1.4.1. Shell building blocks
1.4.1.1. Shell syntax
If input is not commented, the shell reads it and divides it into words and operators, employing quoting rules to define the meaning of each character of input. Then these words and operators are translated into commands and other constructs, which return an exit status available for inspection or processing. The above fork-and-exec scheme is only applied after the shell has analyzed input in the following way:
The shell reads its input from a file, from a string or from the user's terminal.
Input is broken up into words and operators, obeying the quoting rules, see Chapter 3 . These tokens are separated by metacharacters . Alias expansion is performed.
The shell parses (analyzes and substitutes) the tokens into simple and compound commands.
Bash performs various shell expansions, breaking the expanded tokens into lists of filenames and commands and arguments.
Redirection is performed if necessary, redirection operators and their operands are removed from the argument list.
Commands are executed.
Optionally the shell waits for the command to complete and collects its exit status.
1.4.1.2. Shell commands
A simple shell command such as touch file1 file2 file3 consists of the command itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping. A couple of examples:
ls | more
gunzip file.tar.gz | tar xvf -
1.4.1.3. Shell functions
Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed.
Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are explained in Chapter 11 .
1.4.1.4. Shell parameters
A parameter is an entity that stores values. It can be a name, a number or a special value. For the shell's purpose, a variable is a parameter that stores a name. A variable has a value and zero or more attributes. Variables are created with the declare shell built-in command.
If no value is given, a variable is assigned the null string. Variables can only be removed with the unset built-in.
Assigning variables is discussed in Section 3.2 , advanced use of variables in Chapter 10 .
1.4.1.5. Shell expansions
Shell expansion is performed after each command line has been split into tokens. These are the expansions performed:
Brace expansion
Tilde expansion
Parameter and variable expansion
Command substitution
Arithmetic expansion
Word splitting
Filename expansion
We'll discuss these expansion types in detail in Section 3.4 .
1.4.1.6. Redirections
Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment.
1.4.1.7. Executing commands
When executing a command, the words that the parser has marked as variable assignments (preceding the command name) and redirections are saved for later reference. Words that are not variable assignments or redirections are expanded; the first remaining word after
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