15 Updated Shell Scripting Interview Questions and Answers: Freshers and Experienced

9. How Can You Check the Exit Status of a Command?

The exit status of a command can be checked with the help of $? variable. The exit status is measured in numbers. A zero exit status shows success whereas a non-zero exit status represents a potential error.

10. What is the difference between = and == in shell scripting?

In shell scripting, both = and == are used for string comparison, but there are nuances depending on the context.

• =: This operator is used to assign values to variables. For example:

 my_var=”Hello”

• ==: This is often used in conditional expressions within [[ ]] for string comparison, although it can also work with [ (test command) ]. For example:

if [[ $my_var == “Hello” ]]; then

    echo “Matched!”

fi

Note: Use = for assignment and == for comparison in conditional expressions. The double brackets [[ ]] provide more features and avoid issues with word splitting and pathname expansion.

11. What is the Use of trap in Shell Scripting?

In shell scripting, the trap command catches signals and acts on a command upon receipt of that signal. It is primarily used to clean up upon termination or interruption of a script.

Example:

trap ‘echo “Script interrupted”; exit’ SIGINT SIGTERM

while true; do

    echo “Running.”

    sleep 1

done

In the above example, the script will print “Script interrupted” and then exit if it is interrupted with a Ctrl+C by the user.

Note: Use trap to render your scripts robust at interrupt so cleanup of resources or saving of state may take place, as needed.

12. Explain the Process of Substitution. How is It Used?

Process substitution allows you to make a command think that its input comes from or its output goes to a file, whereas actually, it is the output produced by another command. 

Syntax: command <(other_command) 

Example:

diff <(ls dir1) <(ls dir2) 

In the above example, diff compares the results of listing two different directories without using temporary files.

Note: Process substitution can simplify scripts by avoiding the use of an intermediate file and also will optimize performance because it avoids I/O.

13. What is Meant by here Documents? How Can You Use Them?

here documents which are also called Heredocs are a special kind of quoting that allows you to write multiple-line strings directly in your script or command. It is especially useful when you want to pass several lines of text or even commands to utilities like cat, sed, or awk.

Syntax:

command <<EOF

line 1

line 2

EOF

Example:

cat <<EOF > myfile.txt

This is line 1.

This is line 2.

EOF

In the example above, cat creates a file called myfile.txt containing two lines of text.

14. What is Meant by a Crontab? How Many Fields are Present in a Crontab File?

A crontab or cron table is a configuration file that outlines the schedule that the cron daemon should execute and indicates the job to be executed in Unix-like operating systems. It allows for running scripts or commands at given periods-momentarily, daily, weekly, or monthly.

An ordinary crontab file will contain five time and date fields and finally the command to be executed. The fields are as follows:

1. Minute (0-59)
2. Hour (0-23)
3. Day of Month (1-31)
4. Month (1-12)
5. Day of Week (0-7) (Sunday is both 0 and 7)

After the above five fields, you specify the command you want to run.

Note: There are mainly five fields in a crontab file. However, there is also a sixth field that contains the command that needs to be executed.

15. How can you use a shell script to monitor disk space usage and send an alert if the usage exceeds a specified threshold? Provide a detailed explanation of the script components.

To write a shell script that monitors disk usage and sends an alert if the usage is greater than a predefined threshold, follow these steps:

1. Define the Threshold

Define the threshold at which your disk space usage should trigger an alert. You can define this in percentage.

THRESHOLD=90

2. Get Disk Usage Information

To check disk usage, use df. In df, -h is the option for showing output in human-readable format, whereas –output gives specified columns.

USAGE=$(df / | grep / | awk ‘{ print $5 }’ | sed ‘s/%//g’)

– df /: The command to check usage of the root directory.

– grep /: This will filter out the output to get the line containing the root filesystem.

– awk ‘{ print $5 }’: Prints the fifth column, which is the percentage used.

– sed ‘s/%//g’: Removes the percentage sign so that numerical comparison may be done.

3. Compare Usage to Threshold

Check if the disk usage in the current reading is greater than or equal to the threshold set.

if [ “$USAGE” -ge “$THRESHOLD” ]; then

4. Sending Alert

If it is greater than the threshold, send an alert. This may be simply an email notification, or to some logging system.

Sending email via mail:

echo “Disk space usage is critically high: ${USAGE}% used.” | mail -s “Disk Space Alert” admin@example.com

5. Full Script

Herein is how you can piece it together in a shell script:

#!/bin/bash

# Set threshold for disk usage

THRESHOLD=90

# Get present disk usage in percentage

USAGE=$(df / | grep / | awk ‘{ print $5 }’ | sed ‘s/%//g’)

# Check if the current usage is greater than the threshold

if [ “$USAGE” -ge “$THRESHOLD” ]; then

    # Send an alert email.

    echo “Disk space usage is critically high: ${USAGE}% used.” | mail -s “Disk Space Alert” admin@example.com

fi

Explanation of Components:

• Shebang (#!/bin/bash): This line ensures this script runs under the Bash shell.
• Variables: The Script uses two variables, one named THRESHOLD to store the threshold value of disk usage in percentage, and another one named USAGE to store the current usage.
• Command Substitution: We have used command substitution, enabled by the use of the dollar-sign/parentheses notation: $(). This is very handy to capture the output of commands directly into variables.
• Conditional Statement: The conditional statement will check whether the current usage is greater than or equal to the threshold.

Alert Mechanism: The script herein uses the mail command, meaning a mail transfer agent should be installed on the server.

Deployment

Give execute permissions to your script by running the following in your terminal:

“`bash

chmod +x disk_monitor.sh

“`

Then you may want to schedule a cron job to execute this script daily – or at regular periods of your choice:

“`bash

0 0 * * * /path/to/disk_monitor.sh

“`

This will serve to provide you with timely alerts when disk usage starts creating problems, so you may manage the system resources in time.