PS3 The value of this variable is used as the prompt for the select command. Defaults to "'#? '"
PS4 The value is the prompt printed before the command line is echoed when the -x option is set; defaults to "'+ '" .
PWD The current working directory as set by the cd built-in command.
RANDOM Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to this variable seeds the random number generator.
REPLY The default variable for the read built-in.
SECONDS This variable expands to the number of seconds since the shell was started.
SHELLOPTS A colon-separated list of enabled shell options.
SHLVL Incremented by one each time a new instance of Bash is started.
TIMEFORMAT The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed.
TMOUT If set to a value greater than zero, TMOUT is treated as the default timeout for the read built-in. In an interative shell, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt when the shell is interactive. Bash terminates after that number of seconds if input does not arrive.
UID The numeric, real user ID of the current user.
Check the Bash man, info or doc pages for extended information. Some variables are read-only, some are set automatically and some lose their meaning when set to a different value than the default.
