Variables (Linux in a Nutshell, 4th Edition)

7.3.1. Variable Substitution

In the following substitutions, braces ({ }) are optional, except when needed to separate a variable name from following characters that would otherwise be considered part of the name.

Variable	Meaning
`${`var`}`	Value of variable var.
`$0`	Name of the program.
`${`n`}`	Individual arguments on command line (positional parameters); 1 ≤ n ≤ 9.
`$#`	Number of arguments on command line.
`$*`	All arguments on command line.
`$@`	Same as $*, but contents are split into words when the variable is enclosed in double quotes.
`$$`	Process number of current shell; useful as part of a filename for creating temporary files with unique names.
`$?`	Exit status of last command (normally 0 for success).
`$!`	Process number of most recently issued background command.
`$-`	Current execution options (see the set built-in command). By default, hB for scripts and himBH for interactive shells.
`$_`	Initially set to name of file invoked for this shell, then set for each command to the last word of the previous command.

Tables Table 7-16 through Table 7-18 show various types of operators that can be used with bash variables.

Table 7-16. Substitution operators

Operator	Substitution
${varname:-word}	If varname exists and isn't null, return its value; otherwise, return word.
Purpose:	Returning a default value if the variable is undefined.
Example:	${count:-0} evaluates to 0 if count is undefined.
${varname:=word}	If varname exists and isn't null, return its value; otherwise set it to word and then return its value. Positional and special parameters cannot be assigned this way.
Purpose:	Setting a variable to a default value if it is undefined.
Example:	${count:=0} sets count to 0 if it is undefined.
${varname:?message}	If varname exists and isn't null, return its value; otherwise, print varname: followed by message, and abort the current command or script (noninteractive shells only). Omitting message produces the default message "parameter null or not set."
Purpose:	Catching errors that result from variables being undefined.
Example:	{count:?"undefined"} prints "count: undefined" and exits if count is undefined.
${varname:+word}	If varname exists and isn't null, return word; otherwise, return null.
Purpose:	Testing for the existence of a variable.
Example:	${count:+1} returns 1 (which could mean true) if count is defined.
${#varname}	Return the number of characters in the value of varname.
Purpose:	Preparing for substitution or extraction of substrings.
Example:	If ${USER} currently expands to root, ${#USER} expands to 4.

Table 7-17. Pattern-matching operators

Operator	Meaning
${variable#pattern}	If the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest.
${variable##pattern}	If the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest.
${variable%pattern}	If the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest.
${variable%%pattern}	If the pattern matches the end of the variable's value, delete the longest part that matches and return the rest.
${var/pat/sub}	Return var with the first occurrence of pat replaced by sub. Can be applied to $* or $@, in which case each word is treated separately. If pat starts with # it can match only the start of var; if pat ends with % it can match only the end of var.
${var//pat/sub}	Return var with every occurrence of pat replaced by sub.
${variable:n}	Truncate the beginning of the variable and return the part starting with character number n, where the first character is 0.
${variable:n:l}	Starting with character number n, where the first character is 0, return a substring of length l from the variable.

Table 7-18. Expression evaluation

Operator	Meaning
$((arithmetic-expression))	Return the result of the expression. Arithmetic operators are described under Section 7.4.
Example:	TODAY=`date +%-d`; echo $(($TODAY+7)) stores the number of the current day in $TODAY and then prints that number plus 7 (the number of the same day next week).
[[$condition]]	Return 1 if condition is true and 0 if it is false. Conditions are described under the test built-in command.

7.3.2. Built-in Shell Variables

Built-in variables are set automatically by the shell and are typically used inside shell scripts. Built-in variables can use the variable substitution patterns shown earlier. When setting variables, you do not include dollar signs, but when referencing their values later, the dollar signs are necessary.

Tables Table 7-19 through Table 7-22 show the commonly used built-in variables in bash.

Table 7-19. Behavior-altering variables

Variable	Meaning
auto_resume	Allows a background job to be brought to the foreground simply by entering a substring of the job's command line. Values can be substring (resume if the user's string matches part of the command), exact (string must exactly match command), or another value (string must match at beginning of command).
BASH_ENV	Startup file of commands to execute, if bash is invoked to run a script.
CDPATH	Colon-separated list of directories to search for the directory passed in a cd command.
EDITOR	Pathname of your preferred text editor.
IFS	Word separator; used by shell to parse commands into their elements. The default separators are space, tab, and newline.
IGNOREEOF	If nonzero, don't allow use of a single Ctrl-D (the end-of-file or EOF character) to log off; use the exit command to log off.
PATH	Colon-separated list of directories to search for each command.
PROMPT_COMMAND	Command that bash executes before issuing a prompt for a new command.
PS1	Prompt displayed before each new command; see Section 7.5.4 for ways to introduce into the prompt dynamically changing information such as the current working directory or command history number.
PS2	Prompt displayed before a new line if a command is not finished.
PS3	Prompt displayed by select built-in command.
PS4	Prompt displayed by -x debugging (see Section 7.1) and the set built-in command).

Table 7-20. History variables

Variable	Meaning
FCEDIT	Pathname of editor to use with the fc command.
HISTCMD	History number of the current command.
HISTCONTROL	If HISTCONTROL is set to the value of ignorespace, lines beginning with a space are not entered into the history list. If set to ignoredups, lines matching the last history line are not entered. Setting it to ignoreboth enables both options.
HISTFILE	Name of history file on which the editing modes operate.
HISTFILESIZE	Maximum number of lines to store in the history file. The default is 500.
HISTSIZE	Maximum number of commands to remember in the command history. The default is 500.

Table 7-21. Mail variables

Variable	Meaning
MAIL	Name of file to check for incoming mail.
MAILCHECK	How often, in seconds, to check for new mail (default is 60 seconds).
MAILPATH	List of filenames, separated by colons (:), to check for incoming mail.

Table 7-22. Status variables

Variable	Meaning
BASH	Pathname of this instance of the shell you are running.
BASH_VERSION	Version number of the shell you are running.
COLUMNS	Number of columns your display has.
DIRSTACK	List of directories manipulated by pushd and popd commands.
EUID	Effective user ID of process running this shell, in the form of the number recognized by the system.
GROUPS	Groups to which user belongs, in the form of the numbers recognized by the system.
HOME	Name of your home (login) directory.
HOSTNAME	Host the shell is running on.
HOSTTYPE	Short name indicating the type of machine the shell is running on; for instance, i486.
LINES	The number of lines your display has.
MACHTYPE	Long string indicating the machine the shell is running on; for instance, i486-pc-linux-gnu.
OLDPWD	Previous directory before the last cd command.
OSTYPE	Short string indicating the operating system; for instance, linux-gnu.
PPID	Process ID of parent process that invoked this shell.
PWD	Current directory.
SECONDS	Number of seconds since the shell was invoked.
SHELL	Pathname of the shell you are running.
SHLVL	Depth to which running shells are nested.
TERM	The type of terminal that you are using.
UID	Real user ID of process running this shell, in the form of the number recognized by the system.

7.3. Variables