Up and Running with Bash Scripting - 4

Working with variables:
1.Variables are named with alphanumeric characters
2.Names must start with a letter
3.To declare a variable, type its name directly followed with an equals sign, directly followed with a value.Note that there's no space, if you put spaces around the equals sign, you'll get an error when you run the script.
4. If your variable is a string with spaces in it, such a good morning , you'll need to out that string in quotes in the variable declaration.
5. To use these variables, we call them with a dollar sign in front of their name.


Example script:
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#!/bin/bash
# This is a basic bash script
a=Hello
b="Good Morning"
c=16

echo $a
echo $b
echo $c

#These variables also work inside of other variables or strings if they're double quoted.

echo "$b! I have $c apples."

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Adding attributes to variables
Declare i in front of the variable marks that variable as an integer.
example: declare -i d=123   #d is an integer

Declare r before a variable marks it as read-only. That means that it can't be modified later arithmetically or with stream manipulation.

example: declare -r e=456  #e is read-only


some built in variables to be aware of:

a. $HOME
b. $PWD
c. $MACHTYPE
d. $HOSTNAME
e. $SECONDS
f. $0 returns the name of the script

There are bunch of variables that can be handy , refer to tldp.org/LDP/abs/html/internalvariables.html

Command Substitution
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Command substitution can be used to get information back from a command right in the bash script, instead of just running them.

It can be accomplished using a dollar sign with parentheses around a command.

example script:
--
#!/bin/bash
# This is a basic bash script
a=$(pwd)
echo $a


So that's a pretty basic example, but let's take a look at another one. At some point in your script writing you'll want to use the value of some complex command. As an example, let's take a look at finding out how long it takes to get a response from a server. I'm setting the variable a to the result of this command line statement. So if I run this command, I see the result. But now, it's available as a variable within my script that I can use for other operations.

example script:
--
#!/bin/bash
# This is a basic bash script
a=$(ping -c 1 example.com | grep 'bytes from' | cut -d = -f 4)
echo "The ping was $a"