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* Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable. If IFS is null or unset, the parameters are separated by spaces.
@ Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands as a separate word. That is, "$@" is equivalent to "$1""$2" .... When there are no positional parameters, "$@" and $@ expand to nothing (in other words, they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
_ Expands to the current option flags as specified upon invocation, by the set built-in command, or those set by the shell itself (such as the _i flag).
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asynchronous) command.
0 Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, is set to the name of that file. If bash is started with the _c option, then is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the pathname used to invoke bash, as given by argument zero.
_ Expands to the last argument to the previous command, after expansion. Also set to the full pathname of each command executed and placed in the environment exported to that command.

SHELL VARIABLES

The following variables are set by the shell:

PPID The process ID of the shell's parent. PWD The current working directory as set by the cd command. OLDPWD The previous working directory as set by the cd command. REPLY Set to the line of input read by the read built-in command when no arguments are supplied. UID Expands to the user ID of the current user, initialized at shell startup. EUID Expands to the effective user ID of the current user, initialized at shell startup. BASH Expands to the full pathname used to invoke this instance of bash. BASH_VERSION Expands to the version number of this instance of bash. SHLVL Incremented by one each time an instance of bash is started. RANDOM Each time this parameter is referenced, a random integer is generated. The sequence of random numbers may be initialized by assigning a value to RANDOM. If RANDOM is unset, it loses its special properties, even if it is subsequently reset. SECONDS Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it loses its special properties, even if it is subsequently reset. LINENO Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function. When not in a script or function, the value substituted is not guaranteed to be meaningful. When in a function, the value is not the number of the source line that the command appears on (that information has been lost by the time the function is executed), but is an approximation of the number of simple commands executed in the current function. If LINENO is unset, it loses its special properties, even if it is subsequently reset.

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HISTCMD The history number, or index in the history list, of the current command. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
OPTARG The value of the last option argument processed by the getopts built-in command. (See "Shell Built-in Commands," later in this manual page).
OPTIND The index of the next argument to be processed by the getopts built-in command. (See "Shell Built-in Commands.")
HOSTTYPE Automatically set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent.
OSTYPE Automatically set to a string that describes the operating system on which bash is executing. The default is system-dependent.

The following variables are used by the shell. In some cases, bash assigns a default value to a variable; these cases are noted in the following list:

IFS The internal field separator that is used for word splitting after expansion and to split lines into words with the read built-in command. The default value is <space><tab><newline>.
PATH The search path for commands. It is a colon-separated list of directories in which the shell looks for commands. (See "Command Execution," later in this manual page). The default path is system_dependent, and is set by the administrator who installs bash. A common value is /usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:
HOME The home directory of the current user; the default argument for the cd built-in command.
CDPATH The search path for the cd command. This is a colon-separated list of directories in which the shell looks for destination directories specified by the cd command. A sample value is .:~:/usr.
ENV If this parameter is set when bash is executing a shell script, its value is interpreted as a filename containing commands to initialize the shell, as in .bashrc. The value of ENV is subjected to parameter expansion, command substitution, and arithmetic expansion before being interpreted as a pathname. PATH is not used to search for the resultant pathname.
MAIL If this parameter is set to a filename and the MAILPATH variable is not set, bash informs the user of the arrival of mail in the specified file.
MAILCHECK Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for mail, the shell does so before prompting. If this variable is unset, the shell disables mail checking.
MAILPATH A colon-separated list of pathnames to be checked for mail. The message to be printed may be specified by separating the pathname from the message with a question mark (?). $_ stands for the name of the current mailfile. 
          Example:



                   MAILPATH\

                   ='/usr/spool/mail/bfox?"You have

                   mail":~/shell-mail?"$_has mail!"'
bash supplies a default value for this variable, but the location of the user mail files that it uses is system-dependent (for example, /usr/spool/mail/$USER).
MAIL_WARNING If set, and a file that bash is checking for mail has been accessed since the last time it was checked, the message "The mail in mail-file has been read" is printed.
PS1 The value of this parameter is expanded (see "Prompting," later in this manual page) and used as the primary prompt string. The default value is bash\$.
PS2 The value of this parameter is expanded and used as the secondary prompt string. The default is >.

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