Tracing Shell Scripts

To locate problems in your script, the shell provides two options:

-v

and

–x .

The –x and –v options locate many, but not all, errors in a script

When it comes to writing shell scripts, testing each line is crucial to ensure that the script runs smoothly without any errors. To test each line of a shell script, there are several shell options that you can use. In this article, I will discuss the most commonly used shell options for testing each line of a shell script.
Shell Options for Testing Each Line of a Shell Script

1. -n Option
2. -v Option
3. -x Option
4. -e Option
5. -u Option
6. -f Option
7. -s Option
8. -t Option
9. -z Option
10. -a Option
11. -r Option
12. -w Option
13. -x Option
14. -o Option

-n Option: The -n option is used to check if a variable or string is not empty. It returns true if the string is not empty and false if it is empty. Here's an example:

if [ -n "$var" ]; then
    echo "Variable is not empty"
fi

-v Option: The -v option is used to print each line of the script as it is executed. This is useful for debugging and troubleshooting purposes. Here's an example:

set -v
echo "This line will be printed"

-x Option: The -x option is similar to the -v option, but it also prints the value of variables as they are expanded. This is useful for debugging complex scripts. Here's an example:

set -x
var="Hello"
echo "The value of var is: $var"

-e Option: The -e option is used to exit the script if a command fails. This is useful to ensure that the script does not continue running if an error occurs. Here's an example:

set -e
ls /nonexistent_directory
echo "This line will not be executed"

-u Option: The -u option is used to exit the script if an undefined variable is used. This is useful to prevent errors caused by uninitialized variables. Here's an example:

set -u
echo "$undefined_variable"

-f Option: The -f option is used to check if a file exists and is a regular file. This is useful for checking if a file is present before performing operations on it. Here's an example:

if [ -f /path/to/file ]; then
    echo "File exists"
fi

-s Option: The -s option is used to check if a file exists and is not empty. This is useful for checking if a file contains any data before performing operations on it. Here's an example:

if [ -s /path/to/file ]; then
    echo "File is not empty"
fi

-t Option: The -t option is used to check if a file descriptor is associated with a terminal. This is useful for checking if a script is running in an interactive terminal or not. Here's an example:

if [ -t 0 ]; then
    echo "Script is running in an interactive terminal"
fi

-z Option: The -z option is used to check if a variable or string is empty. It returns true if the string is empty and false if it is not empty. Here's an example:

if [

Both options display the lines of your script before they are run, but use different techniques. Looking at the instructions as they are run is called tracing the script.

You may feel confident about part of your script but uncertain about another section. In this case, you would trace only the portion of your code that you suspect is causing a problem. You can turn on the tracing feature inside your script with the set command. Using set v inside your script prints each line before it is run. Use set +v to turn off this feature. You can do the same thing with set x and set +x to show lines of code with their variables translated.
In the following script set x and set v are combined into one statement to turn on both options in the middle of the script: #!

/bin/sh 
echo "Please enter name of new user: " 
read filename 

set xv sort $filename | cat n > tempfile cp tempfile $filename set +xv
echo "

Click the Exercise link below to practice tracing a shell script. Tracing Shell Scripts - Exercise
The next lesson examines errors commonly seen when using the shell’s if statement.