84 77
Jane 2321 56 58 45
James 2122 38 37
John 2537 87 97 95
Keith 2415 30 47
if.awk
#!/bin/awk
{
if ($3 =="" || $4 == "" || $5 == "")
print "Some score for the student",$1,"is
missing";
}
• Invoking script
• awk –f if.awk student-scores
• Output
• Some score for the student James is missing
• Some score for the student Keith is missing
35
SED
36
Sed: Stream-oriented, Non-
Interactive Text Editor
• Looks for patterns one line at a time, like grep. That is, it
operates per line contents of a file
• Non-interactive text editor
• Borrows syntax from the interactive editor ed which accepts
the same commands
• Can be used in many ways such as:
• Text substitution,
• Selective printing of text files,
• In-a-place editing of text files,
• Non-interactive editing of text files, and many more
37
sed Commands
• sed commands have the general form
• [address[, address]][!]command [arguments]
• sed copies each input line into a pattern space
• If the address of the command matches the line in the pattern space,
the command is applied to that line
• If the command has no address, it is applied to each line as it enters
pattern space
• If a command changes the line in pattern space, subsequent
commands operate on the modified line
• When all commands have been read, the line in pattern space is
written to standard output and a new line is read into pattern
space
38
sed Address
• An address can be either a line number or a pattern,
enclosed in slashes ( /pattern/ )
• A pattern is described using regular expressions
• Basic Regular Expression (BRE) as in grep
• If no pattern is specified, the command will be applied
to all lines of the input file
• To refer to the last line use: $ and the first line use: ^
39
sed Command
• command is a single letter
• Example: Deletion: d
• [address1][,address2]d
• Delete the addressed line(s) from the pattern space;
• The removed line(s) are not passed to standard output.
40
Address and Command Examples
• d deletes all lines
• 6d deletes line 6
• /^$/d deletes all blank lines
• 1,10d deletes lines 1 through 10
• 1,/^$/d deletes from line 1 through the first blank line
• /^$/,$d deletes from the first blank line through
the last line of the file
• /^$/,10d deletes from the first blank line through line 10
• /^ya*y/,/[0-9]$/d deletes from the first line that begins
with yay, yaay, yaaay, etc. through
the first line that ends with a digit
41
Examples
• sed -e 'd' /etc/services
• Outputs nothing
• sed -e '1d' /etc/services
• Delete first line and output the rest to standard output
• sed -e '1,10d' /etc/services
• Delete first 10 lines and output the rest
• sed -e '/^#/d' /etc/services
• Delete all lines starting with “#” and output the rest
42
Pattern Substitution
• Syntax: [address(es)]s/pattern/replacement/[flags]
• pattern - search pattern
• replacement - replacement string for pattern
• flags - optionally any of the following
• n a number from 1 to 512 indicating which
occurrence of pattern should be replaced
• g global, replace all occurrences of pattern in pattern space
• p print contents of pattern space
43
Substitute Example
• s/Puff Daddy/P. Diddy/
• Substitute P. Diddy for the first occurrence of Puff Daddy
in pattern space
• s/Tom/Dick/2
• Substitutes Dick for the second occurrence of Tom in the
pattern space
• s/wood/plastic/p
• Substitutes plastic for the first occurrence of wood and
outputs (prints) pattern space
44
Using !
• If an address is followed by an exclamation point (!), the
associated command is applied to all lines that don’t match
the address or address range
• Examples:
• 1,5!d would delete all lines except 1 through 5
• /black/!s/cow/horse/ would substitute “horse” for
“cow” on all lines except those that contained “black”
“The brown cow” -> “The brown horse”
“The black cow” -> “The black cow”
45
Quit
• Quit causes sed to stop reading new input lines and stop
sending them to standard output
• It takes at most a single line address
• Once a line matching the address is reached, the script will be
terminated
• This can be used to save time when you only want to process
some portion of the beginning of a file
• Example: to print the first 100 lines of a file (like head) use:
• sed '100q' filename
• sed will, by default, send the first 100 lines of filename to
standard output and then quit processing
46
Manipulating Files
• sed prints modified content of a file to the screen but does NOT
change the file
• Two ways to fix this:
• Use “-i” option for GNU compatible sed
• Output redirection
• Example
• sed -e 's/coat/bag/g' fileEg01 > fileEg03
• Changes all occurrences of “coat” with “bag” in file “fileEg01”
• The modified output is saved in a new file “fileEg03”
• File “fileEg01” remains unchanged
• sed -i -e 's/coat/bag/g' fileEg01 (Changes the content of the file)
47
Thank You!
