Syntax (Linux in a Nutshell, 3rd Edition)

7.3. Syntax

This subsection describes the many symbols peculiar to bash. The topics are arranged as follows:

Special files
Filename metacharacters
Command-line editing
Quoting
Command forms
Redirection forms
Coprocesses

7.3.1. Special Files

File	Purpose
/etc/profile	Executed automatically at login
$HOME/.bash_profile	Executed automatically at login
$HOME/.bashrc	Executed automatically at shell startup
$HOME/.bash_logout	Executed automatically at logout
$HOME/.bash_history	Record of last session's commands
/etc/passwd	Source of home directories for ~name abbreviations

7.3.2. Filename Metacharacters

Characters	Meaning
*	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 among the enclosed characters.
{str1,...}	Brace expansion: match any of the enclosed strings.
~name	HOME directory of user name.
~+	Current working directory (PWD).
~-	Previous working directory from directory stack (OLDPWD, see also the pushd built-in command).
~+n	The nth entry in the directory stack, counting from the start of the list with the first entry being 0.
~-n	The nth entry in the directory stack, counting from the end of the list with the last entry being 0.

Patterns can be a sequence of patterns separated by |; if any of the subpatterns match, the entire sequence is considered matching. This extended syntax resembles that available to egrep and awk.

7.3.2.1. 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

7.3.3. Command-line Editing

Command lines can be edited like lines in either Emacs or vi. Emacs is the default. See Section 7.6.1, "Line-Edit Mode" later in this chapter for more information.

vi mode has two submodes, insert mode and command mode. The default mode is insert; you can go to command mode by pressing Esc. In command mode, typing a (append) or i (insert) will return you to insert mode.

Some users discover that the Del or Backspace key on the terminal does not delete the character before the cursor, as it should. Sometimes the problem can be solved by issuing one of the following commands (or placing it in your .bashrc file):

stty erase ^?
stty erase ^H

See the stty command in Chapter 3, "Linux Commands" for more information. On the X Window System, an alternative solution is to use the xmodmap command, but this cannot be described easily here because it requires you to do some research about your terminal.

Table 7-1 through Table 7-14 show various Emacs and vi commands.

Table 7-1. Basic Emacs-Mode Commands

Command	Description
Ctrl-B	Move backward one character (without deleting).
Ctrl-F	Move forward one character.
Del	Delete one character backward.
Ctrl-D	Delete one character forward.

Table 7-2. Emacs-Mode Word Commands

Command	Description
Esc b	Move one word backward.
Esc f	Move one word forward.
Esc Del	Kill one word backward.
Esc d	Kill one word forward.
Ctrl-Y	Retrieve (yank) last item killed.

Table 7-3. Emacs-Mode Line Commands

Command	Description
Ctrl-A	Move to beginning of line.
Ctrl-E	Move to end of line.
Ctrl-K	Kill forward to end of line.

Table 7-4. Emacs-Mode Commands for Moving Through the History File

Command	Description
Ctrl-P	Move to previous line.
Ctrl-N	Move to next line.
Ctrl-R	Search backward.
Esc <	Move to first line of history file.
Esc >	Move to last line of history file.

Table 7-5. Completion Commands

Command	Description
Tab	Attempt to perform general completion of the text.
Esc ?	List the possible completions.
Esc /	Attempt filename completion.
Ctrl-X /	List the possible filename completions.
Esc ~	Attempt username completion.
Ctrl-X ~	List the possible username completions.
Esc $	Attempt variable completion.
Ctrl-X $	List the possible variable completions.
Esc @	Attempt hostname completion.
Ctrl-X @	List the possible hostname completions.
Esc !	Attempt command completion.
Ctrl-X !	List the possible command completions.
Esc Tab	Attempt completion from previous commands in the history list.

Table 7-6. Emacs-Mode Miscellaneous Commands

Command	Description
Ctrl-J	Same as Return.
Ctrl-L	Clear the screen, placing the current line at the top of the screen.
Ctrl-M	Same as Return.
Ctrl-O	Same as Return, then display next line in command history.
Ctrl-T	Transpose character left of and under the cursor.
Ctrl-U	Kill the line from the beginning to point.
Ctrl-V	Insert keypress instead of interpreting it as a command.
Ctrl-[	Same as Esc (most keyboards).
Esc c	Capitalize word under or after cursor.
Esc u	Change word under or after cursor to all capital letters.
Esc l	Change word under or after cursor to all lowercase letters.
Esc .	Insert last word in previous command line after point.
Esc _	Same as Esc.

Table 7-7. Editing Commands in vi Input Mode

Command	Description
Del	Delete previous character.
Ctrl-W	Erase previous word (i.e., erase until a blank).
Ctrl-V	Insert keypress instead of interpreting it as a command.
Esc	Enter control mode (see Table 7-8).

Table 7-8. Basic vi Control Mode Commands

Command	Description
h	Move left one character.
l	Move right one character.
b	Move left one word.
w	Move right one word.
B	Move to beginning of preceding nonblank word.
W	Move to beginning of next nonblank word.
e	Move to end of current word.
E	Move to end of current nonblank word.
0	Move to beginning of line.
^	Move to first nonblank character in line.
$	Move to end of line.

Table 7-9. Commands for Entering vi Input Mode

Command	Description
i	Insert text before current character (insert).
a	Insert text after current character (append).
I	Insert text at beginning of line.
A	Insert text at end of line.
r	Replace current character with this text.
R	Overwrite existing text.

Table 7-10. Some vi-Mode Deletion Commands

Command	Description
dh	Delete one character backward.
dl	Delete one character forward.
db	Delete one word backward.
dw	Delete one word forward.
dB	Delete one nonblank word backward.
dW	Delete one nonblank word forward.
d$	Delete to end-of-line.
d0	Delete to beginning of line.

Table 7-11. Abbreviations for vi-Mode Delete Commands

Command	Description
D	Delete to end of line (equivalent to d$).
dd	Delete entire line (equivalent to 0d$).
C	Delete to end of line; enter input mode (equivalent to c$).
cc	Delete entire line; enter input mode (equivalent to 0c$).
X	Delete character backward (equivalent to dl).
x	Delete character forward (equivalent to dh).

Table 7-12. vi Control Mode Commands for Searching the Command History

Command	Description
k or -	Move backward one line.
j or +	Move forward one line.
G	Move to line given by repeat count.
/string	Search backward for string.
?string	Search forward for string.
n	Repeat search in same direction as previous.
N	Repeat search in opposite direction of previous.

Table 7-13. vi-Mode Character-Finding Commands

Command	Description
fx	Move right to next occurrence of x.
Fx	Move left to previous occurrence of x.
tx	Move right to next occurrence of x, then back one space.
Tx	Move left to previous occurrence of x, then forward one space.
;	Redo last character-finding command.
,	Redo last character-finding command in opposite direction.

Table 7-14. Miscellaneous vi-Mode Commands

Command	Description
`~`	Invert (toggle) case of current character(s).
`-`	Append last word of previous command; enter input mode.
Ctrl-L	Clear the screen and redraw the current line on it; good for when your screen becomes garbled.
`#`	Prepend # (comment character) to the line and send it to the history file; useful for saving a command to be executed later, without having to retype it.

7.3.4. Quoting

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

Character	Meaning
`;`	Command separator
`&`	Background execution
`( )`	Command grouping (enter a subshell)
`{ }`	Command block
`\|`	Pipe
`> < &`	Redirection symbols
`* ? [ ] ~ !`	Filename metacharacters
`" ' \`	Used in quoting other characters
`	Command substitution
`$`	Variable substitution (or command substitution)
`newline space tab`	Word separators
`#`	Comment

The following characters can be used for quoting:

Character Action

" "

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.

7.3.4.1. 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

7.3.5. Command Forms

Syntax	Effect
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`)`	Command substitution; nesting is allowed.
cmd1 `&&` cmd2	AND; execute cmd2 only if cmd1 succeeds.
cmd1 `\|\|` cmd2	OR; execute cmd2 only if cmd1 fails.
`{` cmd1 `;` cmd2 `}`	Execute commands in the current shell.

7.3.5.1. 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)         Same as previous using bash command substitution
$ 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"  Echo an error message if the pattern is not found

7.3.6. Redirection Forms

File Descriptor	Name	Common Abbreviation	Typical Default
0	Standard input	stdin	Keyboard
1	Standard output	stdout	Screen
2	Standard error	stderr	Screen

The usual input source or output destination can be changed as shown in Table 7-15.

Table 7-15. I/O Redirectors

Redirector	Function
> file	Direct standard output to file.
< file	Take standard input from file.
cmd1 \| cmd2	Pipe; take standard output of cmd1 as standard input to cmd2.
>> file	Direct standard output to file; append to file if it already exists.
>\| file	Force standard output to file even if noclobber is set.
n>\| file	Force output from the file descriptor n to file even if noclobber is set.
<> file	Use file as both standard input and standard output.
<< 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 text is enclosed in quotes, standard input will not undergo variable substitution, command substitution, etc.
n> file	Direct file descriptor n to file.
n< file	Set file as file descriptor n.
>&n	Duplicate standard output to file descriptor n.
<&n	Duplicate standard input from file descriptor n.
&>file	Direct standard output and standard error to file.
<&-	Close the standard input.
>&-	Close the standard output.
n>&-	Close the output from file descriptor n.
n<&-	Close the input from file descriptor n.

7.3.6.1. Examples

$ cat part1 > book
$ cat part2 part3 >> book
$ mail tim < report
$ grep Chapter part* 2> error_file

$ sed 's/^/XX /' << 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 sends error messages (inaccessible files) to file no_access:

$ find / -print > filelist 2>no_access

7.3.7. Coprocesses

Coprocesses are a feature of bash and do not appear in other shells.

Syntax	Effect
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.

7.3.7.1. Examples

cat memo                     Print contents of file
Sufficient unto the day is 
A word to the wise.
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.