Scripting Classroomnotes 05/Feb/2023

Using Multiple Commands

• If you want to run multiple commands together you can run them on same prompt using semi-colon and also using ampersand (&&)
  
  

Displaying Messages

• The echo command displays a simple text string
  
  
• Refer Here for gnu documentation

Command Substitution

• Commands can be substitute using the following

$()
``

Output redirection

• The basic form of output redirection is greater than >

command > outputfile 

* the >> is also used for output redirection in this if the file exists then the contents are appended and if the file doesnot exist it will create a new file and add/append contents

Input redirection

• The basic form of input redirection is less than <

command < outputfile 

* wc command provides a count of text in the data. It produces three values
* The number of lines
* The number of words
* The number of bytes

Employing Pipes

• Lets find the word count for all the packages installed on ubuntu
  
• Linux has support of pipes where you can pass output of one command as input to other

command1 | command2

Performing Math

• The expr command
  

Operator	Description
ARG1 | ARG2	Return ARG1 if neither argument is null or 0, otherwise Return ARG2
ARG1 & ARG2	Return ARG1 if neither argument is null or 0, Otherwise Return 0
ARG1 < ARG2	Return 1 if ARG1 is less than ARG2
ARG1 <= ARG2	Return 1 if ARG1 is less than or equal to ARG2
ARG1 > ARG2	Return 1 if ARG1 is greater than ARG2
ARG1 >= ARG2	Return 1 if ARG1 is greater than or equal to ARG2
ARG1 + ARG2	Return the sum of ARG1 and ARG2
ARG1 - ARG2	Return the difference of ARG1 and ARG2
ARG1 / ARG2	Return the quotient of ARG1 and ARG2
ARG1 % ARG2	Return the remainder of ARG1 and ARG2
ARG * ARG2	Returns the multipe of ARG1 and ARG2

• Lets write a script to calculate simple interst in Bash
  

Exit Status

• Every command that runs in linux shell uses an exit status to indicate to the shell it has done processing. This value will be a number between 0 to 255.
  
• Refer Herefor the url to view status codes

If-then command

• Basic syntx

if command
then
   commands
fi

• Conditonal expressions of bash Refer Here
• Figure out conditional expressions for
  • equals of text and numbers
  • less than or greater of text and numbers
  • file exists
  • folder exists
• If the variable is not defined assign a default value

#!/bin/bash
DEFAULT_DIST="ubuntu"
if [ -z $DIST ]; then
    echo "DIST has no value"
    DIST=$DEFAULT_DIST
fi
echo $DIST

• If directory doesnot exists create a directory

#!/bin/bash
# if folder does not exists then create
FOLDER='/tmp/dummy'
if [ ! -d "$FOLDER" ]; then
    echo "$FOLDER does not exists so creating"
    mkdir -p $FOLDER
else 
    echo "$FOLDER exists"
fi

Exercise

• Explain what i have done over here . /etc/os-release && echo $ID