[Chapter 8] 8.19 "Special" Characters and Operators

8.19 "Special" Characters and Operators

Before you learn about regular expressions (26.1), you should understand how quoting (8.14) works in UNIX.

Regular expressions use metacharacters. The shells also have metacharacters. Metacharacters are simply characters that have a special meaning. The problem occurs when you want to use a regular expression in a shell script. Will the shell do something special with the character? Or will it be passed unchanged to the program? The $ character is a good example. It could be the beginning of a variable name or it could bepart of a regular expression . (26.2) If you need a regular expression, you must know if any of the characters of the expression are metacharacters, and must know the right way to quote that character so that it is passed to the program without being modified by the shell.

Table 8.3 is a table of special characters and operators in the C shell (csh) and Bourne shell (sh). The chart also includes several combinations of characters just to be complete. As in other parts of this book, the sh entries apply to ksh and bash; the csh entries apply to tcsh.

Table 8.3: List of Special Characters and Their Meanings
Character	Where	Meaning	Article
ESC	csh	Filename completion.	9.8
RETURN	csh, sh	Execute command.	41.2
space	csh, sh	Argument separator.	8.5
TAB	csh, sh	Argument separator.	8.5
TAB	bash	Filename completion.	9.8
#	csh, sh	Start a comment.	44.2
`	csh, sh	Command substitution (backquotes).	9.16
"	sh	Weak quotes.	8.14
"	csh	Weak quotes.	8.15, 8.14
'	sh	Strong quotes.	8.14
'	csh	Strong quotes.	8.15, 8.14 See \.
\	sh	Single-character quote.	8.14
\	csh	Single-character quote.	8.15, 8.14
$`var`	csh, sh	Variable.	6.1, 6.8
${`var`}	csh, sh	Same as `$var`.	6.8
$`var`:`mod`	csh	Edit `var` with modifier `mod`	9.6
${`var`-`default`}	sh	If `var` not set, use `default`.	45.12
${`var`=`default`}	sh	If `var` not set, set it to `default` and use that value.	45.12
${`var`+`instead`}	sh	If `var` set, use `instead`. Otherwise, null string.	45.12
${`var`?`message`}	sh	If `var` not set, print `message` (else default). If `var` set, use its value.	45.12
${`var`#`pat`}	ksh, bash	Value of `var` with smallest `pat` deleted from start.	9.7
${`var`##`pat`}	ksh, bash	Value of `var` with largest `pat` deleted from start.	9.7
${`var`%`pat`}	ksh, bash	Value of `var` with smallest `pat` deleted from end.	9.7
${`var`%%`pat`}	ksh, bash	Value of `var` with largest `pat` deleted from end.	9.7
\|	csh, sh	Pipe standard output.	1.4, 13.1
\|&	csh	Pipe standard output and standard error.	13.5
^	sh only	Pipe character (obsolete).
^	csh, bash	Edit previous command line.	11.5
&	csh, sh	Run program in background.	1.27, 1.28
?	csh, sh	Match one character.	1.16, 15.2
*	csh, sh	Match zero or more characters.	1.16, 15.2
;	csh, sh	Command separator.	8.5
;;	sh	End of case statement.	44.5
~	csh, ksh, bash	Home directory.	14.11
~`user`	csh, ksh, bash	Home directory of `user`.	14.11
!	csh, bash	Command history.	11.2
-	Programs	Start of optional argument.	8.5
-	Programs	Read standard input. (Only certain programs.)	13.13
$#	csh, sh	Number of arguments to script.	44.15
"$@"	sh	Original arguments to script.	44.15
$*	csh, sh	Arguments to script.	44.15
$-	sh	Flags set in shell.	2.11
$?	sh	Status of previous command.	44.7
$$	csh, sh	Process identification number.	8.14
$!	sh	Process identification number of last background job.	7.12
$<	csh	Read input from terminal.	9.11
`cmd1` && `cmd2`	csh, sh	Execute `cmd2` if `cmd1` succeeds.	44.9
`cmd1` \|\| `cmd2`	csh, sh	Execute `cmd2` if `cmd1` fails.	44.9
$(..)	ksh, bash	Command substitution.	45.31, 9.16
((..))	ksh, bash	Arithmetic evaluation.
\. `file`	sh	Execute commands from `file` in this shell.	44.23
:	sh	Evaluate arguments, return true.	45.9
:	sh	Separate values in paths.	6.4, 14.5, 21.8
:	csh	Variable modifier.	9.6
[]	csh, sh	Match range of characters.	1.16, 15.2
[]	sh	Test.	44.20
%`job`	csh, ksh, bash	Identify job number.	12.1
(`cmd`;`cmd`)	csh, sh	Run `cmd;cmd` in a subshell.	13.7
{}	csh, bash	In-line expansions.	9.5
{`cmd`;`cmd`; }	sh	Like `(cmd;cmd)` without a subshell.	13.8
>`file`	csh, sh	Redirect standard output.	13.1
>>`file`	csh, sh	Append standard output.	13.1
<`file`	csh, sh	Redirect standard input.	13.1
<<`word`	csh, sh	Read until `word`, do command and variable substitution.	8.18, 9.14
<<\`word`	csh, sh	Read until `word`, no substitution.	8.18
<<-`word`	sh	Read until `word`, ignoring leading TABs.	8.18
>>! `file`	csh	Append to `file`, even if noclobber set and `file` doesn't exist.	13.6
>! `file`	csh	Output to `file`, even if noclobber set and `file` exists.	13.6
>\| `file`	ksh, bash	Output to `file`, even if noclobber set and `file` exists.	13.6
>& `file`	csh	Redirect standard output and standard error to `file`.	13.5
`m`> `file`	sh	Redirect output file descriptor `m` to `file`.	45.21
`m`>> `file`	sh	Append output file descriptor `m` to `file`.
`m`< `file`	sh	Redirect input file descriptor `m` from `file`.
<&`m`	sh	Take standard input from file descriptor `m`.
<&-	sh	Close standard input.	45.10
>&`m`	sh	Use file descriptor `m` as standard output.	45.21
>&-	sh	Close standard output.	45.21
`m`<&`n`	sh	Connect input file descriptor `n` to file descriptor `m`.	45.22
`m`<&-	sh	Close input file descriptor `m`.	45.21
`n`>&`m`	sh	Connect output file descriptor `n` to file descriptor `m`.	45.21
`m`>&-	sh	Close output file descriptor `m`.	45.21

- BB, JP


8.18 Here Documents		8.20 How Many Backslashes?