Shell Script

\#linux

To create a shell script, use the following steps:

Shebang

#!/bin/bash
hi="Hello World"
echo $hi
read -p "Please enter a number" var
evho $var

Defining and accessing local variable

a="hi"
echo $a
#or
echo ${a}

Accessing environment variable

echo $PATH
#or
set
#or
env
#or
export -p
#or
declare -x
#or
typeset -x

Defining environment variable

EW=1
export EW
#or
export EX=2
#or
declare -x EY=3
#or
typeset -x EZ=4

Deleting environment variable

unset EW

Special Variables

$0, $1, ..., $9, ${10}, ${11}, ... are the arguments to the script
$# means number of command line variables
$? is the exit code of the last command to be run
- 0 means command executed successfully
- other number means false

if expression1
then
    statement
elif expression2
then
    statement
else
    statement
fi

We can use test command to create the boolean expression The test command can also be written using two square brackets: []

Operator	Description	Example
`-z`	True iff length of string is zero	`[ -z "$string" ]`
`-n`	True iff length of string is not zero	`[ -n "$string" ]`
`==`	True iff strings are equal	`[ "$string1" == "$string2" ]`
`!=`	True iff strings are not equal	`[ "$string1" != "$string2" ]`
`\>`	True iff `$string1` is sorted after `$string2`	`[ "$string1" \> "$string2" ]`
`\<`	True iff `$string1` is sorted before `$string2`	`[ "$string1" \< "$string2" ]`

Notice that string are enclosed with double quote so that comparison can work even if there are space inside string2

Operator	Description	Example
`-e`	True iff file exists	`[ -e $file ]`
`-s`	True iff file size > 0	`[ -s $file ]`
`-d`	True iff file is a directory	`[ -d $file ]`
`-f`	True iff file is a plain file	`[ -f $file ]`
`-r`	True iff file has read permission for current user	`[ -r $file ]`
`-w`	True iff file has write permission for current user	`[ -w $file ]`
`-x`	True iff file has execute permission for current user	`[ -x $file ]`

Operator	Description	Example
`-eq`	True iff integers are equal	`[ $int1 -eq $int2 ]`
`-ne`	True iff integers are not equal	`[ $int1 -ne $int2 ]`
`-gt`	True iff first integer is greater than second integer	`[ $int1 -gt $int2 ]`
`-ge`	True iff first integer is greater than or equal to second integer	`[ $int1 -ge $int2 ]`
`-lt`	True iff first integer is less than second integer	`[ $int1 -lt $int2 ]`
`-le`	True iff first integer is less than or equal to second integer	`[ $int1 -le $int2 ]`

case var in
    reg-exp)
        statement
        ;;
    reg-exp)
        statement
        ;;
    *)
        statement
esac

while expression
do
    statement
done

for var in list_of_values
do
    statement
done

for (( <for-initialization> ; <bool-exp> ; <post-processing> ))
do
    statement
done

We can use seq statement to create the iterable list
- seq <start_value> <increment_value> <end_value>
We can use { .. .. } statement to create the iterable list
- {<start_value>..<end_value>..<increment_value>}

e.g.

for var in `seq 0 10 100` # from 0 to 100 increase 10 each time
do
    statement
done

e.g.

for var in {1..100..10} # from 0 to 100 increase 10 each time
do
    statement
done

e.g.

for (( i=0 ; i<5 ; i++ )) # from 0 to 5 increase 1 each time
do
    statement
done

create function

function function_name() {
    # accessing arugment by $1, ..., $9, ${10}, ${11}, ...
    statement
}

calling function

function_name <arugment1> <arugment2> ...

Last modified: 12 June 2025