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Linux nohup and exec

From Ever changing code

nohup and exec do different things.

  • exec replaces the shell with another program. Using exec in a simple background job isn't useful: exec myprogram; more stuff replaces the shell with myprogram and so doesn't run more stuff, unlike myprogram; more stuff which runs more stuff when myprogram terminates; but exec myprogram & more stuff starts myprogram in the background and then runs more stuff, just like myprogram & more stuff.
  • nohup runs the specificed program with the SIGHUP signal ignored. When a terminal is closed, the kernel sends SIGHUP to the controlling process in that terminal (i.e. the shell). The shell in turn sends SIGHUP to all the jobs running in the background. Running a job with nohup prevents it from being killed in this way if the terminal dies (which happens e.g. if you were logged in remotely and the connection drops, or if you close your terminal emulator).
  • nohup also redirects the program's output to the file nohup.out. This avoids the program dying because it isn't able to write to its output or error output. Note that nohup doesn't redirect the input. To fully disconnect a program from the terminal where you launched it, use
nohup myprogram </dev/null >myprogram.log 2>&1 &

Exec explanation 2

In example exec firefox, the shell is no longer running: it has been replaced by firefox. You can think of exec as combining exiting a program and starting a new one, but keeping the same process ID. The terminal keeps running because nothing told it to stop. When you later exit Firefox, the firefox process terminates. The terminal notices that its child process has exited and so it exits in turn.

  • exec & - executes a process as a background process so you may continue using the same terminal for other jobs.
  • nohup - avoids all SIGHUP(terminate signal) and continues execution even if you terminal is closed.
  • exec - process dies when a SIGHUP is received, but nohup process continues.

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