Linux Bible. Christopher Negus

priority of a process.

      Killing processes with kill and killall

      Although usually used for ending a running process, the kill and killall commands can actually be used to send any valid signal to a running process. Besides telling a process to end, a signal might tell a process to reread configuration files, pause (stop), or continue after being paused, just to name a few possibilities.

      Signals are represented by both numbers and names. Signals that you might send most commonly from a command include SIGKILL (9), SIGTERM (15), and SIGHUP (1). The default signal is SIGTERM, which tries to terminate a process cleanly. To kill a process immediately, you can use SIGKILL. The SIGHUP signal can, depending on the program, tell a process to reread its configuration files. SIGSTOP pauses a process, while SIGCONT continues a stopped process.

Different processes respond to different signals. Processes cannot block SIGKILL and SIGSTOP signals, however. Table 6.1 shows examples of some signals (enter man 7 signal to read about other available signals).

      Notice that there are multiple possible signal numbers for SIGCONT and SIGSTOP because different numbers are used in different computer architectures. For most x86 and Power architectures, use the middle value. The first value usually works for Alpha and SPARC, while the last one is for MIPS architecture.

      Using kill to signal processes by PID

      Using commands such as ps and top, you can find processes to which you want to send a signal. Then you can use the process ID of that process as an option to the kill command, along with the signal you want to send.

TABLE 6.1 Signals Available in Linux

Signal	Number	Description
SIGHUP	1	Hang-up detected on controlling terminal or death of controlling process.
SIGINT	2	Interrupt from keyboard.
SIGQUIT	3	Quit from keyboard.
SIGABRT	6	Abort signal from abort(3).
SIGKILL	9	Kill signal.
SIGTERM	15	Termination signal.
SIGCONT	19,18,25	Continue if stopped.
SIGSTOP	17,19,23	Stop process.

For example, you run the top command and see that the bigcommand process is consuming most of your processing power:

        PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 10432 chris 20 0 471m 121m 18m S 99.9 3.2 77:01.76 bigcommand

      Here, the bigcommand process is consuming 99.9 percent of the CPU. You decide that you want to kill it so that other processes have a shot at the CPU. If you use the process ID of the running bigcommand process, here are some examples of the kill command that you can use to kill that process:

       $ kill 10432 $ kill -15 10432 $ kill -SIGKILL 10432

      The default signal sent by kill is 15 (SIGTERM), so the first two examples have exactly the same results. On occasion, a SIGTERM doesn't kill a process, so you may need a SIGKILL to kill it. Instead of SIGKILL, you can use –9 to get the same result.

      Another useful signal is SIGHUP. If, for example, something on your GNOME desktop were corrupted, you could send the gnome-shell a SIGHUP signal to reread its configuration files and restart the desktop. If the process ID for gnome-shell were 1833, here are two ways you could send it a SIGHUP signal:

       # kill -1 1833 # killall -HUP gnome-shell

      Using killall to signal processes by name

      With the killall command, you can signal processes by name instead of by process ID. The advantage is that you don't have to look up the process ID of the process that you want to kill. The potential downside is that you can kill more processes than you mean to if you are not careful. (For example, typing killall bash may kill a bunch of shells that you don't mean to kill.)

Like the kill command, killall uses SIGTERM (signal 15) if you don't explicitly enter a signal number. Also as with kill, you can send any signal you like to the process you name with killall. For example, if you see a process called testme running on your system and you want to kill it, you can simply enter the following:

       $ killall -9 testme

      The killall command can be particularly useful if you want to kill a bunch of commands of the same name.

      Setting processor priority with nice and renice

      When the Linux kernel tries to decide which running processes get access to the CPUs on your system, one of the things it takes into account is the nice value set on the process. Every process running on your system has a nice value between –20 and 19. By default, the nice value is set to 0. Here are a few facts about nice values:

       The lower the nice value, the more access to the CPUs the process has. In other words, the nicer a process is, the less CPU attention it gets. So, a –20 nice value gets more attention than a process with a 19 nice value.

       A regular user can set nice values only from 0 to 19. No negative values are allowed. So a regular user can't ask for a value that gives a process more attention than most processes get by default.

       A regular user can set the nice value higher, not lower. So, for example, if a user sets the nice value on a process to 10 and then later wants to set it back to 5, that action will fail. Likewise, any attempt to set a negative value will fail.

       A regular user can set the nice value only on the user's own processes.

       The root user can set the nice value on any process to any valid value,