Mastering Linux Shell: Basics, Scripts, Variables, and Control Structures

linux shell

Shell is the interface for users to interact with the kernel; the most popular shell is Bash.

Shell is also an interpreted programming language, i.e., shell scripts.

A system can have multiple shells; list installed shells with cat /etc/shells . Different shells may support different command syntax.

shell script execution methods

Run the script by its absolute or relative path after granting execute permission.

./helloworld.sh
/tmp/helloworld.sh

If the script lacks execute permission, run it with sh . sh helloworld.sh Prepend a dot to the script path.

./helloworld.sh
. ./helloworld.sh

The third method runs in the current shell, whereas the first two run in a subshell.

linux variables

Classification

System variables

User-defined variables

Common system variables:

$PATH

$PWD

$USER

$SHELL

Display all variables set The set command shows all variables in the system.

Define variables

Format: NAME=VALUE Notes

No spaces around the equal sign.

Variable names are conventionally uppercase.

Double quotes preserve spaces; single quotes preserve all special characters.

Variable names are case‑sensitive.

Names cannot start with a digit.

Enclose whitespace in single quotes.

Enclose special characters such as $ in single quotes.

STR=helloworld
echo $STR

Unset a variable

unset STR
echo $STR

Define a read‑only variable

readonly B=2
echo $B

Read‑only variables cannot be unset.

Use export VAR to promote a variable to the environment for other shell processes.

Assign command output to a variable A=`ls -al` or A=$(ls -al) Special shell variables $? – exit status of the previous command $$ – PID of the current shell $0 – name of the current script $n – nth positional parameter (n ≥ 1) $# – number of positional parameters $* and $@ – all positional parameters; behavior differs when quoted

Operators

Format: expr m + n or

$((m+n))

expr 1 + 1
echo `expr 1 + 1`

Spaces are required between numbers and operators.

echo $((1+1))

If syntax

if condition
then
    statements
[elif condition
    then statements ...]
[else statements]
fi

if and fi must appear as a pair; elif is the else‑if form.

Test statements

Form 1

Syntax: [ condition ] Spaces are required around condition. A non‑empty result returns true; the exit status can be checked with $? (0 = true, >0 = false).

[ condition ] && echo OK || echo notok

&&

returns true only if both sides are true; || returns true if either side is true.

Form 2

Syntax:

test condition

Common test operators

=

– string equality -lt – less than -le – less than or equal -eq – equal -gt – greater than -ge – greater than or equal -ne – not equal -r – readable -w – writable -x – executable -f – regular file exists -s – file exists and is not empty -d – directory exists -b – block device exists -L – symbolic link exists

for loops

Two styles:

for N in 1 2 3
do
  echo $N
done

or for N in 1 2 3; do echo $N; done or

for N in {1..2}; do echo $N; done

for ((i=0;i<5;i++))
do
  echo $i
done

or

for ((i=0;i<5;i++)); do echo $i; done

while loops

Form 1:

while expression
do
  command
  …
 done

Form 2:

i=1
while ((i<=3))
do
  echo $i
  let i++
done

case syntax

case $1 in
start)
  echo "starting"
  ;;
stop)
  echo "stoping"
  ;;
*)
  echo "Usage: {start|stop}"
esac

;;

works like break in other languages; * is the default case.

Shell custom functions

Syntax:

[ function ] funname [()]
{
    action;
    [return int;]
}

Square brackets indicate optional parts; the function keyword and parentheses cannot both be omitted.

Calling a function:

function_name arguments

function b() {
  echo $1
}
b a

Functions must be defined before they are called because the shell interprets line by line.

Function return values are obtained via $?. An explicit return n (0‑255) can be used; otherwise the exit status of the last command is used.