What s the difference between nohup and ampersand

Question

Both nohup myprocess out  amp  or myprocess out  amp  set myprocess out to run in the background  After I shutdown the terminal  the process is still running  What s the difference between them

User · Answer

Correct me if I m wrong     nohup myprocess out  amp    nohup catches the hangup signal  which mean it will send a process when terminal closed     myprocess out  amp    Process can run but will stopped once the terminal is closed   nohup myprocess out   Process able to run even terminal closed  but you are able to stop the process by pressing ctrl   z in terminal  Crt  z not working if  amp  is existing

User · Answer

Using the ampersand   amp   will run the command in a child process  child to the current bash session   However  when you exit the session  all child processes will be killed   using nohup   ampersand   amp   will do the same thing  except that when the session ends  the parent of the child process will be changed to  1  which is the  init  process  thus preserving the child from being killed

User · Answer

myprocess out  amp  would run the process in background using a subshell  If the current shell is terminated  say by logout   all subshells are also terminated so the background process would also be terminated  The nohup command ignores the HUP signal and thus even if the current shell is terminated  the subshell and myprocess out would continue to run in the background  Another difference is that  amp  alone doesn t redirect the stdout stderr so if there are any output or error  those are displayed on the terminal  nohup on the other hand redirect the stdout stderr to nohup out or  HOME nohup out

User · Answer

nohup catches the hangup signal  see man 7 signal  while the ampersand doesn t  except the shell is confgured that way or doesn t send SIGHUP at all    Normally  when running a command using  amp  and exiting the shell afterwards  the shell will terminate the sub-command with the hangup signal  kill -SIGHUP  lt pid gt    This can be prevented using nohup  as it catches the signal and ignores it so that it never reaches the actual application    In case you re using bash  you can use the command shopt   grep hupon to find out whether your shell sends SIGHUP to its child processes or not  If it is off  processes won t be  terminated  as it seems to be the case for you  More information on how bash terminates applications can be found here   There are cases where nohup does not work  for example when the process you start reconnects the SIGHUP signal  as it is the case here

User · Answer

Most of the time we login to remote server using ssh  If you start a shell script and you logout then the process is killed  Nohup helps to continue running the script in background even after you log out from shell   Nohup command name  amp  eg  nohup sh script sh  amp    Nohup catches the HUP signals  Nohup doesn t put the job automatically in the background  We need to tell that explicitly using

User · Answer

The nohup command is a signal masking utility and catches the hangup signal  Where as ampersand doesn   t catch the hang up signals  The shell will terminate the sub command with the hang up signal when running a command using  amp  and exiting the shell  This can be prevented by using nohup  as it catches the signal  Nohup command accept hang up signal which can be sent to a process by the kernel and block them   Nohup command is helpful in when a user wants to start long running application log out or close the window in which the process was initiated  Either of these actions normally prompts the kernel to hang up on the application  but a nohup wrapper will allow the process to continue  Using the ampersand will run the command in a child process and this child of the current bash session  When you exit the session  all of the child processes of that process will be killed  The ampersand relates to job control for the active shell  This is useful for running a process in a session in the background

User · Answer

There are many cases when small differences between environments can bite you  This is one into which I have ran recently  What is the difference between these two commands   1     nohup myprocess out  amp  2     myprocess out  amp    The answer is the same as usual - it depends   nohup catches the hangup signal while the ampersand does not   What is the hangup signal   SIGHUP - hangup detected on controlling terminal or death of controlling process  value  1    Normally  when running a command using  amp  and exiting the shell afterwards  the shell will terminate the sub-command with the hangup signal  like kill -SIGHUP  PID   This can be prevented using nohup  as it catches the signal and ignores it so that it never reaches the actual application   Fine  but like in this case there are always    buts     There is no difference between these launching methods when the shell is configured in a way where it does not send SIGHUP at all   In case you are using bash  you can use the command specified below to find out whether your shell sends SIGHUP to its child processes or not       shopt   grep hupon   And moreover - there are cases where nohup does not work  For example  when the process you start reconnects the NOHUP signal  it is done inside  on the application code level    In the described case  lack of differences bit me when inside a custom service launching script there was a call to a second script which sets up and launches the proper application without a nohup command   On one Linux environment everything worked smoothly  on a second one the application quit as soon as the second script exited  detecting that case  of course took me much more time then you might think  stuck out tongue     After adding nohup as a launching method to second script  application keeps running even if the scripts will exit and this behavior became consistent on both environments

[linux] What's the difference between nohup and ampersand

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