Category: Documentation

Here is the final summary.

Today I was fuzzied by the help content of the command ‘timeout’, as its name suggest, it a COMMAND, and kill it if still running after a specified period of time. Because of its termination action, I met something related to signals. Googled and finally make a summary here, that is, what’s the differences among HUP, INT, KILL, TERM, USR1 signal.
Everytime when we shutdown our Linux box, it will show “Sending all processes the TERM signal” then “Sending all processes the KILL signal”. Definatly, the TERM signal can be more graceful than KILL, but the best thing we should do now might be looking up the manual page.
In the manual page of kill(7), we get something like this (I’ve ignored things not closely connected to our topic, same below):
Name    Num    Action    Description
————————————————————-
HUP        1        exit
INT        2        exit
KILL        9        exit        cannot be blocked
TERM    15        exit
USR1            exit
Seems not so much information provided. Anyway, we can know they are all related to the process exit action,and the KILL (9) signal cannot be blocked, in other words it cannot be caught by a process so they don’t have the chance to have actions to block it.

Then I searched Google and found some hints from Apache httpd documentation.In the section of Stopping and Restarting, we can see content which have the meaning like this (all signals should be sent to a parent process in this place):
TERM – stop now: The parent immediately attempt to kill off all of its children. It may take it several seconds to complete killing off its children. Then the parent itself exits. Any requests in progress are terminated, and no further requests are served.
USR1 – graceful restart: The parent “advise” the children to exit after their current request (or to exit immediately if they’re not serving anything). The parent re-reads its configuration files and re-opens its log files. As each child dies off the parent replaces it with a child from the new generation of the configuration, which begins serving new requests immediately.
HUP – restart: The parent kill off its children like in TERM, but the parent doesn’t exit. It re-reads its configuration files, and re-opens any log files. Then it spawns a new set of children and continues serving hits.

But things provided still cannot explain our question. Ah, from the last line of manual page of kill(7) we can see there should be another page named ‘signal’. Okay, look up into it and here is what we get :
Signal    Value    Action    Comment
—————————————————
SIGHUP    1        Term    Hangup detected on controlling terminal or death of controlling process
SIGINT    2        Term    Interrupt from keyboard
SIGKILL    9        Term    Kill signal
SIGTERM    15        Term    Termination signal
SIGUSR1    30,10,16    Term    User-defined signal 1
SIGUSR2    31,12,17    Term    User-defined signal 2
Hoo! More information are displayed!

Finally, we can have a short summary:

TERM – Terminate – This signal you send if you want to end a process. It allows the process to clean up nicely though, not like the -9 option (KILL) which just ends everything.
INT – Interrupt – This is a permission which for instance can be used in an NFS environment. If a process hangs, you can interrupt it with say the Ctrl C option.
KILL – exactly what it says. It kills a process without allowing it to clean up, meaning, end all threads, kill child processes etc. It just stops it and can leave either files or other processes in an inconsistant state.
HUP – Hang UP – What this signal does is, when you send it to say the inetd process, it basically tells inetd to go reread it’s configuration file as certain changes have been made which you have to incorporate now. The process doesn’t actually stop, it just, like stated, rereads it’s config.
USR1 – This is a user defined signal. For instance, on our Tru64 systems, we use this signal to tell say the binary log daemon to go save it’s current log file, archive it, and then start a new one. I imagine though it could be used for many other things, thus user defined.

Reference:

1.Manual pages kill(1), signal(7).
2.Apache HTTP Server Documentation Version 2.2 – Stop and Restarting