COMP7004 - Systems Scripting
Lecture 06: Array, Control Flow
Dr. Vincent Emeakaroha
vincent.emeakaroha@mtu.ie
Semester 2, 2026
Manipulating 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
array4.sh
#!/bin/bash
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
echo ${Unix[@]/Ubuntu/Mint}
• Output
Debian Red hat Mint Suse Fedora UTS OpenLinux
• Note: this replacement is not permanent
Manipulating Array: Adding an
Element
• By default, it puts the new element after the last index
• Changes array content permanently
• Example:
• array5.sh
array5.sh
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
Unix=("${Unix[@]}" "Mint" "Solaris")
echo ${Unix[@]}
• Output
Debian Red hat Ubuntu Suse Fedora UTS OpenLinux Mint Solaris
Manipulating Array: Adding an
Element – Start and Specific Position
• To add element at the beginning of array
• Place the new elements before the existing array
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
Unix=("Mint" "Solaris" "${Unix[@]}" )
• To add element at a specific index position. E.g., index 2
• Get all elements before index position2 Unix[0] and Unix[1];
• Add an element to the array;
• Get all elements from index position2 to the last Unix[2], Unix[3], ....
Unix=( "${Unix[@]:0:2}" "new_element" "${Unix[@]:2}" )
Manipulating Array: Removing an
Element
• Command “unset” removes an element and set the value
to “ ”
• Example:
• array6.sh
#!/bin/bash
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
unset Unix[3]
echo ${Unix[@]}
• Output
Debian Red hat Ubuntu Fedora UTS OpenLinux
Manipulating Array: Copying
• Copy by expanding an array into a new one.
• Example:
• array7.sh
#!/bin/bash
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
Linux=("${Unix[@]}")
echo ${Linux[@]}
• Output
Debian Red hat Ubuntu Suse Fedora UTS OpenLinux
Concatenation of Two Arrays
• Expand elements of two arrays and assign to new array.
• Example:
• array8.sh
#!/bin/bash
Unix=('Debian' 'Red hat' 'Ubuntu' 'Suse' 'Fedora' 'UTS' 'OpenLinux');
Shell=('bash' 'csh' 'jsh' 'rsh' 'ksh' 'rc' 'tcsh');
UnixShell=("${Unix[@]}" "${Shell[@]}")
echo ${UnixShell[@]}
echo ${#UnixShell[@]}
• Output
Debian Red hat Ubuntu Suse Fedora UTS OpenLinux bash csh jsh rsh ksh rc tcsh
14
Loading File Content into Array
• A file content can be read line by line into an array.
• Example file content:
• Cat myfile.txt
Hello
How are you
Working
Hard
Big money
Loading File Content into Array: “for” Loop
• array9.sh
#!/bin/bash
IFS=$'\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!
A C-Like “for” Loop
• An alternative form of the for structure is
for (( EXPR1 ; EXPR2 ; EXPR3 ))
do
statements
done
• First, the arithmetic expression EXPR1 is evaluated. EXPR2
is then evaluated repeatedly until it evaluates to 0. Each
time EXPR2 is evaluates to a non-zero value, statements
are executed and EXPR3 is evaluated.
• Example
• for4.sh
for4.sh
#!/bin/bash
echo –n "Enter a number: "; read x
sum=0
for (( i=1 ; $i<$x ; i=$i+1 ))
do
sum=$(($sum+$i))
done
echo "The sum of the first $x numbers is: $sum"
While Statement
• The while structure is used to execute a set of commands
while a specified condition is true.
• The loop terminates as soon as the condition becomes false.
• If condition never becomes false, loop will never exit.
while expression
do
statements
done
• Example
• while.sh
while.sh
#!/bin/bash
echo –n "Enter a number: "; read n
sum=0; i=1
while [ $i -le $n ]
do
sum=$(($sum+$i))
((i=$i+1))
done
echo "The sum of the first $n numbers is: $sum"
“while” Loop: menu.sh
#!/bin/bash
clear ; loop=y
while [ "$loop" == "y " ] ;
do
echo "Menu"; echo "===="
echo "D: print the date"
echo "W: print the users who are currently log on."
echo "P: print the working directory"
echo "Q: quit."
echo
read choice
case $choice in
D | d) date ;;
W | w) who ;;
P | p) pwd ;;
Q | q) loop=n ;;
*) echo "Illegal choice." ;;
esac
echo
done
Continue Statement
• The continue statement causes a jump to the next
iteration of a loop.
• Skips all the remaining commands in that particular loop
cycle.
• It initiates the beginning of the next loop cycle
• Example:
• continue.sh
continue.sh
#!/bin/bash
LIMIT=19
echo
echo "Printing Numbers 1 through 20 (but not 3 and 11)"
a=0
while [ $a -le $LIMIT ]; do
a=$(($a+1))
if [ $a -eq 3 ] || [ $a -eq 11 ]
then
continue
fi
echo -n "$a "
done
Break Statement
• A break command terminates a loop and continue
program execution.
• Breaks out of a loop
• Returns execution back to the main program
• Example
• break.sh
break.sh
#!/bin/bash
LIMIT=19
echo
echo "Printing Numbers 1 through 20, but something happens after 2 … "
a=0
while [ $a -le $LIMIT ]
do
a=$(($a+1))
if [ $a -gt 2 ]
then
break
fi
echo -n "$a "
done
echo; echo; echo
exit 0
Until Statement
• The until structure is very similar to the while structure.
• The until structure loops until the condition is true.
• So basically it is “until this condition is true, do this”.
until [expression]
do
statements
done
• Example
• until.sh
until.sh
#!/bin/bash
echo "Enter a number: "; read x
echo ; echo "Count Down"
until [ $x -le 0 ]
do
echo $x
x=$(($x -1))
sleep 1
done
echo ; echo "GO !"
Debugging Bash Scripts
• Bash provides two options which will give useful
information for debugging
-x : displays each line of the script with variable substitution
and before execution
-v : displays each line of the script as typed before execution
• Usage syntax:
#!/bin/bash –v or #!/bin/bash –x or #!/bin/bash –xv
• Example using for loop
• debug.sh
debug.sh
#!/bin/bash -x
echo –n "Enter a number: "; read x
sum=0
for (( i=1 ; $i<$x ; i=$i+1 ))
do
sum=$(($sum+$i))
done
echo "The sum of the first $x numbers is: $sum"
Output with inputting 3
+ echo $'?\200\223n' 'Enter a number: '
–n Enter a number:
+ read x
3
+ sum=0
+ (( i=1 ))
+ (( 1<3 ))
+ sum=0+1
+ (( i=1+1 ))
+ (( 2<3 ))
+ sum=1+2
+ (( i=2+1 ))
+ (( 3<3 ))
+ echo 'The sum of the first 3 numbers is:3'
The sum of the first 3 numbers is: 3
Thank You!
