Mastering Linux Shell: Key Concepts, Commands, and Scripting Tricks

1. Why is it called a Shell?

Users cannot interact directly with the kernel; they use a "shell" as an interface that translates commands into kernel actions and returns results. The shell is essentially a command interpreter.

Translates user commands for the kernel.

Translates kernel results back to the user.

Different operating systems have different kernels, and multiple shells (sh, bash, csh, ksh, etc.) can run on the same kernel.

2. Shell Prompt (PS1) and Carriage Return (CR)

After logging into a shell, the prompt appears left of the cursor. Ordinary users see $, while administrators see #.

Shell Prompt: input is accepted until a CR (Carriage Return) character is read.

Carriage Return: triggers execution of the entered command.

Technically, the shell splits the input line using IFS, processes meta‑characters, and then reassembles the command.

3. The echo Command

echo

sends its arguments to standard output (stdout), typically the screen.

stdin – standard input stdout – standard output stderr – standard error

echo -n   # suppress trailing newline
 echo -e   # enable backslash escapes

4. Double Quotes vs. Single Quotes

''

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

5. var=value vs. export var=value

Variable definition: name=value (no spaces around =).

Variable reference: echo ${name}. export name=value makes the variable an environment variable, inheritable by child processes.

# Local variable
A=B
# Unset variable
unset A
# Export as environment variable
export A=B

6. exec vs. source

Environment variables flow only from parent to child processes. Executing a script creates a subshell; source runs the script in the current shell.

# Run script in a new subshell
./1.sh
# Run script in the current shell
source 1.sh
# Replace current shell with script
exec 1.sh

7. ( ) vs. { }

( )

groups commands in a subshell; { } groups commands in the current shell.

8. $(()) , $() , and ${} Differences

# Assume variable
file=/dir1/dir2/dir3/my.file.txt
# Non‑greedy left delete
${file#*/}   # dir1/dir2/dir3/my.file.txt
# Greedy left delete
${file##*/}  # my.file.txt
${file##*.}  # txt
# Non‑greedy right delete
${file%/*}   # /dir1/dir2/dir3
${file%.*}   # /dir1/dir2/dir3/my.file
# Greedy right delete
${file%%/*}  # (empty)
${file%%.*}  # /dir1/dir2/dir3/my
# Substring extraction
${file:0:5}   # /dir1
${file:5:5}   # /dir2
# Replace first occurrence
${file/dir/path}   # /path1/dir2/dir3/my.file.txt
# Replace all occurrences
${file//dir/path}  # /path1/path2/path3/my.file.txt
# Parameter defaults
${file-my.file.txt}   # if $file unset, use my.file.txt
${file:-my.file.txt}  # if $file unset or empty, use my.file.txt
${file+my.file.txt}   # if $file set (even empty), use my.file.txt
${file:+my.file.txt}  # if $file non‑empty, use my.file.txt
${file=my.file.txt}   # assign default if unset
${file:=my.file.txt}  # assign default if unset or empty
${file?my.file.txt}   # error if unset
${file:?my.file.txt}  # error if unset or empty

9. $@ vs. $*

"$@"

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

10. && vs. ||

The test command can be written as test expression or [ expression ]. Bash supports testing strings, integers, and files. It returns 0 for true, non‑zero for false. command1 && command2 runs command2 only if command1 succeeds (return 0). command1 || command2 runs command2 only if command1 fails (non‑zero).

A=123
[ -n "$A" ] && ([ "$A" -lt 100 ] || echo "too big")
unset A

11. Redirection: > vs. <

0: STDIN 1: STDOUT 2: STDERR

<

redirects input from a file; > redirects output to a file (stdout or 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
# Example of order: > file clears file before < file reads, causing empty input.
cat < file > file

12. if vs. case

# if example
echo -n "Do you want to continue?(Yes/No):"
read YN
if [ "$YN" = Y -o "$YN" = y -o "$YN" = Yes -o "$YN" = yes -o "$YN" = YES ]; then
    echo "continue"
else
    exit 0
fi
# case example
echo -n "Do you want to continue?(Yes/No):"
read YN
case "$YN" in
    [Yy]|[Yy][Ee][Ss]) echo "continue";;
    *) exit 0;;
esac

13. Loops: for , while , until

# 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.