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 The number of seconds since the time the shell was started.
SHLVL 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 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 is
a security feature that makes it less likely for unattended shells to be accessed by
unauthorized people. (This must be set in the login shell for it to actually cause the
shell to log out the user.)
UID The user ID number assigned to your username. The user ID number is stored in the
/etc/password file.
Setting Your Own Environment Variables
Environment variables can provide a handy way to store bits of information that you use often
from the shell. You can create any variables that you want (avoiding those that are already in use)
so that you can read in the values of those variables as you use the shell.
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