make: Terminal Output
13.2 Synchronized Terminal Output
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Normally GNU 'make' will invoke all commands with access to the same
standard and error outputs that 'make' itself was started with. A
number of tools will detect whether the output is a terminal or
not-a-terminal, and use this information to change the output style.
For example if the output goes to a terminal the tool may add control
characters that set color, or even change the location of the cursor.
If the output is not going to a terminal then these special control
characters are not emitted so that they don't corrupt log files, etc.
The '--output-sync' (
Output During Parallel Output Parallel
Output.) option will defeat the terminal detection. When output
synchronization is enabled GNU 'make' arranges for all command output to
be written to a file, so that its output can be written as a block
without interference from other commands. This means that all tools
invoked by 'make' will believe that their output is not going to be
displayed on a terminal, even when it will be (because 'make' will
display it there after the command is completed).
In order to facilitate tools which would like to determine whether or
not their output will be displayed on a terminal, GNU 'make' will set
the 'MAKE_TERMOUT' and 'MAKE_TERMERR' environment variables before
invoking any commands. Tools which would like to determine whether
standard or error output (respectively) will be displayed on a terminal
can check these environment variables to determine if they exist and
contain a non-empty value. If so the tool can assume that the output
will (eventually) be displayed on a terminal. If the variables are not
set or have an empty value, then the tool should fall back to its normal
methods of detecting whether output is going to a terminal or not.
The content of the variables can be parsed to determine the type of
terminal which will be used to display the output.
Similarly, environments which invoke 'make' and would like to capture
the output and eventually display it on a terminal (or some display
which can interpret terminal control characters) can set these variables
before invoking 'make'. GNU 'make' will not modify these environment
variables if they already exist when it starts.
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