Redirect the background process output to a pager `less` after finish running not to default terminal

background-processless

By default, if you execute a background process ended with &, like ./test &, the output will be printed in the current terminal.
How can I redirect the output to a pager like less when it finishes running other than output directly?

Best Answer

Simplest way: Redirect the output to a file. When the command has finished executing, view the file in less.

./test.sh >test.out &
less test.out

This is under some circumstances also useful even if the command is not run in the background, for example if compiling a large project:

$ make >make.out 2>make.err

$ make >make.log 2>&1

Then you can, in your own time go through the output to look for errors etc. (or send it to the developer as part of a bug report), without having to scroll up and down in your terminal.

Related Solutions

Background Process – Why Process Isn’t Running in Background Using &

When you close a terminal window, the terminal emulator sends a SIGHUP to the process it is running, your shell. Your shell then forwards that SIGHUP to everything it's running. On your local system, this is the ssh. The ssh then forwards the SIGHUP to what it's running, the remote shell. So your remote shell then sends a SIGHUP to all its processes, your backgrounded program.

There are 2 ways around this.

Disassociate the backgrounded program from your shell.
1. Use the disown command after backgrounding your process. This will make the shell forget about it.
2. Prefix your command with nohup (nohup $python program.py &). This accomplishes the same thing, but by using an intermediate process. Basically it ignores the SIGHUP signal, and then forks & executes your program which inherits the setting, and then exits. Because it forked, the program being launched is not a child of the shell, and the shell doesn't know about it. And unless it installs a signal handler for SIGHUP, it keeps the ignore action anyway.
Use logout (or Ctrl+d) instead of closing the terminal window. When you use logout, this isn't a SIGHUP, and so the shell won't send a SIGHUP to any of its children.

Additionally you must make sure that your program doesn't write to the terminal through STDOUT or STDERR, as both of those will no longer exist once the terminal exits. If you don't redirect them to something like /dev/null, the program will still run, but if it tries to write to them, it'll get a SIGPIPE, and the default action of SIGPIPE is to kill the process).

Bash – redirecting output of running background job in bash

Bash can't modify the file descriptors of a running process.

See answers for How to change the output redirection of a running process? (or a similar thread on stackoverflow)

The easiest and only current tool seems to be reredirect:

reredirect is a utility for taking an existing running program and attaching its outputs (standard output and error output) to files or another process. (quoting project homepage)

reredirect -m /dev/null <PID>

Best Answer

Related Solutions

Background Process – Why Process Isn’t Running in Background Using &

Bash – redirecting output of running background job in bash

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