Master the Classic Shell Interview Questions: 13 Essential Concepts Explained

1. Why is it called "shell"?

A shell is the user‑level interface that sits on top of the operating system kernel, translating user commands into kernel actions. It acts as a command interpreter, allowing users to interact with the system via a command line.

Translates user commands to the kernel.

Translates kernel responses back to the user.

2. How does the Shell prompt (PS1) relate to Carriage Return (CR)?

After logging into a shell, the cursor left of the prompt indicates where you can type commands. The prompt (e.g., $ for normal users, # for root) appears before a Carriage Return character, which signals the end of the command line and triggers execution.

Shell prompt: ready for input until a CR is received.

Carriage Return: indicates the command can be executed.

3. What does echo actually do?

echo sends its arguments to standard output (stdout), typically displaying them on the screen.

stdin – standard input

stdout – standard output

stderr – standard error

echo -n   # suppress newline
echo -e   # enable backslash escape

4. Difference between double quotes (") and single quotes (')

''

(single quotes) – hard quoting; disables all variable and command expansion. "" (double quotes) – soft quoting; allows variable expansion ( $var) while preserving spaces.

5. What changes when you add export to a variable assignment?

Assigning name=value creates a shell variable. Adding export ( export name=value ) promotes it to an environment variable, making it visible to child processes.

# Local variable
A=B
unset A
# Environment variable
export A=B

6. How do exec and source differ?

Environment variables flow only from parent to child. source runs a script in the current shell, allowing it to modify the current environment. exec replaces the current shell process with the script, so changes do not affect the original shell.

# Run script in a subshell
./script.sh
# Run script in the current shell
source script.sh
# Replace current shell with script
exec script.sh

7. Difference between ( ) and { } ?

( )

creates a subshell to execute a command group, isolating variable changes. { } groups commands in the current shell, so any variable modifications persist.

8. How do $() , backticks, and ${} differ?

$()

and backticks perform command substitution. ${var} and $var perform variable substitution.

# Example variable
file=/dir1/dir2/dir3/my.file.txt
# Remove leading path components
${file#*/}   # dir1/dir2/dir3/my.file.txt
${file##*/}  # my.file.txt
# Remove file extension
${file%.*}  # /dir1/dir2/dir3/my.file

9. Difference between "$@" and "$*" ?

"$@"

expands to separate quoted arguments (e.g., "p1" "p2 p3" "p4"). "$*" expands to a single quoted string containing all arguments (e.g., "p1 p2 p3 p4").

10. How do && and || differ?

cmd1 && cmd2

executes cmd2 only if cmd1 succeeds (returns 0). cmd1 || cmd2 executes cmd2 only if cmd1 fails (non‑zero return).

11. What do > and < do?

< redirects a file to standard input. > redirects standard output (or standard error with 2> ) to a file.

# Redirect stdout and stderr
ls my.file no.such.file 1>file.out 2>file.err
# Merge stderr into stdout
ls my.file no.such.file 1>file.out 2>&1
# Discard output
ls my.file no.such.file > /dev/null 2>&1

12. When to use if versus case ?

# if example
read YN
if [ "$YN" = Y ] || [ "$YN" = y ] || [ "$YN" = Yes ]; then
    echo "continue"
else
    exit 0
fi

# case example
read YN
case "$YN" in
    [Yy]|[Yy][Ee][Ss]) echo "continue";;
    *) exit 0;;
esac

13. Differences among for , while , and until loops

# for loop
for ((i=1;i<=10;i++)); do
    echo "num is $i"
done

# while loop
num=1
while [ "$num" -le 10 ]; do
    echo "num is $num"
    num=$((num+1))
 done

# until loop
num=1
until [ "$num" -gt 10 ]; do
    echo "num is $num"
    num=$((num+1))
 done

break

exits a loop. return exits a function. exit terminates the script or shell.