Master Shell Scripting: 25 Essential Q&A for Linux Administrators

Shell Scripting Basics – Q&A

Q: What is a Shell script and is it necessary?

A Shell script is a plain‑text file that contains one or more commands. System administrators often need to run several commands to complete a task, so they place those commands in a script to automate routine work.

Q: What is the default login shell and how can it be changed for a specific user?

The default login shell on most Linux systems is /bin/bash. It can be changed with the chsh command, e.g.:

# chsh <username> -s <new_default_shell>
# chsh linuxtechi -s /bin/sh

Q: What types of variables can be used in a Shell script?

Two kinds of variables are available:

System‑defined variables (usually uppercase, viewable with set).

User‑defined variables (created by the user; their values can be displayed with echo $<var_name>).

Q: How can standard output and error output be redirected to the same location?

Two common methods:

2>&1   # ls /usr/share/doc > out.txt 2>&1

&>    # ls /usr/share/doc > out.txt

Q: How is the if syntax nested in a Shell script?

Basic nested if example:

if [ Condition ]
then
    command1
    command2
    ...
else
    if [ condition ]
    then
        command1
        command2
        ...
    else
        command1
        command2
        ...
    fi
fi

Q: What is the purpose of the $? variable?

$?

holds the exit status of the previously executed command; 0 indicates success, any non‑zero value indicates failure. Example:

# ls /usr/bin/shar
/usr/bin/shar
# echo $?
0
# ls /usr/bin/share
ls: cannot access /usr/bin/share: No such file or directory
# echo $?
2

Q: How to compare two numbers in a Shell script?

Use test operators such as -gt:

#!/bin/bash
x=10
y=20
if [ $x -gt $y ]
then
    echo "x is greater than y"
else
    echo "y is greater than x"
fi

Q: What does the break command do?

break

exits the innermost loop (e.g., while or until).

Q: What does the continue command do?

continue

skips the remainder of the current loop iteration and proceeds with the next iteration.

Q: What is the syntax of a case statement?

Basic syntax:

case word in
value1)
    command1
    command2
    ...
    ;;
value2)
    command1
    command2
    ...
    ;;
esac

Q: What is the syntax of a while loop?

Basic syntax:

while [ test_condition ]
do
    commands…
done

Q: How to make a script executable?

Use chmod:

# chmod a+x myscript.sh

Q: What does #!/bin/bash mean?

It is the shebang line that tells the system to execute the script with /bin/bash.

Q: What is the syntax of a for loop?

Basic syntax:

for variable in list_of_items
do
    command1
    command2
    ...
    done

Q: How to debug a Shell script?

Run the script with the -x option ( sh -x myscript.sh) or with -nv for a dry‑run.

Q: How to compare strings in a Shell script?

Use the test command (e.g., [ "$str1" = "$str2" ]).

Q: What special variables exist in the Bourne shell (bash)?

Built‑in Variable

Explanation

$0

Script name

$1

First positional parameter

$2

Second positional parameter

…

…

$9

Ninth positional parameter

$#

Number of positional parameters

$*

All positional parameters as a single word

Q: How to test files in a Shell script?

Use the test command with various operators: Test Usage -d filename True if the file exists and is a directory -e filename True if the file exists -f filename True if the file exists and is a regular file -r filename True if the file exists and is readable -s filename True if the file exists and is not empty -w filename True if the file exists and is writable -x filename True if the file exists and is executable

Q: How to write comments in a Shell script?

Comments start with #. Example:

#!/bin/bash
# This is a comment
 echo "I am logged in as $USER"

Q: How to read input from the terminal?

Use the read command:

#!/bin/bash
echo "Please enter your name"
read name
echo "My Name is $name"

Q: How to unset a variable or its value?

Use unset <variable_name>.

# unset VAR_NAME

Q: How to perform arithmetic operations?

Two methods:

Using expr (e.g., expr 5 + 2).

Using the arithmetic expansion syntax $[ expression ] (e.g., result=$[16 + 4]).

Q: What is the basic format of a do‑while loop?

Example:

do
{
    statements
} while (condition)

Q: How to define a function in a Shell script?

Define a named code block and call it by name:

diskusage () { df -h ; }

Q: How to use bc (Bash calculator) inside a script?

Example:

variable=`echo "scale=2; 5/2" | bc`