Process Lab: Processes and Variables

NAME:____________________________________________________________________

User-defined variables are local to the shell or process in which they are set.

Processes cannot access or change variables in the environment of another process.
Every process has an operating environment inherited from its parent process.
User-defined variables are not by default inherited from parent to child
To make variables available to any child process, you export them.

Syntax:

Set the value of the variable, then export:

$ var=value

$ export var

If you don't give export an argument, you will see the variables that are already exported.
$ export
How many lines of output did the export command produce? __________
What command did you use to count them? _________________________

Type each of the commands in the table into the command line
The right column describes what you are doing

`$ echo $$`	This shows the PID of current process
`$ x=4`	Set the valueof x to 4
`$ export x`	Export the value of x to subsequent children
`$ bash`	Create a subshell
`$ echo $$`	Display the PID of current process

What is the process ID (PID) of your subshell?__________
What command did you type to find it? ________________________

Use the echo command to find the value of x.

$ echo $x

What is the value of x? ___________

Change the value of x to 400 in subshell.

What command do you use to assign 400 to the variable x? ______________________________

What command do you use to check the value of your x variable? _________________________

Exit the subshell by typing either exit or <Ctrl>d.

What command do you use to check the shell's process ID?______________________________

What command do you use to check the current value of x?____________________

Did the parent process inherit the new value of x?___________________________

Why or why not?________________________________________________________________

___________________________________________________________________

Shell Scripts

A shell script is a collection of system commands stored in a text file.
The shell reads and executes them in sequence.
You can execute any Linux command in a shell script.
You can use any text editor to create a shell script.

Creating a Simple Shell Script

Create a simple shell script which returns the date. This can be done without an editor by using the cat command and redirecting output to a file.

$ cat > datescript
date
<Ctrl>d

Invoking Shell Scripts

There are several ways to execute a script.

===> 1. Use a shell to run the command datescript

$ bash datescript

Tue Oct 16 17:35:07 PDT 2001

Here you have invoked a subshell, bash, that runs only one command, datescript
The file datescript must be readable but does not have to be executable

The shell needs to be able to open the script file to read the commands within in

===> 2. Make the shell script executable and run it directly

The file must be readable and executable in order to execute it directly.

$ chmod 755 datescript
The permission characters would look like this:
-rwxr-xr-x
/ | \
owner group other
Users must have both read and execute permission on the script file to invoke it like this:
$ datescript
bash: datescript: command not found
Why do you get the "command not found" error message?
__________________________________________________________________________
What is the definition of your current path?
$ echo $PATH
__________________________________________________________________________

Is the current directory in your path? __________________________
$ pwd
Preceed your command with a ./ to tell bash to look in the current directory.
$ ./datescript
Tue Oct 16 17:55:23 PDT 2001

===> 3. Use the dot (.) or source Command

By useing the source command, you will execute in the current shell not a subshell
The dot or source command allows you to retain changes in the current shell
You only need read permissions to the shell script using this method of execution

Typically the source (.) command is used after modifying .profile or .bashrc file

Test this theory by creating a datescript that contains an assignment variable.

Below we assigning the value "my birthday" to the variable today.

Create a file that looks like this one. Here I am using the cat command to show you what the file should look like. (The cat command should not be part of your file!)

$ cat datescript

	date
	today="my birthday"
	echo $today

What command do you use to make the script executable?_____________________________

Execute the script the normal way and notice the output:
$ ./datescript
What is the value of the today variable? _____________________________________

Check the value of today in your current shell. Remember it was set in the script.
$ echo $today
Does the current shell have a value defined for today?____________________
Why or why not? _____________________________________________________
___________________________________________________________________

Now source the script. There is a space after the period character below.

Look at the output of the script and then check the value of today in your current shell.

$ . datescript

$ echo $today

Can you explain what has happened?_____________________________________

__________________________________________________________________

Return Codes from Commands

After a command has completed (or attempted to complete) it sends a return code (or exit code) to the parent process.

The return code is stored in the parent process built in '?' environment variabe.
Successful completion returns a value of 0
Unsuccessful completion returns a number between 1-255

To access the return code type the following right after sending the command for execution:
$ echo $?

Example:

$ date
Tue Oct 16 18:31:33 PDT 2001
$ echo $?
0

Run your datescript shell script and find out the return code. What is it? __________________

Change to the directory above your current directory.
$ cd ..
Now attempt to execute datascript even though it doesn't exist in the current directory.
$ ./datescript
What is the return code?____________

What do you think the two different return codes mean?_______________________________________________________
_____________________________________________________________