Lab No. 9: Processes and Shell Script Basics¶

Processes¶

A process is simply a running instance of a program. A program is composed of instructions that are executed by the CPU. When a process starts, its instructions are stored in memory, and also, memory is allocated for the process. The process has “safe access” to the chunk of memory that it gets allocated, which essentially means that other processes would not be able to modify the memory allocated to the process, and also that the process would not modify memory allocated to other processes.

Chapter 10 of TLC covers processes. Please go ahead and read that chapter in its entirety, making sure that you have understood all the examples. The example in the section “Controlling Processes” which executes the xlogo command will not work, but instead use the sleep 1000 command. Consult the man pages of sleep so you can understand what is the purpose of this command (you can probably make a pretty good guess by its name). Also, the xload command will not work in blue. So, go ahead and read Chapter 10 and once you are done come back and continue with the next paragraph in this document.

OK, so you are back from Chapter 10. There are a few things that need some clarification. One thing you might be wondering is: How are processes created?

Essentially, every process is created by another process. As explained in the book, when a Linux system boots, there is one special process that starts first, and that is known as the init process, and it is assigned a process ID of 1, so it is also called PID1. Every other process is a child of PID1. You can picture now that (just like the filesystem) the processes in a Linux system are organized in a tree structure. When a new process is created, the operating system makes a copy of the parent process (in a process called forking) and then the specific command is executed by reading the instructions of the command and loading it into memory, and it gets a process id assigned to it. This is what’s known as the fork-and-exec mechanism.

Let’s see some examples (please make sure you are logged in blue.cs.sonoma.edu:

By now, you should know that the ps command is used to list processes. In the following example, you can see the processes your user is running:

[user@blue ~]$ ps f
  PID TTY      STAT   TIME COMMAND
15853 pts/0    Ss     0:00 -bash
21441 pts/0    R+     0:00  \_ ps f

You can see that your current shell (bash) is the parent process of the ps f command. Also, notice that each process thas a “PID” or Process ID associated with it. Of, course, the process ID numbers will differ from what you see here. So just to set the record straigh, in that example process 15853 is the parent process of process 21441, and process 21441 is a child process of process 21441. One thing that should be obvious at this point is that one process can have many child processes, and one child process can only have one parent.

The BASHPID variable stores the value of the current shell process ID:

[user@blue ~]$ echo $BASHPID
15853

Let’s try now to create some child processes. You can start a new shell session within the current shell session:

[user@blue ~]$ bash
[user@blue ~]$ echo $BASHPID
40791
[user@blue ~]$ ps f
  PID TTY      STAT   TIME COMMAND
15853 pts/0    Ss     0:00 -bash
40791 pts/0    S      0:00  \_ bash
39492 pts/0    R+     0:00      \_ ps f

Notice how $BASHPID prints the process ID of the second instance of bash, and that PID 39492 (ps f) is a child of the new shell with PID 40791, instead of the shell we were on before (PID 15853). You can see this with more detail if your run ps jf which includes a column with the parent process ID (PPID):

[user@blue ~]$ ps jf
 PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND
15852 15853 15853 15853 pts/0    40855 Ss    1135   0:00 -bash
15853 40791 40791 15853 pts/0    40855 S     1135   0:00  \_ bash
40791 40855 40855 15853 pts/0    40855 R+    1135   0:00      \_ ps jf

At this point, if you type the exit command (once) you will terminate the second shell and you will be back at the original shell.

The environment¶

Recall how on Lab 04 we learned about environment variables. Before learning how to write Shell scripts, we need to understand how the environment is set and how it can be altered. From your text book read Chapter 11, from pages 128 until page 134. Pay special attention to the section where the PATH environment variable is explained. Once you have read that part come back to this document.

Shell Scripts¶

A shell script is basically a file that contains a series of commands. Shell scripts are plain text files, so they can be created with any text editor. They can be even conveniently created from the command line if they are simple enough.

For this part of the Lab you need to read (and execute all examples) from Chapter 24 of TLC. Once you are done, come back to this document.

#!/bin/bash

# This is my first shell script with arguments
echo "The first argument you entered is: " $1

[user@blue ~]$ ./args.sh Linux
The first argument you entered is: Linux

vehicles
│
├── public_bicycles
├── public_cars
├── public_planes
├── bicycles
├── cars
└── planes