Bash Scripting Cheat Sheet - Part 2

While Loop

A while loop executes a set of commands repeatedly as long as a specified condition is true. The syntax of a while loop is as follows:

while [condition]
do
    [commands]
done

Here, [condition] is a test command that returns a status of 0 (true) or 1 (false). If the status is true, the [commands] will be executed. The [commands] can be a single command or a block of commands enclosed in {}.

Example

Suppose we want to print numbers from 1 to 5. We can use the following while loop:

i=1
while [ $i -le 5 ]
do
    echo $i
    i=$((i+1))
done

In this example, the [condition] is [ $i -le 5 ], which means the loop will execute as long as the value of $i is less than or equal to 5. The [commands] in this case are two commands: echo $i and i=$((i+1)). The first command prints the value of $i to the screen, and the second command increments the value of $i by 1.

When we run this script, we get the following output:

1
2
3
4
5

For Loop

A for loop is used to iterate over a sequence of values. The syntax of a for loop is as follows:

for var in [sequence]
do
    [commands]
done

Here, [sequence] is a list of values that var will take on successively. The [commands] will be executed for each value of var.

Example

Suppose we want to print the numbers from 1 to 5 using a for loop. We can use the following script:

for i in 1 2 3 4 5
do
    echo $i
done

In this example, the [sequence] is the list of values 1 2 3 4 5. The [commands] in this case is the command echo $i, which will print the value of i to the screen.

When we run this script, we get the following output:

1
2
3
4
5

Simplify the for loop [sequence]

In bash, {1...5} is a brace expansion that generates a sequence of values from 1 to 5, inclusive. This is equivalent to writing 1 2 3 4 5. You can use this syntax in a for loop to iterate over the sequence of values. For example, the following script will print the numbers from 1 to 5 using a for loop with brace expansion:

for i in {1..5}
do
    echo $i
done

When we run this script, we get the following output:

1
2
3
4
5

Note that the syntax {1...5} is incorrect and will result in an error. The correct syntax is {1..5}.

Some practical example

Q. How do I zip all the files inside the logfile directory?

Suppose we have a directory called logfile that contains several log files, and we want to compress all the files in the directory into a single archive. We can use the following script:

#!/bin/bash

for file in /path/to/logfile/*
do
        gzip "$file"
done

The above code is a bash script that compresses all the files in a directory called logfile into a single archive. It uses a for loop to iterate over all the files in the directory, and the gzip command to compress each file. The * wildcard is used to match all files in the directory, and the $file variable is used to refer to each file in turn. When the script is run, it will compress each file in the directory using the gzip command. The resulting compressed files will be stored in the same directory as the original files.

To write the text "hello" to a file named output.txt using bash, you can use the following command:

echo "hello" > output.txt

This will create a new file called output.txt in the current directory, and write the text "hello" to the file. If the file already exists, the command will overwrite the existing file with the new text.

Functions

A function is a block of code that can be called repeatedly throughout a script. Functions are useful for encapsulating common tasks or operations that need to be repeated multiple times with different inputs.

Defining a Function

To define a function in bash, you can use the following syntax:

function_name () {
    [commands]
}

Here, function_name is the name of the function, and [commands] is the block of code that defines the function. The function can take arguments, which are passed to it as positional parameters.

Example

Here is an example bash function that checks the exit code of a command and prints an error message if the exit code is not zero:

check_exit_code () {
    if [ $? != 0 ]
    then
        echo "error"
    fi
}

This function uses the $? variable to check the exit code of the previous command. If the exit code is not zero, the function prints the error message "error" to the screen. You can call this function after running a command to check its exit code. For example:

ls /path/to/some/directory/
check_exit_code

In this example, the ls command is run to list the files in a directory. The check_exit_code function is then called to check the exit code of the ls command. If the command failed, the function will print the error message "error" to the screen.

Note that you can replace the ls command with any other command whose exit code you want to check.

Another Example

Suppose we want to define a function that takes two arguments and returns their sum. We can use the following script:

#!/bin/bash

add () {
    sum=$(($1 + $2))
    echo $sum
}

result=$(add 2 3)
echo "The sum is: $result"

In this example, we define a function called add that takes two arguments and returns their sum. The function uses the $1 and $2 variables to refer to the first and second arguments, respectively. It then calculates the sum of the arguments and stores the result in the sum variable. Finally, the function uses the echo command to print the value of sum to the screen.

We then call the add function with arguments 2 and 3, and store the result in a variable called result. Finally, we use the echo command to print the value of result to the screen.

When we run this script, we get the following output:

The sum is: 5

Note that the add function can be called with different arguments to calculate the sum of different numbers.

Passing Arguments to a Function

To pass arguments to a function in bash, you can use the positional parameters $1, $2, $3, and so on. These variables refer to the first, second, third, and subsequent arguments passed to the function, respectively.

Returning Values from a Function

To return a value from a function in bash, you can use the return command followed by a value. The value can be a string, a number, or any other data type that can be stored in a variable.

Example

Suppose we want to define a function that takes two arguments and returns their product. We can use the following script:

#!/bin/bash

multiply () {
    product=$(($1 * $2))
    return $product
}

multiply 2 3
result=$?
echo "The product is: $result"

In this example, we define a function called multiply that takes two arguments and returns their product. The function uses the $1 and $2 variables to refer to the first and second arguments, respectively. It then calculates the product of the arguments and stores the result in the product variable. Finally, the function uses the return command to return the value of product to the calling program.

We then call the multiply function with arguments 2 and 3. The function returns the product of the arguments, which is stored in the special variable $?. We then use the echo command to print the value of $?, which is the value returned by the multiply function.

When we run this script, we get the following output:

The product is: 6

Note that the multiply function can be called with different arguments to calculate the product of different numbers.

Read input from the user

The read command is used to read input from the user and store it in a variable. The syntax of the read command is as follows:

read variable_name

Here, variable_name is the name of the variable to store the input in. When the read command is executed, it waits for the user to enter input, and then stores the input in the specified variable.

Example

Suppose we want to write a script that prompts the user to enter their name and then prints a greeting. We can use the following script:

#!/bin/bash

echo "What is your name?"
read name
echo "Hello, $name!"

In this example, we use the echo command to print a message to the screen, and the read command to read the user's input. We then use the echo command again to print a greeting that includes the user's name.

When the user runs this script, they will be prompted to enter their name. After they enter their name, the script will print a greeting that includes their name.

Note that the read command can be used to read input other than text, such as numbers or filenames. To read a number, you can use the -n option to specify the number of characters to read, and the -p option to prompt the user for input. For example:

read -p "Enter a number: " -n 2 number

This command will prompt the user to enter a two-digit number, and store the number in the number variable. To read a filename, you can use the -e option to enable tab completion. For example:

read -e -p "Enter a filename: " filename

This command will prompt the user to enter a filename, and enable tab completion for file paths.

Case Statement

A case statement is used to match a variable against a list of patterns, and execute the corresponding commands if a match is found. The syntax of a case statement is as follows:

case variable in
pattern1)
    [commands1]
    ;;
pattern2)
    [commands2]
    ;;
...
esac

Here, variable is the variable to match against the patterns, and pattern1, pattern2, and so on are the patterns to match. The ;; symbols are used to separate the commands for each pattern.

Example

Suppose we want to write a script that installs a Linux distribution based on user input. We can use a case statement to match the user input against a list of distributions, and execute the corresponding commands to install the selected distribution. Here is an example script:

#!/bin/bash

echo "Which Linux distribution do you want to install?"
echo "1. Ubuntu"
echo "2. Debian"
echo "3. Fedora"
echo "4. CentOS"

read choice

case $choice in
    1)
        echo "Installing Ubuntu..."
        # Commands to install Ubuntu
        ;;
    2)
        echo "Installing Debian..."
        # Commands to install Debian
        ;;
    3)
        echo "Installing Fedora..."
        # Commands to install Fedora
        ;;
    4)
        echo "Installing CentOS..."
        # Commands to install CentOS
        ;;
    *)
        echo "Invalid choice!"
        ;;
esac

In this example, we use the echo command to print a list of Linux distributions to the screen, and the read command to read the user input. We then use a case statement to match the user input against the list of distributions, and execute the corresponding commands to install the selected distribution.

When the user runs this script, they will be prompted to select a Linux distribution to install. If they enter a valid choice (1, 2, 3, or 4), the script will execute the corresponding commands to install the selected distribution. If they enter an invalid choice, the script will print an error message.

Note that the case statement can be used to match any variable against a list of patterns, not just user input.

at command

The at command in bash is used to run a command or script at a specified time in the future. The syntax of the at command is as follows:

at [-f file] [-m] [-q queue] [-t time] [-v] [time]

Here, time is the time at which the command or script should be run. The time can be specified in a number of different formats, such as now, noon, or midnight, or as a specific date and time.

The [options] specify additional options for the at command. For example, the -f option can be used to specify a file containing the command or script to be run, and the -m option can be used to send an email to the user when the job is complete.

Example

Suppose we want to run a script called backup.sh at 3:00 AM tomorrow. We can use the following command:

echo "/path/to/backup.sh" | at 3am tomorrow

In this example, we use the echo command to write the command /path/to/backup.sh to the standard input of the at command. We then use the at command to run the command at 3:00 AM tomorrow.

When the at command runs the script, it will execute the commands in the script as if they were typed directly into the terminal. The script can perform any task, such as backing up files or sending email notifications.

Note that the at command can be used to run any command or script, not just bash scripts. For example, you can use the at command to schedule a shutdown or reboot of the system, or to run a Python script or other program.

Scheduling Jobs in Bash

In bash, you can use the cron utility to schedule jobs to run at specified intervals. The cron utility is a time-based job scheduler that runs commands at specified intervals.

To schedule a job in cron, you need to create a crontab file that specifies the jobs to run and the intervals at which they should be run. The syntax of a crontab file is as follows:

*     *     *     *     *  command to be executed
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- day of the week (0 - 6) (Sunday = 0)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of the month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

Here, the first five fields specify the time and date when the command should be run, and the last field specifies the command to be run. The asterisk (*) symbol is used as a wildcard to match any value.

For example, the following crontab entry would run the command /path/to/command every day at 3:30 AM:

30 3 * * * /path/to/command

To edit your crontab file, you can use the crontab -e command. This will open your crontab file in the default text editor. You can then add, edit, or delete entries as needed.

Note that the syntax of the crontab file can be complex, so it is a good idea to test your entries using the date command before adding them to your crontab file.

Example

Suppose we want to schedule a job to run a script called backup.sh every day at 5:00 AM. We can use the following crontab entry:

0 5 * * * /path/to/backup.sh

In this example, the crontab entry specifies that the command /path/to/backup.sh should be run every day at 5:00 AM.

When the cron daemon runs the crontab entry, it will execute the command /path/to/backup.sh at the specified time. The backup.sh script can perform any task, such as backing up files or sending email notifications.

Note that the crontab entry can be modified to run the command at different intervals, such as every hour or every week. For example, the following crontab entry would run the command every hour:

0 * * * * /path/to/command

This entry specifies that the command /path/to/command should be run every hour at the start of the hour (when the minute field is 0).

Common Issues

One common issue when using cron is that the PATH environment variable may not be set correctly. This can cause cron to be unable to find the commands specified in the crontab file.

To fix this issue, you can set the PATH environment variable in your crontab file. For example, you can add the following line to your crontab file to set the PATH variable to include the directories /usr/bin and /usr/local/bin:

PATH=/usr/bin:/usr/local/bin

This will ensure that cron can find the commands specified in your crontab file.

Another common issue is that the output of the command may not be redirected correctly. By default, the output of the command is sent to the email address of the user who created the crontab file. This can result in large amounts of email being sent to the user's inbox.

To fix this issue, you can redirect the output of the command to a file instead of sending it to the user's email address. For example, you can add the following line to your crontab file to redirect the output of the command to a file called output.log:

0 5 * * * /path/to/backup.sh > /path/to/output.log 2>&1

This will ensure that the output of the command is stored in the file output.log instead of being sent to the user's email address. The 2>&1 syntax is used to redirect both standard output and standard error to the same file.

Conclusion

In this section, we have discussed how to schedule jobs in bash using the cron utility. We have seen how to create a crontab file, and how to specify the intervals at which commands should be run. We have also discussed some common issues that can arise when using cron, and how to fix them.

With this knowledge, you should be able to schedule jobs in bash with confidence and ensure that your scripts are run at the correct intervals.

Summary

Examples

While Loop

i=1
while [ $i -le 5 ]
do
    echo $i
    i=$((i+1))
done

For Loop

for i in 1 2 3 4 5
do
    echo $i
done

Functions

check_exit_code () {
    if [ $? != 0 ]
    then
        echo "error"
    fi
}

Read input from the user

echo "What is your name?"
read name
echo "Hello, $name!"

Case Statement

case $choice in
    1)
        echo "Installing Ubuntu..."
        # Commands to install Ubuntu
        ;;
    2)
        echo "Installing Debian..."
        # Commands to install Debian
        ;;
    3)
        echo "Installing Fedora..."
        # Commands to install Fedora
        ;;
    4)
        echo "Installing CentOS..."
        # Commands to install CentOS
        ;;
    *)
        echo "Invalid choice!"
        ;;
esac

At command

echo "/path/to/backup.sh" | at 3am tomorrow

Cron utility

0 5 * * * /path/to/backup.sh