Shell Scripting - Loops & Command Line Arguments

These topics are covered in today's session by Pranav Jambare

Loops in Linux

• Loops in shell scripting allow you to execute a series of commands repeatedly, based on a condition or a list of items.

Types of Loops

While Loop

• The while loop repeatedly executes the commands inside the loop as long as the condition remains true.

• If the condition becomes false, the loop stops executing and moves to the next part of the script.

#Syntax:
while <command>;
do
    # Statement execute if command is true
done

#Example: (Create a sample.sh and edit the file with the given script)
#Printing numbers from 1 to 20
#!/bin/bash
A=1
while [ ${A} -le 20 ];
do
    echo "A = ${A}"
    A=`expr ${A} + 1` 
done

#After running script...
A = 1
A = 2
...
A = 19
A = 20

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Until Loop

• The until loop is similar to the while loop, but it continues executing the commands inside the loop until the condition becomes false.

• Once the condition is true, the loop stops executing.

#Syntax:
until <command>;
do
    # Statement execute if command is false
done

#Example: (Create a sample.sh and edit the file with the given script)
#Printing numbers from 20 to 1
#!/bin/bash
A=20
while [ ${A} -ge 1 ];
do
    echo "A = ${A}"
    A=`expr ${A} - 1` 
done

#After running script...
A = 20
A = 19
...
A = 2
A = 1

---

For Loop

• The for loop iterates over each item in the list, and the variable takes the value of each item in each iteration.

• The commands inside the loop are executed for each item in the list.

#Syntax:
for var in word1 word2 .. wordn;
do
    # Statement executed till element end
done

#Example:
#Printing the names of students...
#!/bin/bash
A=1
for studName in Viraj Amogh Suyash;
do
    echo "The name of student ${A} is : ${studName}"
    A=`expr ${A} + 1`
done

#After running the script...
The name of student 1 is : Viraj
The name of student 2 is : Amogh
The name of student 3 is : Suyash

#Example:
#To print alphabets from A to Z
#!/bin/bash
for alpha in {A..Z};
do
    echo "${alpha}"
done

#After running the script...
A
B...
..
Y
Z

#Example:
#To print the names of students from a file
#!/bin/bash
for name in $(cat /home/viraj/namesFile);
do
    echo "The name is : ${name}"
done

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Command Line Arguments

• Command Line Arguments are inputs or parameters provided to a command or script when it is executed in the terminal.

• $1 $2 $3 --> The first command-line argument is represented by $1, the second one by $2, and so on.

• $0 --> Represents the name of the script itself.

• $@ --> Passing arguments one-by-one

• $# --> Total number of command line arguments provided

• $* --> Represents all the command-line arguments as a single string

• $$ --> Represents the process ID (PID) of the currently running shell or script

• $? --> Holds the status of the last executed command

• $! --> Hold the process ID (PID) of the last executed command

#Example:
#!/bin/bash
for names in $1 $2;
do
    echo "Your name is : ${names}"
    echo "The name of script is : $0"
    echo "Passing args one-by-one : $@"
    echo "Total number of args provided : $#"
    echo "Args as a single string : $*"
    echo "PID of current script : $$"
    echo "Status of last executed cmd: $?"
    echo "PID of last executed cmd: $!"
done

#After running script...
Your name is : Viraj
The name of script is : sampleFile.sh
Passing args one-by-one : Viraj Amogh
Total number of args provided : 2
Args as a single string : Viraj Amogh
PID of current script : 4920
Status of last executed cmd : 0
PID of last executed cmd : --

Your name is : Amogh
The name of script is : sampleFile.sh
Passing args one-by-one : Viraj Amogh
Total number of args provided : 2
Args as a single string : Viraj Amogh
PID of current script : 4920
Status of last executed cmd : 0
PID of last executed cmd : --

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Shell Scripting Question

Q. Create 5 directories and take user input to read name of those directories. Now create two files in each directory and take user input to name those files. Now take user input to add some content in each file.

#Answer:
#!/bin/bash
for dirName in {1..5};
do 
    echo "Enter the name of directory : "
    read dir
    mkdir ${dir}

    for fileName in 1 2;
    do 
        echo "Enter the name of file : "
        read file

        touch /home/viraj/${dir}/${file}

        echo "Enter the content for file : "
        read content >> /home/viraj/${dir}/${file}
    done
done

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