2 Create Simple Shell Scripts

2.1 : Use scripting to automate system maintenance 
------------

bash is a command interpreter
Remove .sh to scripts if you want other programs to execute them

#!/bin/bash   - shebang on first line

sample.sh

#!/bin/bash

if test -f /tmp/archive.tar.gz;then
    mv /tmp/archive.tar.gz /tmp/archive.tar.gz.OLD
    tar acf /tmp/archive.tar.gz /etc/dnf
else
    tar acf /tmp/archive.tar.gz /etc/dnf
fi


--
2.2 : Conditional Logic
------------------------

if [ $rocket_status = "failed"]
then
    rocker_debug $mission_name
elif [ $rocket_Status = "success" ]
then
    echo "This is successful"
else
    echo "The state is not failed no success"
fi

Conditional Operators

[ STRING1 = STRING2 ]

= is only used to compare strings
[ "abc" = "abc" ]

!= is not equal
[ "abc" != "abc" ]

numbers equals
[ 5 -eq 5 ]
[ 5 -ne 5 ]
[ 5 -gt 5 ]
[ 5 -lt 5 ]

Enhanced patterns

[[ STRING1 = STRING2 ]]

[[ "abcd" = *bc* ]]
[[ "abc" = ab[cd] ]]
[[ "abe"= "ab[cd]" ]]
[[ "abc" > "bcd" ]]
[[ "abc" < "bcd" ]]

[COND1] && [COND2] - OR OPERATORS
[[ COND1 && COND2 ]]

[ COND1 ] || [ COND2]
[[ COND1 || COND2 ]]


[ -e FILE ] - if file exists
[ -d FILE ] - if directory exist
[ -s FILE ] - if file exist and size > 0
[ -x FILE ] - if the file is executable
[ -w FILE ] - if the file is writeable


2.3 : Processing shell exit codes
----------------------------------

EXIT STATUS = 0 --> SUCCESS
EXIT STATUS > 0 --> FAILURE

$? stores the exit status



2.4: Loops
-----------

for loop: 

for mission in <list of missions>
do
    create-and-launch-rocket lunar_mission
done  


for mission in 'cat mission.txt'
do
    create_and_launch_rocket $mission
done



while loop :

while [ $rocket_status = "launching" ]
do
    sleep 2
    rocket_status=rocket-status $mission_name

done



2.5: Process script inputs
--------------------------

$1, $2

#!/bin/bash
read -p "Enter a mission name:" mission_name

#!/bin/bash
mission_name=$1