Shells terminology

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The terms and definitions in this table are helpful in understanding shells.

Item Description
blank A blank is one of the characters in the blank character class defined in the LC_CTYPE category. In the POSIX shell, a blank is either a tab or space.
built-in command A command that the shell executes without searching for it and creating a separate process.
command A sequence of characters in the syntax of the shell language. The shell reads each command and carries out the desired action either directly or by invoking separate utilities.
comment Any word that begins with pound sign (#). The word and all characters that follow it, until the next newline character, are ignored.
identifier A sequence of letters, digits, or underscores from the portable character set, starting with a letter or underscore. The first character of an identifier must not be a digit. Identifiers are used as names for aliases, functions, and named parameters.
list A sequence of one or more pipelines separated by one of the following symbols: semicolon (;), ampersand (&), double ampersand (&&), or double bar (||). The list is optionally ended by one of the following symbols: semicolon (;), ampersand (&), or bar ampersand (|&).
;
Sequentially processes the preceding pipeline. The shell carries out each command in turn and waits for the most recent command to complete.
&
Asynchronously processes the preceding pipeline. The shell carries out each command in turn, processing the pipeline in the background without waiting for it to complete.
|&
Asynchronously processes the preceding pipeline and establishes a two-way pipe to the parent shell. The shell carries out each command in turn, processing the pipeline in the background without waiting for it to complete. The parent shell can read from and write to the standard input and output of the created command by using the read -p and print -p commands. Only one such command can be active at any given time.
&&
Processes the list that follows this symbol only if the preceding pipeline returns an exit value of zero (0).
||
Processes the list that follows this symbol only if the preceding pipeline returns a nonzero exit value.
The semicolon (;), ampersand (&), and bar ampersand (|&) have a lower priority than the double ampersand (&&) and double bar (||). The ;, &, and |& symbols have equal priority among themselves. The && and || symbols are equal in priority. One or more newline characters can be used instead of a semicolon to delimit two commands in a list.
Note: The |& symbol is valid only in the Korn shell.
metacharacter Each metacharacter has a special meaning to the shell and causes termination of a word unless it is quoted. Metacharacters are: pipe (|), ampersand (&), semicolon (;), less-than sign (<), greater-than sign (>), left parenthesis ((), right parenthesis ()), dollar sign ($), backquote (`), backslash (\), right quote ('), double quotation marks ("), newline character, space character, and tab character. All characters enclosed between single quotation marks are considered quoted and are interpreted literally by the shell. The special meaning of metacharacters is retained if not quoted. (Metacharacters are also known as parser metacharacters in the C shell.)
parameter assignment list Includes one or more words of the form Identifier=Value in which spaces surrounding the equal sign (=) must be balanced. That is, leading and trailing blanks, or no blanks, must be used.
Note: In the C shell, the parameter assignment list is of the form setIdentifier=Value. The spaces surrounding the equal sign (=) are required.
pipeline A sequence of one or more commands separated by pipe (|). Each command in the pipeline, except possibly the last command, is run as a separate process. However, the standard output of each command that is connected by a pipe becomes the standard input of the next command in the sequence. If a list is enclosed with parentheses, it is carried out as a simple command that operates in a separate subshell.

If the reserved word ! does not precede the pipeline, the exit status will be the exit status of the last command specified in the pipeline. Otherwise, the exit status is the logical NOT of the exit status of the last command. In other words, if the last command returns zero, the exit status will be 1. If the last command returns greater than zero, the exit status will be zero.

The format for a pipeline is as follows: 
[!] command1 [ | command2 ...]
Note: Early versions of the Bourne shell used the caret (^) to indicate a pipe.
shell variable A name or parameter to which a value is assigned. Assign a variable by typing the variable name, an equal sign (=), and then the value. The variable name can be substituted for the assigned value by preceding the variable name with a dollar sign ($). Variables are particularly useful for creating a short notation for a long path name, such as $HOME for the home directory. A predefined variable is one whose value is assigned by the shell. A user-defined variable is one whose value is assigned by a user.
simple command A sequence of optional parameter assignment lists and redirections, in any sequence. They are optionally followed by commands, words, and redirections. They are terminated by ;, |, &, ||, &&, |&, or a newline character. The command name is passed as parameter 0 (as defined by the exec subroutine). The value of a simple command is its exit status of zero if it terminates normally or nonzero if it terminates abnormally. The sigaction, sigvec, or signal subroutine includes a list of signal-exit status values.
subshell A shell that is running as a child of the login shell or the current shell.
wildcard character Also known as a pattern-matching character. The shell associates them with assigned values. The basic wildcards are ?, *, [set], and [!set]. Wildcard characters are particularly useful when performing file name substitution.
word A sequence of characters that does not contain any blanks. Words are separated by one or more metacharacters.