Shell Scripting Basics.txt

Shell Scripting Basics


bash_profile vs bashrc
------------------------------


Checking Your Environment
-----------------------------------
env - prints all exported (globally available) variables
set - prints all locally scoped variables


simple-script.sh

# creates a subshell which is separate from the parent shell from which it was launched
#!/bin/bash 

# built-in shell util that says breakout of the shell when a failure/error is raised (exit status not 0)
set -e 

echo 'Hello World' // executes this command and exits


Exit Status
---------------
0 - successful
1 - failure/error

> ls
> echo $? // displays last exit status


Escaping Characters
------------------------------


Using /dev/null
---------------------
- generally the blackhole of a system
- when any file/output is sent here it is gone forever

ls -al >> /dev/null 
# no output will be shown


Reading Input
--------------------

echo "Enter your First Name: "
read FIRSTNAME

echo " Your first name is: $FIRSTNAME"


Accepting Arguments
---------------------------
- arguments are positional by nature
- you can take each argument (in the order they were passed in) and assign them to variables

#!/bin/bash 

USERNAME=$1 // argument 1 
PASSWORD=$2 // argument 2

echo "The following username is $USERNAME and $PASSWORD

$ args.sh Lowell Password1
The following Username is Lowell and Password is Password1


Flow Control 
-----------------

echo "Guess a number between 1 and 5: "
read GUESS

if [ $GUESS -eq 3 ]
  then 
      echo "You Guessed the Correct Number!"
fi 

========

FILENAME=$1
echo "Testing for the existence of a file called $FILENAME"

# -a, -e, -f all check for existence (true or false) of something
if [ -e $FILENAME ]
  then
       echo "File $FILENAME does exist!"