shell-scripting/shell-scripting.odt at master · nishijayaraj/shell-scripting · GitHub

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Running a file
================
using sh command
1. We can directly run the file
 sh example.sh
 sh example

2. using ./

  * For that we need to be in the directory where the file resides.
  * set executable permisiion using
    chmod +x file name.
  * File can be run as follows,
     ./example.sh
     ./basic


 Exit Status
=============
Whenever a command is run its success status can be determined using its exit status value
 0 -------> Success
 1 -------> Failure

eg : mv demo.txt test.txt
     echo $?
     Display  0 if the file is already there
     Otherwise iyt would display 1

Variables
===============

1. System Varibales or Environment variables
   Not supposed to change without any valid reason
   printenv ---This command will display all Env variables

2. USer Defined Variables

3. Multi line comments
    <<"COMMENT"
    This long comment text includes ${parameter:=expansion}
    `command substitution` and $((arithmetic++ + --expansion)).
    COMMENT

4. Expressions

   s=`expr 1 + 3`
   echo $s

(  Back quotes NOT single quotes, and also spaces on either sides of + is important)
But the preferable way to do this :

t=$(( 1+3 ))         /* here spaces are not at all important , if we want we can give them* /
echo $t

5 Quotes


6. If -else
if [ expression ]
then
   Statement(s) to be executed if expression is true
else
   Statement(s) to be executed if expression is not true
fi


The indentation is very important.

The semicolon is needed only when the end of line is missing:
if [ "a" == "a" ] ; then echo "true" ; fi
Without semicolons, you get Syntax error.


7. Multi level if-else

if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to elif statement
           elif condition1
           then
                       condition1 is zero (true - 0)
                       execute all commands up to elif statement
           elif condition2
           then
                       condition2 is zero (true - 0)
                       execute all commands up to elif statement
           else
                       None of the above condtion,condtion1,condtion2 are true (i.e.
                       all of the above nonzero or false)
                       execute all commands up to fi
           fi