3.1 Shell Syntax

When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (`#'), and the rest of that line.

Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing.

3.1.1 Shell Operation

The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following:

1. Reads its input from a file (see section Shell Scripts), from a string supplied as an argument to the `-c' invocation option (see section Invoking Bash), or from the user's terminal.
2. Breaks the input into words and operators, obeying the quoting rules described in Quoting. These tokens are separated by metacharacters. Alias expansion is performed by this step (see section Aliases).
3. Parses the tokens into simple and compound commands (see section Shell Commands).
4. Performs the various shell expansions (see section Shell Expansions), breaking the expanded tokens into lists of filenames (see section Filename Expansion) and commands and arguments.
5. Performs any necessary redirections (see section Redirections) and removes the redirection operators and their operands from the argument list.
6. Executes the command (see section Executing Commands).
7. Optionally waits for the command to complete and collects its exit status (see section Exit Status).

3.1.2 Quoting

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.

Each of the shell metacharacters (see section Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (@pxref{History Interaction}), the history expansion character, usually `!', must be quoted to prevent history expansion. @xref{Bash History Facilities}, for more details concerning history expansion.

There are three quoting mechanisms: the escape character, single quotes, and double quotes.

3.1.2.1 Escape Character

A non-quoted backslash `\' is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of newline. If a \newline pair appears, and the backslash itself is not quoted, the \newline is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

3.1.2.2 Single Quotes

Enclosing characters in single quotes (`'') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

3.1.2.3 Double Quotes

Enclosing characters in double quotes (`"') preserves the literal value of all characters within the quotes, with the exception of `$', ``', `\', and, when history expansion is enabled, `!'. The characters `$' and ``' retain their special meaning within double quotes (see section Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: `$', ``', `"', `\', or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an `!' appearing in double quotes is escaped using a backslash. The backslash preceding the `!' is not removed.

The special parameters `*' and `@' have special meaning when in double quotes (see section Shell Parameter Expansion).

3.1.2.4 ANSI-C Quoting

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

\a

alert (bell)

\b

backspace

\e

an escape character (not ANSI C)

\f

form feed

\n

newline

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\'

single quote

\nnn

the eight-bit character whose value is the octal value nnn (one to three digits)

\xHH

the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)

\cx

a control-x character

The expanded result is single-quoted, as if the dollar sign had not been present.

3.1.2.5 Locale-Specific Translation

A double-quoted string preceded by a dollar sign (`$') will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.

Some systems use the message catalog selected by the LC_MESSAGES shell variable. Others create the name of the message catalog from the value of the TEXTDOMAIN shell variable, possibly adding a suffix of `.mo'. If you use the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR variable to the location of the message catalog files. Still others use both variables in this fashion: TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.

3.1.3 Comments

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see section Bash Builtin Commands), a word beginning with `#' causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells. See section Interactive Shells, for a description of what makes a shell interactive.