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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

/etc/profile

Executed automatically at login.

$HOME/.profile

Executed automatically at login.

/etc/passwd

Source of home directories for ~name abbreviations.

$ENV

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.

[abc...]

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

[!abc...]

Match any character not enclosed as above.

In the Korn shell:

?(pattern)

Match zero or one instance of pattern.

*(pattern)

Match zero or more instances of pattern.

+(pattern)

Match one or more instance of pattern.

@(pattern)

Match exactly one instance of pattern.

!(pattern)

Match any strings that don't contain pattern.

~

HOME directory of the current user.

~name

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.

4.2.2.1 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.

|

Pipe.

> < &

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.

4.2.3.1 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.

4.2.4.1 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

File

Common

Typical

Descriptor

Name

Abbreviation

Default

0

Standard Input

stdin

Keyboard

1

Standard Output

stdout

Terminal

2

Standard Error

stderr

Terminal

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

4.2.5.1 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.

4.2.5.2 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.

4.2.5.3 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.

4.2.5.4 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.
> END_ARCHIVE
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.

4.2.6.1 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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