Fossman Media | Bash

Shebang (local)

If the script is being run locally on the machine that created it, the following command can be use to find the path to the Bash interpreter. After finding the path, add it to the shebang/hashbang at the top of the script.

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Find path to interpreter

Shebang (portable)

Use the following example to specify the intended interpreter by the environment variable. This can be useful if the path to the interpreter is unknown.

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Using environment variable

Basic User Input

The following example uses a user input to get information from the user. Ater this information has been collected, it will be stored in a variable called any_value and shown to the user.

#!/usr/bin/bash

# user input
echo -n "Enter any value: "
read any_value

# print out entered value
echo "value entered: $any_value"

# exit this script
exit

Screenshot
User input example

if / else

The following is an example scipt using a if/else statement. The user will first be asked to select a mode. If read is selected the message "mode was set to read" will be shown. If any other value is entered, the message "mode was not set to read" will be shown.

#!/usr/bin/bash

# user input
echo -n "choose mode (read/write): "
read mode

# if / else
if [ $mode == read ]; then
    echo "mode was set to read"
else
    echo "mode was not set to read"
fi

# exit this script
exit

Screenshot
if/else statement example

else / if

The following is an example scipt using a else/if statement. The user will first be asked to select a mode. If read is selected the message "mode was set to read" will be shown. If write is selected the message "mode was set to write" will be shown. If any other value is entered, the message "invalid value" will be shown.

#!/usr/bin/bash

# user input
echo -n "choose mode (read/write): "
read mode

# else / if
if [ $mode == read ]; then
    echo "mode was set to read"
elif [ $mode == write ]; then
    echo "mode was set to write"
else
    echo "invalid value"
fi

# exit this script
exit

Screenshot
else/if statement example

if / and

The following is an example scipt using a if/and statement. The user will first be asked to provide a username and password. If the username is root and the password is password, they will be presented with a welcome message. If the username entered is root and the password is not password, they will get a message saying the password is invalid. Finally... if the user is not root, they will be denied access and threatened.

#!/usr/bin/bash

echo ""
echo -n "Username: "
read username
echo -n "Password: "
read password
echo ""

# if / and
if [ $username == "root" ] && [ $password == "password" ]; then
    echo "Welcome $username"
elif [ $username == "root" ] && [ $password != "password" ]; then
    echo "password entered is not valid"
else
    echo "only root has access"
    echo "this indicent will be reported"
fi

# exit script
exit

Screenshot
if/and statement example

if / or

The following is an example scipt using a if/or statement. The user will first be asked to provide a username and password. If the username is root or tux and the password is password, they will be presented with a welcome message. If the username entered is root or tux and the password is not password, they will get a message saying the password is invalid. Finally... if the user is not root or tux, they will be denied access and threatened.

#!/usr/bin/bash

echo ""
echo -n "Username: "
read username
echo -n "Password: "
read password
echo ""

# if / or
if [ $username == 'root' ] || [ $username == 'tux' ] && [ $password == "password" ]; then
    echo "Welcome $username"
elif [ $username == 'root' ] || [ $username == 'tux' ] && [ $password != "password" ]; then
    echo "password entered is not valid"
else
    echo "only root and tux have access"
    echo "this incident will be reported"
fi

# exit script
exit

Screenshot
if/and statement example

For Loops

The following code snippets are examples of for loops

Specified Range (iteration not shown)
Complete a task for a predetermined amount of iterations. In the example below... the sentence "just some text" will be printed out to the screen 10 times.

#!/usr/bin/bash

for i in $(seq 10); do
    echo "just some text"
    sleep 1
done

# exit script
exit

Specified Range (iterations shown)
Similar to the example above, the following code snippet will complete a task for a predetermined amount of iterations. The sentence "just some text" will be printed out to the screen 10 times. Showing each iteration can be done by adding the $i to the end of the echo command. This would be the variable created in the for loop representing each iteration. In this context, the "i" can be considered as an item or iteration. Any other character or name can also be used, as the "i" is only representative

#!/usr/bin/bash

for i in $(seq 10); do
 echo "just some text:" $i
 sleep 1
done

# exit script
exit

Specified Iterations (iterations shown)
In this example, we can define where the iteration will start and where it will end. Each iteration will be shown, starting at 1 and ending at 10.

#!/usr/bin/bash

for i in {1..10}; do
 echo "just some text:" $i
 sleep 1
done

# exit script
exit

Specified Iterations (iterations shown 5-10)
Similar to the example above, we can specify where the iterations will start and when they will end. In this example the 1st iteration starts at 5 and ends at 10. Resulting in 6 iterations.

#!/usr/bin/bash

for i in {5..10}; do
 echo "just some text:" $i
 sleep 1
done

# exit script
exit

Specified Iterations (iterations shown 1, 4, 7 and 10)
In addition to specifying where the iterations will start and end, we can also specify the size of each step. In the example below, the iterations will start at 1, end at 10 and will have a step count of 3. Resulting in 4 iterations (1,4,7 and 10)

#!/usr/bin/bash

for i in {1..10..3}; do
 echo "just some text:" $i
 sleep 1
done

# exit script
exit

Arrays (show contents)
For loops can also be used to iterated through an array. The items in said array can be anything from strings to functions and even other arrays. In the example below, a list of employees will be printed out to the screen from an array of employees. ${Employees[@] expands the array to include all elements.

#!/usr/bin/bash

Employees=("John" "Alice" "Tux" "May" "Jessie" "Jade" "Louis" "Sara" "Donald")

echo ""
echo "Employees"
echo "---------"

for i in ${Employees[@]}; do
 echo "$i"
 sleep 1
done

# exit script
exit

While Loops

The following is an example script using a while loop. These are used to execute a block of code repeatedly while a specified condition remains true. It evaluates the condition before each iteration based on changing conditions within the program.

While Loop (Basic Example)
In the example below, the while loop will execute its commands while the "valid" condition remains true or when the iteration count reaches 5. After which, the script will then exit. Each iteration will display it's value on the screen until it reaches 5 and sleep for 1 second. Resulting in the numbers 1 through 5 being printed on the screen.

#!/usr/bin/bash

count=1

while true; do
    echo currnet count: $count
if [ $count -eq 5 ]; then
    break
fi

((count++))
sleep 1

done

# exit script
exit

Screenshot
basic while loop

While Loop (with user input)
In the example below, the while loop will execute its commands while the value of "guess" is not equal to 5 (the magic number) At each iteration, the user will be prompted to enter a number. When the magic number has been guessed correctly the number will be shown along with some positive feedback. After which, the program will exit.

#!/usr/bin/bash

magic_number=5
guess=0

echo ""
echo "Guess the magic number (1-10)"
echo ""

while [ $guess != $magic_number ]; do
    echo -n "Enter Number => "
    read guess

done

echo ""
echo "You guessed correctly!"
echo "The magic number is $magic_number"

# exit script
exit

Screenshot
while loop with user input