Home » Why use “nohup &” rather than “exec &”

Why use “nohup &” rather than “exec &”

Solutons:


What’s better, a fish or a bicycle? 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 & => 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.

The shell built in command exec <command> replaces the shell with <command>, no new process, no new PID is created. After completion of <command> normally your terminal will close. By running it in the background first a subshell is created, which then similarly is immediately replaced by <command>.

The nohup <command> command will run <command> but immume to hangups (kill -s 1) so it will not be terminated when the shell, the terminal from which it was started is, is closed. By running it in the background first a subshell is created and the command runs in the background, returning you to the prompt.

In scripting the immediate effect is more or less the same though, <command> is started by your script and the script will continue without waiting for <command> to start, to send output or to complete.

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