Section 6.2. Invoking the Shell

6.3. Syntax

This section describes the many symbols peculiar to the Bash and Korn shells . The topics are arranged as follows :

• Special files

• Filename metacharacters

• Quoting

• Command forms

• Redirection forms

• Coprocesses (Korn shell only)

6.3.1. Special Files

Both shells read one or more startup files. Some of the files are read only when a shell is a login shell.

The Korn shell reads these files:

1. /etc/profile . Executed automatically at login, first.

2. ~/.profile . Executed automatically at login, second.

3. $ENV. Specifies the name of a file to read when a new Korn shell is created. ( ksh88 : all shells, ksh93 : interactive shells only.) The value is a variable ( ksh93 : and command and arithmetic) substituted in order to determine the actual filename. Login shells read $ENV after processing /etc/profile and $HOME/.profile .

Bash reads these files:

1. /etc/profile . Executed automatically at login, first.

2. The first file found from this list: ~/.bash_profile , ~/.bash_login , or ~/.profile . Executed automatically at login, second.

3. ~/.bashrc is read by every shell, after the login files. However, if invoked as sh , Bash instead reads $ENV, just as the Korn shell does.

For both shells, the getpwnam( ) and getpwuid( ) functions are are the sources of home directories for ~name abbreviations. (On single-user systems, the user database is stored in /etc/passwd . However on networked systems, this information may come from NIS, NIS+, or LDAPnot your workstation password file.)

6.3.2. Filename Metacharacters

In the Korn shell, or Bash with the extglob option on:

This pattern can be a sequence of patterns separated by , meaning that the match applies to any of the patterns. This extended syntax resembles that available in egrep and awk . In the Korn shell, but not in Bash, if & is used instead of , all the patterns must match. & has higher precedence than .

ksh93 and Bash support the POSIX [[= c =]] notation for matching characters that have the same weight, and [[. c .]] for specifying collating sequences. In addition, character classes, of the form [[: class :]] , allow you to match the following classes of characters.

Bash and ksh93 also accept the [:word:] character class, which is not in POSIX. [[:word:]] is equivalent to [[:alnum:]_] .

6.3.2.1 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

$

pr !(*.ocore)  lp


Print files that are not object files or core dumps

NOTE

On modern systems, ranges such as [D-R] are not portable; the system's locale may include more than just the uppercase letters from D to R in the range.

6.3.3. Quoting

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

These characters can be used for quoting:

" "

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

$

Variable (or command and arithmetic) substitution will occur.

'

Command substitution will occur.

"

This marks the end of the double quote.

' '

Everything between ' and ' is taken literally except for another ' . You cannot embed another ' within such a quoted string.

\

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

$" "

Not ksh88 . Just like " " , except that locale translation is done.

$' '

Not ksh88 . Similar to ' ' , but the quoted text is processed for the following escape sequences:

6.3.3.1 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

6.3.4. Command Forms

6.3.4.1 Examples

$

nroff file > file.txt &


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 '(yesno)' `cat list`


Specify a list of files to search

$

egrep '(yesno)' $(cat list)


POSIX version of previous

$

egrep '(yesno)' $(< list)


Faster, not in POSIX

$

grep XX file && lp file


Print file if it contains the pattern;

$

grep XX file  echo "XX not found"


Otherwise, echo an error message

6.3.5. Redirection Forms

The usual input source or output destination can be changed, as seen in the following sections.

6.3.5.1 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

The contents of the shell script up to a line identical to text become the standard input for cmd ( text can be stored in a shell variable). This command form is sometimes called a Here document . Input is usually typed at the keyboard or in the shell program. Commands that typically use this syntax include cat , ex , and sed . (If <<- is used, leading tabs are stripped from the contents of the here document, and the tabs are ignored when comparing input with the end-of-input text marker.) If any part of text is quoted, the input is passed through verbatim. Otherwise, the contents are processed for variable, command, and arithmetic substitutions.

cmd <<< word

Supply text of word , with trailing newline, as input to cmd . (This is known as a here string , from the free version of the rc shell.) Not ksh88 .

cmd <> file

Open file for reading and writing on the standard input. The contents are not destroyed . ^[*]

^[*] With < , the file is opened read-only, and writes on the file descriptor will fail. With <> , the file is opened read-write; it is up to the application to actually take advantage of this.

cmd > file

Send output of cmd to file (overwrite), even if the shell's noclobber option is set.

6.3.5.2 Redirection using file descriptors

6.3.5.3 Multiple redirection

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

Bash allows multi-digit file descriptor numbers . The other shells do not.

6.3.5.4 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 sends output (files found) to filelist and error messages (inaccessible files) to file no_access :

$

find / -print > filelist 2>no_access

6.3.6. Coprocesses

Coprocesses are a feature of the Korn shell only.

Moving the coprocess input and output file descriptors to standard file descriptors allows you to open multiple coprocesses.

6.3.6.1 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.