When I look in
man bash none of these are defined. However, random posts online refer to them; where do they come from? Or are they just a convention?
When I look in
They are just a convention as much as any other convention.
PAGER are mentioned in the standards as belonging to variables you'd be unwise to conflict with since they are widely used. See Chapter 8, Section 1:
It is unwise to conflict with certain variables that are frequently exported by widely used command interpreters and applications:
... EDITOR ... PAGER ... VISUAL ...
Various programs respect various combinations of them:
man 1 crontab (POSIX):
The following environment variables shall affect the execution of crontab: EDITOR Determine the editor to be invoked when the -e option is specified. The default editor shall be vi.
2. The editor specified by the policy is run to edit the temporary files. The sudoers policy uses the SUDO_EDITOR, VISUAL and EDITOR environment variables (in that order). If none of SUDO_EDITOR, VISUAL or EDITOR are set, the first program listed in the editor sudoers(5) option is used.
man 1 man (POSIX):
ENVIRONMENT VARIABLES The following environment variables shall affect the execution of man: ... PAGER Determine an output filtering command for writing the output to a terminal. Any string acceptable as a command_string operand to the sh -c command shall be valid. When standard output is a terminal device, the reference page output shall be piped through the command. If the PAGER variable is null or not set, the command shall be either more or another paginator utility documented in the system documentation.
It is not surprising that the
bash manual doesn't mention them, as none of the bash builtins that I can think of make use of any of these. However, they are widely used in other utilities, and these three are just the ones that I commonly use.
BROWSER variable is not in the same league as
PAGER - it is not mentioned by the standards. However, some programs may use them, like
man 1 man (Debian):
BROWSER If $BROWSER is set, its value is a colon-delimited list of com- mands, each of which in turn is used to try to start a web browser for man --html. In each command, %s is replaced by a filename containing the HTML output from groff, %% is replaced by a single percent sign (%), and %c is replaced by a colon (:).
It is convention going as far back as I can remember, like 30+ years.
The BSD 4.2 Manual page for mail of 1 April 1983 mentions
PAGER isn't mentioned in BSD 4.2's verion of "man":
If the standard output is a teletype, or if the flag - is given, man pipes its output through cat(1)
But it is in in 4.3 Reno circa 1990.