Process information

$$

Process ID of shell

$PPID

Process ID of shell’s parent process.

Quirk Warning:For subshells, the value of PPID is inherited from the parent shell. Thus, PPID is only the parent of the outermost shell process.

$?

Exit status of last command.

$_

Name of last command.

$!

Process ID of last process run in the background using ampersand (&) operator. This is commonly used in conjunction with the wait builtin.

$PATH

A colon-delimited list of locations where trusted executables are installed. Any executable in one of these locations can be executed without specifying a complete path.

Field and record parsing

$IFS

Input Field Separators (uses are explained in Variable Expansion and Field Separators)

User information

$HOME

The user’s home directory.

$UID

The user’s ID.

Security Warning:This value can be modified by the calling script, so it should not be used for authentication purposes.

$USER

The user’s (short) login name.

Security Warning:This value can be modified by the calling script, so it should not be used for authentication purposes.

Miscellaneous Variables

$#

Number of arguments passed to the shell. This variable is described further in Handling Flags and Arguments.

$@

Complete list of arguments passed to the shell, separated by spaces.. This variable is described further in Handling Flags and Arguments.

$*

Complete list of arguments passed to the shell, separated by the first character of the IFS (input field separators) variable. This variable is described further in Handling Flags and Arguments.

$-

A list of all shell flags currently enabled.

$PWD

The current working directory. Equivalent to executing the pwd command.