UNIX/Linux: Notes on Sobell, Chapter 9, Bourne Again Shell

Transcription

1 These are some notes to help you through Chapter 9 of A Practical Guide to Linux, by Mark Sobell. This is the chapter on the BASH shell, and it is long, complicated, and difficult. But it covers how the shell works, and you should have a good idea what is in this chapter. HOWEVER, you donʼt need to understand every detail. Some of what is covered in this chapter should be understood now, but much else can be saved to reference later. You should skim the whole chapter, but not try to remember every detail. In the following, Iʼll try to summarize what is in each section, and note which sections you should try to understand in detail, and which ones you can skim over and take note of, so that you can re-read them when (and if) you need the content. The sections I mark with an asterisk (*) are particularly important, and you should be sure you understand them. * Startup Files What files are executed when a (bash) shell is started. Note that on the CSM server, we are mainly concerned with /etc/bashrc, /etc/profile, $HOME/.bashrc, and $HOME/.bash_profile. There are other files bash looks for, but we generally are not using them. -- login vs. non-login: /etc/profile and.profile are only executed when you first login and your first shell is started, while non-login are started every time you run a script, or use parentheses on a command line -- interactive vs non-interactive shells: If you start a shell directly (for instance, by typing bash (or some other shell name), you are running an interactive shell, one that assumes you are sitting at the terminal and typing in standard input of that shell. When you run a script, it is also in a shell (a subshell of the shell you are in), but it is assumed non-interactive. The script itself may initiate communication with you at the terminal, but the shell it is running in doesnʼt (normally) see your aliases, local variables, etc. *. (or source ) to run a script: Why and when do you use this method of running a script? Mainly to test startup scripts. You should NOT use it to run normal scripts. But startup scripts need to set local variables, so need to run in the current shell, not a sub-shell. So to be able to test the startup scripts without logging out, you need to use the. method.

2 * Symbols for commands There are several symbols that act like commands or are shorthand for commands. They are mostly described later, but listed here. You should know all of them. (The... inside is replaced by something different in each symbol.) There are page references in the book for more details about each of these symbols. (...) - Run stuff in a subshell $(...) - command substitution ((...)) - arithmetic $((...)) - arithmetic [... ] - test command [[... ]] - test command with extra syntax Redirecting You can ignore most of this section, we covered it in the first half of the class. * Writing a simple shell script This is a very good section on what you need to do to get started with scripts. Separating and Grouping You donʼt need the details of this section, just the general ideas. It mentions ; and <CR> to separate commands, \ to continue a command on another line, as a pipe, & to put a command into the background, and (...) to group parts of a command line. Mentions that the NEWLINE (which for us is a carriage return, CR ) is a command terminator; it tells the shell to start looking at what you have typed, and start the command line going.

3 Job Control Again, you donʼt need the details, just the general ideas. Talks about the jobs command, to see what you have running, and fg and bg to put a process into the foreground or background. What job control does is manage processes. Here is what a process is: Directory stack You can skip this whole section for now. You might want to look at it if you do a lot of UNIX or Linux work. * Parameters & Variables You need to know how to create and use variables, and what ones the system creates for you. You need to know that parameters are a kind of variable that the system creates for scripts, and that many environment variables are initially set for you in the startup process. You need to know how to quote the variables, and what the different quotes do. You do not need to know about the attributes until you are using a UNIX system more heavily than in this class. There is a list of the most common system variables (keyword variables) at the end of this section, as well as a list of the special characters the shell looks at. Processes Look at the slides, that should be all the information you need at this time. The fundamental unit of multiuser UNIX/Linux operation is the process, which is just a program while it is executing in the UNIX system. The running program is called a "process", and it "does something" like "this process will open files" or "this process is printing my term paper". UNIX and Linux support multiple users by allowing multiple processes to execute concurrently. There may be hundreds of processes in concurrent execution at the same time. Almost nothing gets done in a UNIX or Linux system

4 without creating a process to perform the task. Of course, since there are only one or two central processors (CPUs), the system creates the illusion of serving many users simultaneously by switching the CPU, very rapidly, from one process to the next. History You can ignore this whole section, but as you use LInux more, you should come back and learn how to edit your history. Readline Library If you want to get a better idea of what you can type on the command line, you can look at this section. Our default is the emacs editing mode. * Aliases and Functions You need to know how to make and use both aliases and functions, and how to avoid using them (sometimes). You should know where they are defined. Bash options Command line options - Skip for now. But you can see all the things you could set in your shell in the list of bash features. Processing the command line Read through this, donʼt memorize it. You should have a general idea of what kind of things the shell does for you, and be able to look up the details if you need them.

5 *Chapter Sumary The end of this chapter has a good, short summary of everything covered in the chapter, and you should be sure you understand it all. There are also some lists of symbols, standard environment variables, in the chapter.