Is it possible to pre-pend each STDERR with a given string

Question

I am writing a program to interact with a linux machine through the serial port, and I am using pexpect.spawn as my main communication channel as follows:

proc = pexpect.spawn("cu dir -l /dev/ttyUSB0 -s 115200", logfile = *someFile*)

and I am sending commands to the machine with the sendline("cmd") method, and at the end of each session I parse the log file to see how the commands behaved.

I would like to be able to distinguish between lines that were printed to stdout and stderr from my log file, but currently I have no way of doing that. Is that a way to globally prepend each line printed to stderr with a given string?

Answer 1

You don't mention how you capture stdout and stderr, but one simple way distinguish the stdout and stderr is to simply place stdout and stderr in different files. For example:

./command.py >stdout-log 2>stderr-log

Answer 2

I think this is a limitation of pexpect. You're basically dealing with a black box command prompt, so pexpect has no knowledge about whether a string returned to the console (effectively) is stdout or stderr, just that something came back. Can you safely assume a limited set of message and error formats in your system so that you could write some regex-based post-processor?