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What Is the Shell

This chapter is from the book

This chapter is from the book

The Shell's Responsibilities

Now you know that the shell analyzes each line you type in and initiates execution of the selected program. But the shell also has other responsibilities, as outlined in Figure 3.8.

Program Execution

The shell is responsible for the execution of all programs that you request from your terminal.

Each time you type in a line to the shell, the shell analyzes the line and then determines what to do. As far as the shell is concerned, each line follows the same basic format:

program-name arguments

The line that is typed to the shell is known more formally as the command line. The shell scans this command line and determines the name of the program to be executed and what arguments to pass to the program.

Figure 3.8Figure 3.8 The shell's responsibilities.

The shell uses special characters to determine where the program name starts and ends, and where each argument starts and ends. These characters are collectively called whitespace characters, and are the space character, the horizontal tab character, and the end-of-line character, known more formally as the newline character. Multiple occurrences of whitespace characters are simply ignored by the shell. When you type the command

mv tmp/mazewars games

the shell scans the command line and takes everything from the start of the line to the first whitespace character as the name of the program to execute: mv. The set of characters up to the next whitespace character is the first argument to mv: tmp/mazewars. The set of characters up to the next whitespace character (known as a word to the shell)—in this case, the newline—is the second argument to mv: games. After analyzing the command line, the shell then proceeds to execute the mv command, giving it the two arguments tmp/mazewars and games (see Figure 3.9).

Figure 3.9Figure 3.9 Execution of mv with two arguments.

As mentioned, multiple occurrences of whitespace characters are ignored by the shell. This means that when the shell processes this command line:

echo      when  do      we   eat?

it passes four arguments to the echo program: when, do, we, and eat? (see Figure 3.10).

Figure 3.10Figure 3.10 Execution of echo with four arguments.

Because echo takes its arguments and simply displays them at the terminal, separating each by a space character, the output from the following becomes easy to understand:

$ echo     when  do     we  eat?
when do we eat? 

The fact is that the echo command never sees those blank spaces; they have been "gobbled up" by the shell. When we discuss quotes in Chapter 6, "Can I Quote You on That?," you'll see how you can include blank spaces in arguments to programs.

We mentioned earlier that the shell searches the disk until it finds the program you want to execute and then asks the Unix kernel to initiate its execution. This is true most of the time. However, there are some commands that the shell knows how to execute itself. These built-in commands include cd, pwd, and echo. So before the shell goes searching the disk for a command, the shell first determines whether it's a built-in command, and if it is, the shell executes the command directly.

Variable and Filename Substitution

Like any other programming language, the shell lets you assign values to variables. Whenever you specify one of these variables on the command line, preceded by a dollar sign, the shell substitutes the value assigned to the variable at that point. This topic is covered in complete detail in Chapter 5, "And Away We Go."

The shell also performs filename substitution on the command line. In fact, the shell scans the command line looking for filename substitution characters *, ?, or [...] before determining the name of the program to execute and its arguments. Suppose that your current directory contains the files as shown:

$ ls

Now let's use filename substitution for the echo command:

$ echo *      List all files
mrs.todd prog1 shortcut sweeney

How many arguments do you think were passed to the echo program, one or four? Because we said that the shell is the one that performs the filename substitution, the answer is four. When the shell analyzes the line

echo *

it recognizes the special character * and substitutes on the command line the names of all files in the current directory (it even alphabetizes them for you):

echo mrs.todd prog1 shortcut sweeney

Then the shell determines the arguments to be passed to the command. So echo never sees the asterisk. As far as it's concerned, four arguments were typed on the command line (see Figure 3.11).

Figure 3.11Figure 3.11 Execution of echo.

I/O Redirection

It is the shell's responsibility to take care of input and output redirection on the command line. It scans the command line for the occurrence of the special redirection characters <, >, or >> (also << as you'll learn in Chapter 13, "Loose Ends").

When you type the command

echo Remember to tape Law and Order > reminder

the shell recognizes the special output redirection character > and takes the next word on the command line as the name of the file that the output is to be redirected to. In this case, the file is reminder. If reminder already exists and you have write access to it, the previous contents are lost (if you don't have write access to it, the shell gives you an error message).

Before the shell starts execution of the desired program, it redirects the standard output of the program to the indicated file. As far as the program is concerned, it never knows that its output is being redirected. It just goes about its merry way writing to standard output (which is normally your terminal, you'll recall), unaware that the shell has redirected it to a file.

Let's take another look at two nearly identical commands:

$ wc -l users
   5 users
$ wc -l < users

In the first case, the shell analyzes the command line and determines that the name of the program to execute is wc and it is to be passed two arguments: -l and users (see Figure 3.12).

Figure 3.12Figure 3.12 Execution of wc -l users.

When wc begins execution, it sees that it was passed two arguments. The first argument, -l, tells it to count the number of lines. The second argument specifies the name of the file whose lines are to be counted. So wc opens the file users, counts its lines, and then prints the count together with the filename at the terminal.

Operation of wc in the second case is slightly different. The shell spots the input redirection character < when it scans the command line. The word that follows on the command line is the name of the file input is to be redirected from. Having "gobbled up" the < users from the command line, the shell then starts execution of the wc program, redirecting its standard input from the file users and passing it the single argument -l (see Figure 3.13).

Figure 3.13Figure 3.13 Execution of wc -l < users.

When wc begins execution this time, it sees that it was passed the single argument -l. Because no filename was specified, wc takes this as an indication that the number of lines appearing on standard input is to be counted. So wc counts the number of lines on standard input, unaware that it's actually counting the number of lines in the file users. The final tally is displayed at the terminal—without the name of a file because wc wasn't given one.

The difference in execution of the two commands is important for you to understand. If you're still unclear on this point, review the preceding section.

Pipeline Hookup

Just as the shell scans the command line looking for redirection characters, it also looks for the pipe character |. For each such character that it finds, it connects the standard output from the command preceding the | to the standard input of the one following the |. It then initiates execution of both programs.

So when you type

who | wc -l

the shell finds the pipe symbol separating the commands who and wc. It connects the standard output of the former command to the standard input of the latter, and then initiates execution of both commands. When the who command executes, it makes a list of who's logged in and writes the results to standard output, unaware that this is not going to the terminal but to another command instead.

When the wc command executes, it recognizes that no filename was specified and counts the lines on standard input, unaware that standard input is not coming from the terminal but from the output of the who command.

Environment Control

The shell provides certain commands that let you customize your environment. Your environment includes your home directory, the characters that the shell displays to prompt you to type in a command, and a list of the directories to be searched whenever you request that a program be executed. You'll learn more about this in Chapter 11, "Your Environment."

Interpreted Programming Language

The shell has its own built-in programming language. This language is interpreted, meaning that the shell analyzes each statement in the language one line at a time and then executes it. This differs from programming languages such as C and FORTRAN, in which the programming statements are typically compiled into a machine-executable form before they are executed.

Programs developed in interpreted programming languages are typically easier to debug and modify than compiled ones. However, they usually take much longer to execute than their compiled equivalents.

The shell programming language provides features you'd find in most other programming languages. It has looping constructs, decision-making statements, variables, and functions, and is procedure-oriented. Modern shells based on the IEEE POSIX standard have many other features including arrays, data typing, and built-in arithmetic operations.

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