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linuxbashshellbackground-process

Running a process in the background with input/output redirection

I'm curious to know if it makes a difference where the '&' operator is used in code when a process has input/output redirection to run a process in the background

What are the differences/are there any differences between these lines of code in terms of running the process in the background. If there are, how can I determine what the differences are going to be?

setsid python script.py < /dev/zero &> log.txt &

setsid python script.py < /dev/zero & > log.txt &

setsid python script.py < /dev/zero > log.txt &

setsid python script.py & < /dev/zero > log.txt
Solution

  • Control operator

    There are two uses of & here. One is as a so-called control operator. Every command is terminated by a control operator such as &, ; or <newline> . The difference between them is that ; and <newline> run the command in the foreground and & does it in the background.

    setsid python script.py < /dev/zero & > log.txt &
setsid python script.py & < /dev/zero > log.txt

    These two lines, therefore, actually execute two commands each. The first is equivalent to the two commands:

    setsid python script.py < /dev/zero &
> log.txt &

    And the second is equivalent to:

    setsid python script.py &
< /dev/zero > log.txt

    If you're wondering, yes, > log.txt and < /dev/zero > log.txt are both legal commands. Lacking a command name, they simply process the redirections: each one creates an empty file called log.txt.

    Redirection

    setsid python script.py < /dev/zero &> log.txt &

    This version with &> is different from the one with & >. &> without a space is a special redirection operator in bash that redirects both stdout and stderr.

    setsid python script.py < /dev/zero > log.txt &

    This final version is similar to the previous one except it only redirects stdout to log.txt. stderr continues to go to the terminal.