Syntax (Linux in a Nutshell, 4th Edition)

8.3.3. Quoting

Quoting disables a character's special meaning and allows it to be used literally, as itself. The characters in the following table have special meaning to tcsh.

Characters	Description
;	Command separator
&	Background execution
( )	Command grouping
\|	Pipe
* ? [ ] ~	Filename metacharacters
{ }	String expansion characters (usually don't require quoting)
> < & !	Redirection symbols
! ^	History substitution, quick substitution
" ' \	Used in quoting other characters
`	Command substitution
$	Variable substitution
newline space tab	Word separators

The characters that follow can be used for quoting:

" ": Everything between " and " is taken literally except for the following characters, which keep their special meaning:
$: Variable substitution will occur.
`: Command substitution will occur.
": The end of the double quote.
\: Escape next character.
!: The history character.
newline: The newline character.
' ': Everything between ' and ' is taken literally except for ! (history).
\: The character following a \ is taken literally. Use within " " to escape ", $, and `. Often used to escape itself, spaces, or newlines. Always needed to escape a history character (usually !).

8.3.3.1. Examples

% echo 'Single quotes "protect" double quotes'
Single quotes "protect" double quotes

% echo "Well, isn't that "\""special?"\"
Well, isn't that "special"?

% echo "You have `ls|wc -l` files in `pwd`"
You have 43 files in /home/bob

% echo The value of \$x is $x
The value of $x is 100

8.3.5. Redirection Forms

File descriptor	Name	Common abbreviation	Typical default
0	Standard input	stdin	Keyboard
1	Standard output	stdout	Screen
2	Standard error	stderr	Screen

The usual input source or output destination can be changed with redirection commands listed in the following sections.

8.3.5.1. Simple redirection

Command	Action
cmd > file	Send output of cmd to file (overwrite).
cmd >! file	Same as preceding, even if noclobber is set.
cmd >> file	Send output of cmd to file (append).
cmd>>! file	Same as preceding, even if noclobber is set.
cmd < file	Take input for cmd from file.
cmd << text	Read standard input up to a line identical to text (text can be stored in a shell variable). Input usually is typed on the screen or in the shell program. Commands that typically use this syntax include cat, echo, ex, and sed. If text is enclosed in quotes, standard input will not undergo variable substitution, command substitution, etc.

8.3.5.2. Multiple redirection

Command	Action
cmd >& file	Send both standard output and standard error to file.
cmd >&! file	Same as preceding, even if noclobber is set.
cmd >>& file	Append standard output and standard error to end of file.
cmd >>&! file	Same as preceding, even if noclobber is set.
cmd1 \|& cmd2	Pipe standard error together with standard output.
`(`cmd> f1) >& f2	Send standard output to file f1 and standard error to file f2.
cmd \| tee files	Send output of cmd to standard output (usually the screen) and to files. (See the example in Chapter 3 under tee.)

8.3.5.3. Examples

% cat part1 > book                       Copy part1 to book
% cat part2 part3 >> book                Append parts 2 and 3 to same file as part1
% mail tim < report                      Take input to message from report
% cc calc.c >& error_out                 Store all messages, including errors
% cc newcalc.c >&! error_out             Overwrite old file
% grep Unix ch* |& pr                    Pipe all messages, including errors
% (find / -print > filelist) >& no_access  Separate error messages from list of files
% sed 's/^/XX /' << "END_ARCHIVE"           Supply text right after command
This is often how a shell archive is "wrapped",
bundling text for distribution. You would normally
run sed from a shell program, not from the command line.
"END_ARCHIVE"

8.3.3.1. Examples

8.3. Syntax

8.3.1. Special Files

8.3.2. Filename Metacharacters

8.3.2.1. Examples

8.3.3. Quoting

8.3.4. Command Forms

8.3.4.1. Examples

8.3.5. Redirection Forms

8.3.5.1. Simple redirection

8.3.5.2. Multiple redirection

8.3.5.3. Examples