What does this line of bash script do?

Question

nohup tensorflow_model_server \
  --rest_api_port=8501 \
  --model_name=fashion_model \
  --model_base_path="${MODEL_DIR}" >server.log 2>&1

I saw this in tensorflow serving guide here. I want to know the meaning of > server.log 2>&1. I kind of know that it's about saving stdout to a file, and saving stderr to another file which I don't know what it is.

Answer 1

This redirection is meant to also write the stderr output into the server log.

When you use a redirection like

some command > file.txt

It only saves output written by the program to stdout, the standard output stream usually connected to the console.

Most programs, however, adhere to the good practice of writing error messages to a different output stream, stderr, which while also printed on the console, can be "catched" and redirected separately in order to ensure error messages don't get overlooked in "verbose" normal output of the program.

This can however also become a problem: if you only redirect the "normal" output of some lengthy (and thus usually unattended) compile process, e.g., into a file, warnings and errors will only be printed to the console. If now you later come back to inspect the results, you will have standard "everything ok" output in your log file, and seemingly uncorrelated error output in your console.

Therefore, it can be helpful to "also redirect" the error output to the file, so that all messages are "in context".

The mechanism here is to

• first redirect stdout to a file (the command > file part, which explicitly could be written as command 1>file, the 1 indicating the file descriptor of the stdout stream)

• redirect the stderr in a "merging" way to stdout, too (that is why the syntax is 2>&1, the & meaning "copy" the file descriptor), well-knowing that the stdout is already redirected to a file.

A good place for further reading might be

• the Bash reference manual
• the Bash programming introduction How-To