List of Variables and Metacharacters in Linux - Shell Programming

Shell program is series of Linux commands. Shell script is just like batch file is MS-DOS but have more power than the MS-DOS batch file. Shell script can take input from user, file and output them on screen. Useful to create our own commands that can save our lots of time and to automate some task of day today life.

Variables in Linux

Sometimes to process our data/information, it must be kept in computers RAM memory. RAM memory is divided into small locations, and each location had unique number called memory location/address, which is used to hold our data. Programmer can give a unique name to this memory location/address called memory variable or variable (Its a named storage location that may take different values, but only one at a time).

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In Linux, there are two types of variable:

System variables – Created and maintained by Linux itself. This type of variable defined in CAPITAL LETTERS.
User defined variable (UDV) – Created and maintained by user. This type of variable defined in lower LETTERS.

Predefined Variables

Like most shells, Bash defines some variables when it starts-

S.No.	Name	Value
1	$-	The current shell options assigned from the command line or by the built-in set command discussed later.
2	$$	The process ID of this shell.
3	$!	The process ID of the last background command.
4	$#	The number of positional parameters.
5	$?	The exit value of the last command.
6	$@	An individually quoted list of all the positional parameters.
7	$_	The last parameter of the previous command.
8	$BASH	The full pathname of the Bash executable.
9	$BASH_ENV	Location of Bash’s startup file (default is ~/.bashrc).
10	$BASH_VERSINFO	A read-only array of version information.
11	$BASH_VERSION	Version string.
12	$DIRSTACK	Array defining the directory stack (discussed later).
13	$ENV	If this variable is not set, the shell searches the user’s home directory for the “.profile” startup file when a new login shell is created. If this variable is set, then every new shell invocation runs the script specified by ENV.
14	$EUID	Read-only value of effective user ID of user running Bash.
15	$HISTFILE	Location of file containing shell history (default ~/.bash_history).
16	$HISTFILESIZE	Maximum number of lines allowed in history file (default is 500).
17	$HISTSIZE	Maximum number of commands in history (default is 500).
18	$HOSTNAME	Hostname of machine where Bash is running.
19	$HOSTTYPE	Type of host where Bash in running.
20	$IFS	When the shell tokenizes a command line prior to its execution, it uses the characters in this variable as delimiters. IFS usually contains a space, a tab, and a newline character.
21	$LINES	Used by select to determine how to display the selections.
22	$MAILCHECK	How often (seconds) to check for new mail.
23	$OLDPWD	The previous working directory of the shell.
24	$OSTYPE	Operating system of machine where Bash is running.
25	$PPID	The process ID number of the shell’s parent.
26	$PPID	Read-only process ID of the parent process of Bash.
27	$PS1	This contains the value of the command-line prompt, and is $ by default. To change the command-line prompt, simply set PS1 to a new value.
28	$PS2	This contains the value of the secondary command-line prompt that is displayed when more input is required by the shell, and is > by default. To change the prompt, set PS2 to a new value.
29	$PS3	The prompt used by the select command, #? by default.
30	$PWD	The current working directory of the shell.
31	$RANDOM	A random integer.
32	$REPLY	Set by a select command.
33	$SHLVL	Level of shell (incremented once each time a Bash process is started, it shows how deeply the shell is nested).
34	$UID	Read-only value of user ID of user running Bash.

How to define User defined variables (UDV)

To define UDV use following syntax

Syntax: variablename=value

NOTE: Here ‘value’ is assigned to given ‘variablename’ and Value must be on right side = sign For

e.g.

$no=20 # this is OK
$20=no # Error, NOT Ok, Value must be on right side of = sign.

To define variable called ‘vari’ having value Bus

$vari=CAR

To define variable called n having value 20

$n=20

Rules for naming variable name (Both UDV and System Variable)

Variable name must begin with Alphanumeric character or underscore character (_), followed by one or more Alphanumeric character, For e.g. Valid shell variable are as follows HOME
Don’t put spaces on either side of the equal sign when assigning value to variable.
Variables are case-sensitive, just like filename in Linux.
You can also define NULL variable (NULL variable is variable which has no value at the time of definition) For e.g.
Do not use ?,* etc, to name your variable names.

Metacharacters

Some characters are processed specially by the shell, and are known as metacharacters. All shells share a core set of common metacharacters.

S.No.	Name	Value
1	>	Output redirection; writes standard output to a file.
2	>>	Output redirection; appends standard output to a file.
3	<	Input redirection; reads standard input from a file.
4	*	File substitution wildcard; matches zero or more characters.
5	?	File substitution wildcard; matches any single character.
6	[…]	File substitution wildcard; matches any character between backets.
7	` command `	Command substitution; replaced by the output from command.
8	\|	Pipe symbol; sends the output of one process to the input of another.
9	;	Used to sequence commands.
10	\|\|	Conditional execution; executes a command if the previous one failed.
11	&&	Conditional execution; executes a command if the previous one succeeded.
12	(…)	Groups commands.
13	&	Runs a command in the background.
14	#	All characters that follow up to a newline are ignored by the shell and programs (i.e., a comment).
15	$	Expands the value of a variable.
16	\	Prevents special interpretation of the next character.
17	<<tok	Input redirection; reads standard input from script up to tok.

When you enter a command, the shell scans it for metacharacters and processes them specially. When all metacharacters have been processed, the command is finally executed. To turn off the special meaning of a metacharacter, precede it by a \ character.

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