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UNIX in a Nutshell: System V Edition

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Previous: 4.1 Overview of FeaturesChapter 4
The Bourne Shell and Korn Shell
Next: 4.3 Variables

4.2 Syntax

This subsection describes the many symbols peculiar to the Bourne and Korn shell. The topics are arranged as follows:

4.2.1 Special Files


Executed automatically at login.


Executed automatically at login.


Source of home directories for ~name abbreviations.


Specifies the name of a file to read when a new Korn shell is created.

4.2.2 Filename Metacharacters


Match any string of zero or more characters.


Match any single character.


Match any one of the enclosed characters; a hyphen can be used to specify a range (e.g., a-z, A-Z, 0-9).


Match any character not enclosed as above.

In the Korn shell:


Match zero or one instance of pattern.


Match zero or more instances of pattern.


Match one or more instance of pattern.


Match exactly one instance of pattern.


Match any strings that don't contain pattern.


HOME directory of the current user.


HOME directory of user name.


Current working directory (PWD).


Previous working directory (OLDPWD).

The pattern above can be a sequence of patterns separated by |, meaning that the match applies to any of the patterns. This extended syntax resembles that available to egrep and awk. Examples

$ ls new*	List new and new.1.
$ cat ch?	Match ch9 but not ch10.
$ vi [D-R]*	Match files that begin with uppercase D through R.
$ cp !(Junk*|Temp*)*.c ..	Korn shell only.  Copy C source files
		except for Junk and Temp files.

4.2.3 Quoting

Quoting disables a character's special meaning and allows it to be used literally, as itself. The following characters have special meaning to the Bourne and Korn shells:


Command separator.


Background execution.

( )

Command grouping.



> < &

Redirection symbols.

* ? [ ] ~ + - @ !

Filename metacharacters.

" ' \

Used in quoting other characters.


Command substitution.


Variable substitution (or command substitution).

newline space tab

Word separators.

The characters below can be used for quoting:

" "

Everything between " and " is taken literally, except for the following characters that keep their special meaning:


Variable substitution will occur.


Command substitution will occur.


This marks the end of the double quote.

' '

Everything between ' and ' is taken literally except for another '.


The character following a \ is taken literally. Use within " " to escape ", $, and `. Often used to escape itself, spaces, or newlines. 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

4.2.4 Command Forms

cmd &

Execute cmd in background.

cmd1 ; cmd2

Command sequence; execute multiple cmds on the same line.

(cmd1 ; cmd2)

Subshell; treat cmd1 and cmd2 as a command group.

cmd1 | cmd2

Pipe; use output from cmd1 as input to cmd2.

cmd1 `cmd2`

Command substitution; use cmd2 output as arguments to cmd1.

cmd1 $(cmd2)

Korn-shell command substitution; nesting is allowed.

cmd1 && cmd2

AND; execute cmd1 and then (if cmd1 succeeds) cmd2.

cmd1 || cmd2

OR; execute either cmd1 or (if cmd1 fails) cmd2.

{ cmd1 ; cmd2 }

Execute commands in the current shell. Examples

$ nroff file &	Format in the background.
$ cd; ls	Execute sequentially.
$ (date; who; pwd) > logfile	All output is redirected.
$ sort file | pr -3 | lp		Sort file, page output, then print.
$ vi `grep -l ifdef *.c`		Edit files found by grep.
$ egrep '(yes|no)' `cat list`	Specify a list of files to search.
$ egrep '(yes|no)' $(cat list)	Korn shell version of previous.
$ egrep '(yes|no)' $(<list)	Same, but faster.
$ grep XX file && lp file		Print file if it contains the pattern,
$ grep XX file || echo "XX not found"	otherwise, echo an error message.

4.2.5 Redirection Forms









Standard Input




Standard Output




Standard Error



The usual input source or output destination can be changed as follows: Simple Redirection

cmd > file

Send output of cmd to file (overwrite).

cmd >> file

Send output of cmd to file (append).

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 is usually typed on the screen or in the shell program. Commands that typically use this syntax include cat, echo, ex, and sed. (If <<- is used, leading tabs are ignored when comparing input with end-of-input text marker.) This command form is sometimes called a "Here" document. Redirection Using File Descriptors

cmd >&n

Send cmd output to file descriptor n.

cmd m>&n

Same, except that output that would normally go to file descriptor m is sent to file descriptor n instead.

cmd >&-

Close standard output.

cmd <&n

Take input for cmd from file descriptor n.

cmd m<&n

Same, except that input that would normally come from file descriptor m comes from file descriptor n instead.

cmd <&-

Close standard input. Multiple Redirection

cmd 2>file

Send standard error to file; standard output remains the same (e.g., the screen).

cmd > file 2>&1

Send both standard error and standard output to file.

(cmd > f1) 2>f2

Send standard output to file f1; standard error to file f2.

cmd | tee files

Send output of cmd to standard output (usually the terminal) and to files. (See the example in Section 2 under tee.)

No space should appear between file descriptors and a redirection symbol; spacing is optional in the other cases. Examples

$ cat part1 > book
$ cat part2 part3 >> book
$ mail tim < report
$ sed 's/^/XX /g' << END_ARCHIVE
> 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.
XX This is often how a shell archive is "wrapped",
XX bundling text for distribution.  You would normally
XX run sed from a shell program, not from the command line.

To redirect standard output to standard error:

$ echo "Usage error:  see administrator" 1>&2

The following command will send output (files found) to filelist and send error messages (inaccessible files) to file no_access:

$ (find / -print > filelist) 2>no_access

4.2.6 Coprocesses

Coprocesses are a feature of the Korn shell only.

cmd1 | cmd2 |&

Coprocess; execute the pipeline in the background. The shell sets up a two-way pipe, allowing redirection of both standard input and standard output.

read -p var

Read coprocess input into variable var.

print -p string

Write string to the coprocess.

cmd <&p

Take input for cmd from the coprocess.

cmd >&p

Send output of cmd to the coprocess. Examples

ed - memo |&	Start coprocess.
print -p /word/	Send ed command to coprocess.
read -p search	Read output of ed command into variable search.
print "$search"	Show the line on standard output.
A word to the wise.

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