Common Shell Variables

Variable	Description
BASH	This contains the full pathname of the bash command. This is usually /bin/bash
BASH_VERSION	This is a number representing the current version of the bash command.
EUID	This is the effective user ID number of the current user. It is assigned when the shell starts, based on the user’s entry in the /etc/passwd file
FCEDIT	If set, this variable indicates the text editor used by the fc command to edit history commands. If this variable isn’t set, the vi command is used
HISTFILE	This is the location of your history file. It is typically located at $HOME/. bash_history
HISTFILESIZE	This is the number of history entries that can be stored. After this number is reached, the oldest commands are discarded. The default value is 1000
HISTCMD	This returns the number of the current command in the history list
HOME	This is your home directory. It is your current working directory each time you log in or type the cd command with any options
HOSTTYPE	This is a value that describes the computer architecture on which the Linux system is running. For Intel-compatible PCs, the value is i386, i486, i586, i686, or something like i386-linux. For AMD 64-bit machines, the value is x86_64
MAIL	This is the location of your mailbox file. The file is typically your username in the /var/spool/mail directory
OLDPWD	This is the directory that was the working directory before you changed to the current working directory
OS TYPE	This name identifies the current operating system. For Fedora Linux, the OSTYPE value is either linux or linux-gnu, depending on the type of shell you are using. (Bash can run on other operating systems as well)
PATH	This is the colon-separated list of directories used to find commands that you type. The default value for regular users varies for different distributions but typically includes the following: /bin:/usr/bin:/ usr/local/bin:/usr/bin/X11:/usr/X11R6/bin:~/bin You need to type the full path or a relative path to a command that you want to run which is not in your PATH. For the root user, the value also includes: /sbin, /usr/sbin, and /usr/local/sbin
PPID	This is the process ID of the command that started the current shell (for example, the Terminal window containing the shell)
PROMPT_COMMAND	This can be set to a command name that is run each time before your shell prompt is displayed. Setting PROMPT_COMMAND=date lists the current date/time before the prompt appears
PS1	This sets the value of your shell prompt. There are many items that you can read into your prompt (date, time, username, hostname, and so on). Sometimes a command requires additional prompts, which you can set with the variables PS2, PS3, and so on
PWD	This is the directory that is assigned as your current directory. This value changes each time you change directories using the cd command
RANDOM	Accessing this variable causes a random number to be generated. The number is between 0 and 99999
SECONDS	This is the number of seconds since the time the shell was started
SHLVL	This is the number of shell levels associated with the current shell session. When you log in to the shell, the SHLVL is 1. Each time you start a new bash command (by, for example, using su to become a new user, or by simply typing bash), this number is incremented
TMOUT	This can be set to a number representing the number of seconds the shell can be idle without receiving input. After the number of seconds is reached, the shell exits. This security feature makes it less likely for unattended shells to be accessed by unauthorized people. (This must be set in the login shell for it actually to cause the shell to log out the user)