Metacharacters for the POSIX shell

Argument and command separators

Metacharacter	Meaning
Blank Newline Tab	Argument separators, depending on the value of the IFS variable
Newline	End of command
|	Pipe symbol
;	End of command
&	End of command; a command terminated with this symbol is run as a background process
||	ORIF; the next command is executed only if the preceding command returns a non-zero exit status
&&	ANDIF; the next command is executed only if the preceding command returns an exit status of zero

Command grouping

Metacharacter	Meaning
(command_list)	Execute command_list in a subshell
{ command_list;}	Collect the output of all commands from command_list

Execute command and replace with output

Metacharacter	Meaning
`command`	Replace command with its output
$(command)	Replace command with its output

File name generation

Metacharacter	Meaning
*	As a pattern on its own: replaced by the list of all file names in the current directory that do not begin with a period (.). As part of a pattern: replaced by no, one or more character(s) depending on the file names which match the pattern.
?	As a pattern on its own: replaced by the list of all file names in the current directory that consist of exactly one character, except for a period (.). As part of a pattern: replaced by no, one or more character(s) depending on the file names which match the pattern.
[s]	Replaced by exactly one of the characters which are not contained in the string s depending on the file names which match the pattern.
[c1-c2]	Replaced by any one character from the range between c1 and c2 inclusive to match file names in the current directory. c1 and c2 must be ordinary characters. The characters which collate between c1 and c2 are determined by the EBCDIC collating sequence. Expressions of type [s] and type [c1-c2] can be combined: [s1c1-c2s2]
[!s]	Replaced by exactly one of the characters which are not contained in the string s depending on the file names which match the pattern.
[!c1-c2]	Replaced by exactly one of the characters which do not lie in the range c1 to c2 depending on the file names which match the pattern (see [c1-c2]). Expressions of type [!s] and type[!c1-c2] can be combined: [!s1c1-c2s2]

Redirection of standard output

Metacharacter	Meaning
[n]> file	Redirect standard output [or file descriptor n] to file, deleting original contents
[n]>> file	Redirect standard output [or file descriptor n] to file, retaining original contents (append)
[n]> &digit	Redirect standard output [or file descriptor n] to the file associated with file descriptor digit
[n]> &-	Close standard output [or file descriptor n]

Redirection of standard input

Metacharacter	Meaning
[n] <file	Redirect standard input [or file descriptor n] to file
[n] <<argument	Start a "here" document or redirect to file descriptor n
[n] <<-argument	Start a "here" document or redirect to file descriptor n; stripping leading tabs
[n] <&digit	Redirect standard input to the file [or file descriptor n] associated with file descriptor digit
[n] <&-	Close standard input [or file descriptor n]
[n] <>file	file [or file descriptor n] is opened as the standard input for reading and writing

Shell variables and parameters

Metacharacter	Meaning
name=value	Assign a value to the variable name
$name	Value of the variable name; keyword parameter
${name}	Like $name; the braces separate the variable name from following figures or characters
${name-default_value}	Replace with default_value if name is undefined
${name=default_value}	Assign default_value if name is undefined
${name?default_value}	Shell terminates process execution with error message parameter : default_value if name is undefined
${name+default_value}	Replace with the null string if name is undefined; replace with default_value if name is defined
${name:-default_value}	Replace with default_value if name is undefined or if its value is the null string
${name:=default_value}	Assign default_value if name is undefined or if its value is the null string
${name:?default_value}	Shell terminates process execution with error message parameter : default_value if name is undefined or if its value is the null string
${name:+default_value}	Replace with the null string if name is undefined or if its value is the null string; replace with default_value if name is undefined or if its value is the null string
${#name}	Replace with the length of the string formed by name. If name is one of the characters * or commercial at @ then the substitution is undefined
${name%pattern}	If the POSIX shell pattern pattern matches the end of the value of name, then the substitute text is formed from the value of name without the deleted segment which corresponds to pattern. The shortest matching pattern is deleted.
${name%%pattern}	If the POSIX shell pattern pattern matches the end of the value of name, then the substitute text is formed from the value of name without the deleted segment which corresponds to pattern. The shortest matching pattern is deleted.
${name#pattern}	If the POSIX shell pattern pattern is to match the start of the value of name, then the substitute text is formed from the value of name from which the segment which corresponds to pattern has been deleted. The shortest matching pattern is deleted.
${name##pattern}	If the POSIX shell pattern pattern is to match the start of the value of name, then the substitute text is formed from the value of name from which the segment which corresponds to pattern has been deleted. The shortest matching pattern is deleted.
$0	0th command-line argument, i.e. the name of the command, the shell script or the current shell
$1, $2, ... , $9	Positional parameters
$* "$*"	All command-line arguments All command-line arguments as a single argument
$@ "$@"	All command-line arguments All command-line arguments as separate arguments
$#	Number of command-line arguments, excluding $0
$$	Process ID (PID) of the current shell
$!	PID of the last command run in the background
$?	Exit status of the last executed command that was not run in the background
$-	All options set in the current shell

Shell functions

Metacharacter	Meaning
name() { command_list;}	Shell function; when name is invoked, the commands from command_list are executed

Quoting metacharacters

Metacharacter	Meaning
\ '...' "..."	Escapes the following metacharacter Escapes all metacharacters between quotes; treat as a single argument Does not escape the metacharacters $ `...` \ Treat as a single argument

Miscellaneous

Metacharacter	Meaning
# ;;	Comment character in shell scripts Command list terminator within case constructs
~ ~- ~+	Tilde substitution
(( )) $((expression))	Arithmetic evaluation
[[ ]]	Conditional expressions