| #
| Number of arguments given to current process.
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| -
| Options given to shell on invocation.
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| ?
| Exit status of previous command.
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| $
| Process ID of shell process.
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| _
| Last argument to previous command.
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| !
| Process ID of last background command.
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| CDPATH
| List of directories for cd command to search.
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| COLUMNS
| Width of display in columns (for editing modes and select).
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| EDITOR
| Used to set editing mode; also used by mail and other programs.
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| ERRNO
| A
| Error number of last system call that failed.
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| ENV
| Name of file to run as environment file when shell is invoked.
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| FCEDIT
| Default editor for fc command.
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| FPATH
| Search path for autoloaded functions.
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| IFS
| Internal field separator: list of characters that act as word separators. Normally set to SPACE, TAB, and NEWLINE.
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| HISTFILE
| Name of command history file.
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| HISTSIZE
| Number of lines kept in history file.
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| HOME
| Home (login) directory.
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| LINENO
| Number of line in script or function that just ran.
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| LINES
| Height of display in lines (for select command).
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| MAIL
| Name of file to check for new mail.
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| MAILCHECK
| How often (in seconds) to check for new mail.
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| MAILPATH
| List of file names to check for new mail, if MAIL is not set.
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| OLDPWD
| Previous working directory.
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| OPTARG
| Argument to option being processed by getopts.
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| OPTIND
| Number of first argument after options.
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| PATH
| Search path for commands.
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| PS1
| Primary command prompt string.
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| PS2
| Prompt string for line continuations.
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| PS3
| Prompt string for select command.
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| PS4
| Prompt string for xtrace option.
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| PPID
| Process ID of parent process.
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| PWD
| Current working directory.
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| RANDOM
| Random number between 0 and 32767 (2-1).
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| REPLY
| User's response to select command; result of read command if no variable names given.
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| SECONDS
| Number of seconds since shell was invoked.
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| SHELL
| Full pathname of shell.
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| TMOUT
| If set to a positive integer, number of seconds between commands after which shell automatically terminates.
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| VISUAL
| Used to set editing mode. |