"Special" Characters and Operators

Before you learn about regular expressions (), you should understand how quoting () 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 . () 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 tutorial, the sh entries apply to ksh and bash; the csh entries apply to tcsh.

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

- BB, JP