Understanding Linux Shell: Basics, Scripts, and Common Commands

linux shell

The shell is an interface for users to interact with the kernel; the most widely used shell is Bash.

The shell is also an interpreted programming language, known as shell scripting.

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

Shell script execution methods

Run a 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 Prefix the script path with a dot to execute it in the current shell.

./helloworld.sh
. ./helloworld.sh

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

Variables in Linux

Categories

System variables

User‑defined variables

Common system variables include:

$PATH

$PWD

$USER

$SHELL

Display all current variables set The set command lists all variables.

Defining variables

Format: NAME=value Notes:

No spaces around the equals 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.

If the value contains whitespace, enclose it in single quotes.

To include special characters like $, use single quotes.

STR=helloworld
echo $STR

Unsetting a variable

unset STR
echo $STR

Defining a read‑only variable

readonly B=2
echo $B

Read‑only variables cannot be unset.

Using export makes a variable global for other shell processes.

Assigning command output to a variable A=`ls -al` or A=$(ls -al) Special shell variables $? – exit status of the previous command. $$ – current process ID. $0 – name of the current script. $n – nth positional parameter (n ≥ 1). $# – number of positional parameters, often used in loops. $* and $@ – list of all parameters; quoted forms preserve spacing differently.

Operators

Arithmetic format: expr m + n or $((m+n)).

expr 1 + 1
echo `expr 1 + 1`

Note: there must be spaces between numbers and operators.

echo $((1+1))

if syntax

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

if and fi must match; elif is used for else‑if.

Test expressions

First form

Syntax: [ condition ] Spaces are required around condition. Non‑empty strings evaluate to true; result 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.

Second form

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 loop

Syntax examples:

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 Second form using C‑style syntax:

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

or on one line:

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

while loop

First form:

while expression
do
  command
  …
 done

Second form with a counter:

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

case syntax

Format:

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; function keyword and parentheses cannot both be omitted.

Calling a function:

function_name arguments

function b() {
  echo $1
}
b a

A function must be defined before it is called because the shell interprets scripts line by line.

Function return values are accessed via $?. You can explicitly return a numeric value (0‑255); otherwise the exit status of the last command is used.