In Linux you can send signals to a process id to trigger actions for the program. Useful scenario for this is to renew an IP address on a device that uses udhcpc. You should be able to change udhcpc for other programs, you’ll just need to read the help for that specific program.<\/p>\n\n\n\n
In the udhcpc help it says <\/p>\n\n\n\n
Signals:\n USR1 Renew lease\n USR2 Release lease<\/pre>\n\n\n\nBut how do we send those signals to udhcpc? Answer, use the kill command.<\/p>\n\n\n\n
kill: kill [-s sigspec | -n signum | -sigspec] pid | jobspec \u2026 or kill -l [sigspec]\n Send a signal to a job.\n\nSend the processes identified by PID or JOBSPEC the signal named by SIGSPEC or SIGNUM. If neither SIGSPEC nor SIGNUM is present, then SIGTERM is assumed. <\/code>\n\nOptions: <\/code>\n-s sig SIG is a signal name <\/code>\n-n sig SIG is a signal number <\/code>\n-l list the signal names; if arguments follow `-l' they are <\/code>\nassumed to be signal numbers for which names should be listed <\/code>\n-L synonym for -l <\/code>\n\nKill is a shell builtin for two reasons: it allows job IDs to be used<\/code>instead of process IDs, and allows processes to be killed if the limit on processes that you can create is reached<\/code>.\nExit Status:<\/code>\nReturns success unless an invalid option is given or an error occurs. <\/code><\/pre>\n\n\n\nWe see from above that we can pass a signal name in using the -s option.<\/p>\n\n\n\n
So to send USR1 signal to udhcp we do the following<\/p>\n\n\n\n
kill -s USR1 pid_of_udhcpc<\/pre>\n\n\n\nReplace pid_of_udhcpc with the actual pid or use the following command to find the pid<\/p>\n\n\n\n
kill -s USR1 $(pgrep udhcpc)<\/pre>\n\n\n\n“pgrep udhcpc” prints the pid of the searched for process.<\/p>\n\n\n\n
Helpful links
https:\/\/www.thegeekstuff.com\/2011\/02\/send-signal-to-process\/<\/a>
https:\/\/www.linux.org\/threads\/kill-signals-and-commands-revised.11625\/<\/a><\/p>\n\n\n\n<\/p>\n","protected":false},"excerpt":{"rendered":"
In Linux you can send signals to a process id to trigger actions for the program. Useful scenario for this is to renew an IP address on a device that uses udhcpc. You should be able to change udhcpc for … Continue reading