A metacharacter is a character that performs a special function in the
shell, like expanding a file name or redirecting output. In fact, the
metacharacters in this article fall into two categories: wildcard
characters for file name expansion and redirection of input/output and
process.
File name expansion characters include
* Asterisk *
* Brackets [ ]
* Brackets-Hyphen [ - ]
* Question mark ?
Input/output and process redirection characters include
* Ampersand &
* Back quote `
* Backslash \\
* Braces { }
* Double greater than > >
* Greater than >
* Less than <
* Parentheses ( )
* Semicolon ;
* Slash /
* Vertical bar |
The article is divided into two sections (one for each type of character).
All metacharacters are listed alphabetically within their sections.
FILE NAME EXPANSION METACHARACTERS
----------------------------------
The Asterisk
------------
The asterisk (*) substitutes in a file or directory name for zero or more
characters, except a leading period (like ".ash").
Example: The command
ls *ash
returns file names ending with the letters "ash" like "ash", "bash",
"cash", "mash", and "splash". However, it won't return strings that have
characters after the "ash" like "bash.tmp" or "bashful.
Brackets
--------
Brackets ([ ]) cause the shell to look for a match for each character
between the brackets, one at a time. It does not match the period
character.
Example: ls [bm]ash returns file names "bash" and "mash", but not "cash" or
"bash.tmp".
A variation is brackets with a hyphen separating two characters. It causes
the shell to match any character within the range of these characters in a
file or directory name.
Example: ls [a-c]ash returns file names that have letters in the range from
"a" through "c" as the first character of their name and that are followed
by the letters "ash". Thus, "ash", "bash", and "cash" match, but not
"dash" or "mash".
You can place a bracket metasequence anywhere in a string.
Question Mark
-------------
The question mark (?) substitutes for any ONE character in the same
position in a file or directory name. The question mark does not expand
file names, nor does not match a leading period.
Example: "ls ?ash" returns only file names that have one character followed
by the characters "ash". Thus, "bash", and "cash" match, but not "ash" or
"splash".
REDIRECTION METACHARACTERS
--------------------------
Ampersand
---------
The ampersand (&), placed at the end of a line, causes the task it follows
to run in the background. When using the ampersand, the shell returns the
task's PID (Process ID Number). Note: In the Bourne shell, you cannot
retrieve a task from the background.
Example: cat /etc/passwd &
Back Quotes
------------
The back quotes (` `) contain any UNIX command. When the shell executes
the line containing the back-quoted command, it replaces the command and
with the command's output in the string. Without the back quotes, the
shell would treat a command like an ordinary string.
For example,
echo today is date
returns "echo today is date", whereas
echo today is `date`
returns "today is " followed by the system date.
Backslash
----------
The backslash (\\), preceding a metacharacter, causes the shell to interpret
the character as a regular character rather than as a metacharacter.
Typically, this can stop a variable from returning its value.
For example, type in the following three lines of code. And notice that the
third line does not return the variable's value.
x=hi
echo $x
echo \\$x
Dollar Sign
-----------
The dollar sign ($) causes the shell to evaluate or display the value of
the variable the dollar sign precedes. The following to lines output "hi",
the contents of variable x.
x=hi
echo $x
Double Quotes
---------------
The double quotes (" ") serve three purposes. First, it lets you put tabs
and spaces in a string as you assign the string to a variable. Second,
when you display the contents of a variable, surrounding the variable name
in double quotes preserves tabs and multiple contiguous spaces and tabs.
Try this example:
y=me and you
The shell refuses the command when it hits the first space. Try this
(including multiple spaces after "and"):
y="me and you"
echo $y
echo "$y"
The third purpose for double quotes is that they can "seal in" command
characters, so that these characters appear as literals. For example,
typing
echo today's
causes the system to display a greater than sign. It's waiting for further
input, input that must conclude with a single quote. The command
echo "today's"
just prints the word.
Greater Than and Double-Greater-Than
------------------------------------
Both these commands take data from a source command and send that data to
the target file as if it were standard output. If the file doesn't exist
the command creates one. The difference is that greater-than always
*writes* to the target file, thus erasing any data already in the file.
The double-greater-than character always *appends* the data to the file.
To see these characters at work, try this command-line sequence:
First, check to see if you have a file named "doc." If you do, use a
different file name.
ls doc
date > doc
cat doc
who am i >> doc
cat doc
cal 11 1993 > doc
cat doc
Less Than
----------
The less than (<) causes a file to be treated as a standard input for the
command. The example writes the file names in the current directory into
doc, It then hands the contents of doc to the sort command for a reverse
sort to the screen
ls > doc
sort -r < doc
Parentheses
-------------
The parentheses -- ( ) -- group several commands for execution in a
subshell with command output passed as standard input to the next command
on a pipeline. Notice the difference when you use parentheses and when you
don't in this example:
date; who am i > doc
cat doc
(date; who am i) > doc
cat doc
Without parentheses, the shell executes the date command without sending
its output to doc. The shell pipes the who am i output, because it is
immediately adjacent to the greater than.
Semicolon
-----------
The semicolon (;) causes commands on the current line to be executed
sequentially. Note the example for parentheses and the example below:
date ; ls
Single Quotes
--------------
Put single quotes (') around special characters when you do not want the
shell to interpret them.
Note: if a back quote occurs within the command, it must be escaped with a
"\\" (backslash). Otherwise, the usual quoting conventions apply within the
command.
Vertical Bar
------------
The vertical bar (|) causes the standard output for the first command and
to be treated as the standard input for the second command. That is, the
vertical bar combines commands into a pipeline, passing data from one
command to another without an intervening file. The example passes the
listing for the current directory directly to the sort command for a
reverse sort printed to the screen:
ls | sort -r
Article Change History:
24 Jun 1993 - Revised for technical accuracy.