Book Notes Topic 2

City College of San Francisco - CS160B
Unix/Linux Shell Scripting
Module: Shell Basics

Book Notes on Topic 2

Chapter 11:

on current versions of Redhat Linux systems, the variable TZ is not set. Instead, the timezone information can be accessed from either the file /etc/timezone or the command timedatectl
the variable USER is duplicated in the variable LOGNAME
IF you modify your startup files, be careful. Bad syntax errors at login can effectively disable your account.
The ~/.bash_profile file is only read at login when logging in remotely. If you login at the GUI, the ~/.profile file is read instead
Think of an environment variable as having a special mark that means: when a new process starts, copy me. Thus, whenever a new process starts, it gets a copy of the current value of all its parent's environment variables. Hence once a variable has been exported, it does not need to be re-exported, even if its contents change. This is also true of child processes that get a copy of the environment variable - once it is in the environment, it is in the environment for the current process, and for all child processes.
You cannot take a variable out of the environment (there is un unexport command). You must destroy the variable using unset, then re-create it. All variables start out local to the process that created them.

Chapter 7:

This section on expansions is crucial to how the shell works. It discusses briefly each type of expansion that is done on the command-line during command processing. Remember, the shell works in a big loop that goes like this:

read command (this step completes when the user types ENTER)
expand command (this is where all the shell magic occurs!)
execute command (the shell looks for the command and starts it, giving it the commandline)
wait for command to exit (the command executes until it is finished)
start over at 1.

the section details the things that hapen in the expand commandstep. As we will see, a lot happens! The result of this step may be a command that looks nothing like what the user typed! Each remaining page of this module of online notes discusses these expansions in detail.

Chapter 25:

Ignore the project discussed in this chapter. Concentrate instead on the extra information about variables and here documents. (I dont know why he separated out the information on variables between this chapter and Chapter 7, but he did.)

Chapter 13:

This chapter on customizing the prompt is optional. If you want a short introduction, just read the first two pages and look over Table 13-1. These are the most useful (and portable) prompt-formatting sequences.

This page was made entirely with free software on Linux:
Kompozer and LibreOffice