105-2-Customize-or-write-simple-scripts

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105.2 Customize or write simple scripts
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Description

Candidates should be able to customize existing scripts, or write simple new Bash scripts.

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Key Knowledge Areas:
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    & Use standard sh syntax (loops, tests)
    & Use command substitution
    & Test return values for success or failure or other information provided by a command
    & Perform conditional mailing to the superuser
    & Correctly select the script interpreter through the shebang (#!) line
    & Manage the location, ownership, execution and suid-rights of scripts

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Terms and Utilities
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    & for
    & while
    & test
    & if
    & read
    & seq
    & exec

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Shell Scripts
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Are a way of automating tasks.

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Shebang
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If a line starts with #! it is called shebang and tells the shell what interpreter to use for running this script.

Note: Normally a # at the beginning of a script is for showing comments. Do not confuse it with Shebang (#!)

In many cases we run shells with #!/bin/bash or #!/bin/sh
We can use the command we already know in our shell scripts. A sample is:

    #!/bin/bash

    echo 
    echo "We are learning! Wowww..."
    echo

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Variables
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Already seen in some parts. You can define variables like this VARNAME=VALUE. A sample:

    #!/bin/bash

    NAME=mojtaba

    echo 
    echo "$NAME is learning! Wowww..."
    echo

Note: you can also do NAME="mojtaba"

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Command substitution
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Sometimes you need to have a variable with the result of something to a variable. In this case you can use $() construct:

    FILES=$(ls -1)

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executing scripts
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If the file is executable, we can run them using the ./script_name.sh if we are in the same directory, or give the complete path or include their directory ine $PATH variable. As you can see they are just normal programs.

Another way is giving our scripts name as a parameter to the bash or sh commands.

Note: you know that for making a file executable we can do chmod 755 filename or chmod +x fiename.

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Conditions
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Up to now, we were just running commands one by one. That is not very programmatic. If we are going to have some logic in our programs, we need conditions and loops. First we will cover conditions, using the if command. Its usage is like this:

    if [condition]
    then
       do something 
       do another thing
    else
       do new things
       even funnier things
    fi

Note: else part is optional, if, then, fi is enough.

Conditions can be TRUE or FALSE. A very simple conditions is if [ "Linux" = "Linux" ]. Silly? I know but we will change it soon but for now, learn the syntax! Specially the spaces and = for checking if two strings are equal.

    #!/bin/bash

    kernel=$(uname -s)
    if [ $kernel = "Linux" ]
    then
        echo YES. You are using a Linux
    else
        echo "Not a linux :("
    fi
Note spaces and using doublequotes (") on seccond echo because it has ( character which will be interpreted by bash if we do not enclose the string in a doublequote.
The actual checking of the condition is done by test command which is writter as [ some test ]. There are the other options:

    conditions	what is means
    "a" = "b"	if two strings are equal (here it will return False)
    "a" != "b"	string a is not equal to string b
    4 -lt 40	if 4 is lower than 40 (True)
    5 -gt 15	if 5 is greater than 15 (False)
    5 -ge 5	if 5 is greater or equal 5
    5 -le 3	if 5 is lower or equal to 3
    9 -ne 2	9 is not equal with 2 (True)
    -f FILENAME	if file FILENAME exists
    -s FILENAME	if file exists and its size is more than 0
    -x FILENAME	file exists and is executable

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read
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Using read we can read the user input. Look at this:

    1 #!/bin/sh
      2 
      3 echo "what is your name?"
      4 read NAME
      5 
      6 echo "Hello $NAME"
      7 
      8 if [ $NAME = "mojtaba" ]
      9 then
     10      echo "Oh I know you!"
     11  else
     12      echo "I wish I knew you"
         13 fi
     14 
     15 echo "Bye"

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for loop
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Generaly loops are used to run a specific set of commands more than once. The syntax is like this:

    for VAR in SOME_LIST;
    do
      some stuff with $VAR
      some other stuff
    done

Note the in, ;, do and done.

On each loop, the VAR will be equal to one of the SOME_LIST elements. SOME_LIST can be numbers, name of files, words, ...

    for NUM in 1 2 3 4 5 6;
    do
        echo $NUM
    done

But what if you needed to go 1 to 42? We have the seq command which can be used like seq 1 42 or a shorthand like {1..42}.

Good part is we can use non-numbers too!

    for FILE in $(ls);
    do
        echo $FILE
        wc $FILE
    done

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while loop
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This is another kind of loop but loops while a conditions is TRUE. This is the syntax:

    while [condition]
    do
        do something
        do anohter thing
    done

Note: If your condition will remains true all the time, the while loop will run forever. This is called an infinite loop

This is sample:

    VAR=52

    while [ $VAR -gt 42 ]
    do
        echo VAR is $VAR and it is still greater than 42
        let VAR=VAR-1
    done

we will have an infinite loop if we use let VAR=VAR+1. Ctrl+C will help us to break the loop.

Note the let usage! If you just just say VAR=1 and then VAR=$VAR+1, then VAR will be equal to 1+1 as an string!.

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mailing the root user
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For sending mail, you need to install mailutils. Then the mail command will send emails. You can send the mail to the root user by issuing this command:

    mojtaba@funlife:~$ mail root
    Cc: 
    Subject: Hi there root
    hello there. This is my mail

And root will get this email. She can read it using mail command.

If you need to send emails in a script, just do:

    $ echo "Body!" | mail -s "Subject" root

This can be easily embeded as poart of your scripts!