kill

How to “pause” (suspend) an active process

 in GNU/Linux  tagged CONT / kill / pid / resume / SIGCONT / SIGSTOP / SIGTSTP / stop / suspend / TSTP by

Recently, we were executing the following time-wasting application and we wanted to pause it somehow and release the CPU that was being used temporarily for other tasks.
Unfortunately, the process was not executing on an active console, so we could not press CTRL+Z and suspend it.
Conveniently, the kill command provides us with the suspend functionality as long as we know the PID of the process to be suspended.

Using ps x, we found the PID of the application even though it was not attached to an active console.

Then to suspend the application, we used

kill -TSTP "$PID";

which instructed the process to stop by sending it the SIGTSTP signal.

Fortunately, our application did not block the signal and it was suspended.

Note: In case an application ignores the SIGTSTP signal, you can still force it to suspend by sending it the SIGSTOP signal as follows

kill -STOP "$PID";

After we were done, we resumed the execution of the process by sending the SIGCONT signal to it

kill -CONT "$PID";

 

Status of an executing dd 1

 in Bash  tagged dd / kill / pgrep / status / watch by

Recently, we were cloning a large hard disk on another using dd.
This operation took a really long time, at some point we got curious on what the status of the execution was.
Due to the minimal output dd offers, there was no indication for us whether the system was still copying and if it had a long way to go or not.

Fortunately, the developers of dd added a feature where sending a USR1 signal to a running dd process makes it print I/O statistics to standard error and then resume copying.

To achieve that we used a second terminal and followed these steps:

  1. We used pgrep to look up the running process based on its name and get the dd running process ID (PID): pgrep ^dd$ .
  2. We passed that PID to kill -USR1 which triggered the printing of the statistics on the terminal where dd was executing: kill -USR1 $(pgrep ^dd$).

Solution

kill -USR1 $(pgrep ^dd$);

Bonus

Additionally, we wanted to have dd statistics printed automatically every minute.
To achieve that, we used watch.  watch executes a program periodically, showing it’s output in full-screen.
We defined the interval in seconds using the parameter -n. (Please note that, the command will not allow less than 0.1 second interval.)

In the end, our command became as follows:

watch -n 60 kill -USR1 $(pgrep ^dd$)

The above command was sending a USR1 signal to dd via the kill application every minute (60 seconds) forcing it to print on standard output the I/O statistics.

Example

On terminal 1, we executed the command dd if=/dev/sda of=/dev/sdb;, which will copy disk sda over sdb.

On terminal 2, we executed the command kill -USR1 $(pgrep ^dd$);, which forced dd to print I/O statistics back on terminal 1.

0+49728 records in
7218+0 records out
3695616 bytes (3.7 MB) copied, 2.85812 s, 1.3 MB/s
0+78673 records in
11443+0 records out
5858816 bytes (5.9 MB) copied, 4.49477 s, 1.3 MB/s
0+99003 records in
14386+0 records out
7365632 bytes (7.4 MB) copied, 5.75575 s, 1.3 MB/s
^C0+172104 records in
24918+0 records out
12758016 bytes (13 MB) copied, 10.197 s, 1.3 MB/s

Bash: Close a range of open sockets by killing the PIDs that are holding them open

 in Bash / GNU/Linux  tagged awk / kill / lsof / pid / port / socket / tail by

Sometimes you want to use a specific port number but some other process(es) is using it. To get the control of the port you need to terminate all other processes and free it.
To find out which process(es) you need to kill, use lsof -i :port. It will return a list of each command and PID that is using the specific port. After that kill those PID using kill -s 9.

The following script will accept a range of ports to free, and for each it will try to kill all processes that are holding them blocked.

low=12345;
high=12350;
for i in `seq $low $high`; do
  lsof -i :$i | tail -n +2 | awk '{system("kill -s 9 " $2)}';
done

Using tail -n +2 we skip the first line of the input which would be the header information.
The system method will invoke a new sh shell and execute the command in it.
Using kill -s 9 we signal the processes that they have to terminate immediately.