Managing Processes Process: A running program

Transcription

1 Managing Processes Process: A running program User Process: The process initiated by a User while logged into a terminal (e.g. grep, find, ls) Daemon Process: These processes are usually initiated on system startup (but can be started manually) Parent process: Child process: Every process can start an unlimited number of processes (called child processes) and as such count as their parent process. The processes initiated by another process. Each child process can have only one parent. Process Attributes A process has a series of characteristics, which can be viewed with the ps command: PID: The process ID, a unique identification number used to refer to the process. PPID: Nice number: The parent process ID, the number of the process (PID) that started this process. the degree of friendliness of a process toward other processes (do confuse with process priority, which is calculated based on this number and recent CPU usage of the process). Terminal or TTY: terminal to which the process is connected. RUID and EUID: name of the real and effective user. The real owner is the user issuing the command, the effective user is the one determining access to system resources. RUID and EUID are usually the same, and the process has the same access rights the issuing user has. Displaying Process Information The ps command is one of the tools for visualizing processes. This command has several options which can be combined to display different process attributes. Common options used with the ps command:

2 With no options specified, ps only gives information about the current shell and interactive or user processes: PID TTY TIME CMD 4245 pts/7 00:00:00 bash 5314 pts/7 00:00:00 ps This does not give much information - generally, at least a hundred processes are running on the system. A good strategy is to select particular processes out of the list of all processes, using the grep command in a pipe, as in this line, which will select and display all processes owned by a particular user (or those with a specific name or those with a specific terminal, and so on) ps -ef grep username [name e.g. bash] [terminal e.g. tty2] Note: ps only gives a momentary state of the active processes, it is a one-time recording. top program: displays a more precise view by updating the results given by ps (with a bunch of options) once every five seconds, generating a new list of the processes causing the heaviest load periodically, meanwhile integrating more information about the swap space in use and the state of the CPU, from the proc file system: top - 11:32:46 up 2:56, 1 user, load average: 0.07, 0.02, 0.00 Tasks: 69 total, 1 running, 68 sleeping, 0 stopped, 0 zombie Cpu(s): 0.0% us, 0.0% sy, 0.0% ni, 100.0% id, 0.0% wa, 0.0% hi, 0.0% si Mem: k total, k used, k free, 68472k buffers Swap: k total, 0k used, k free, k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root S :00.95 init 2 root S :00.00 ksoftirqd/0 3 root RT S :00.00 watchdog/0 4 root S :00.00 events/0 5 root S :00.00 khelper 6 root S :00.00 kthread 8 root S :00.01 kblockd/0 9 root S :00.00 kacpid 88 root S :00.00 khubd 143 root S :00.00 pdflush 144 root S :00.03 pdflush 146 root S :00.00 aio/0 145 root S :00.00 kswapd0 233 root S :00.00 kseriod 309 root S :00.00 kpsmoused 325 root S :00.08 kjournald 392 root S :00.42 udevd uptime command: gives the first line of information yielded by the top command. The data for these programs are stored among others in /var/run/utmp (information about currently connected users) and in the virtual file system /proc: [steve@localhost proc]$ ls -a asound dma iomem loadavg net sysrq-trigger buddyinfo driver ioports locks partitions sysvipc bus execdomains irq mdstat pci tty cmdline fb kallsyms meminfo self uptime cpuinfo filesystems kcore misc slabinfo version crypto fs keys modules stat vmstat devices ide key-users mounts swaps zoneinfo acpi diskstats interrupts kmsg mtrr sys pstree command: displays the relations between processes

3 Killing Processes Although a total of 64 kill signals are listed in the text (pg. 443), the five most common kill signals are listed below. Signal name Signal number Meaning SIGHUP 1 For daemons: reread the configuration file. SIGINT 2 Interrupt the process (Ctrl + c). A process can ignore this signal. SIGQUIT 3 Initiates a core dump (Ctrl +\). SIGTERM 15 Terminate the process in an orderly way. SIGKILL 9 Interrupt the process. A process can not ignore this signal. To send a kill signal to a process, you specify the kill signa. To send as an option to the kill command, followed by the appropriate PID of the process. [root@server1 root]# kill Process Forking An process starts an instance of itself (with a new PID and becomes the parent) to execute a job and the new instance will exit on termination of the job. While the job is being executed the process will assume a WAIT state if the job is running in the foreground, otherwise the parent will continue to run. Running Process in Background Process name + & Running Processes in Foreground Process name + % Background processes can be moved to the foreground using the fg command: fg %1 A foreground process can be moved to the background: First, the process must be stopped: Ctrl + z Second, the process can be moved to the background with the bg command: bg %1 Jobs Command After a process has been started, you can use the ps command to view the PID or the jobs command to view the background job ID, as shown in the following output. Process Priorities

4 Scheduling Commands Two different commandscan be used to schedule commands: the at daemon (atd) and the cron daemon (crond). atd: crond: The at daemon can be used to schedule a command to execute once in the future. The cron daemon is used to schedule a command to execute repeatedly in the future. With atd With crond The cron daemon is better suited for repetitive tasks because it uses configuration files called cron tables to specify when a command should be executed. Cron tables have six fields separated by space or tab characters. The first five fields specify the times to run the command and the sixth field is the absolute pathname to the command to be executed. As with the at command, you can place commands in a shell script and schedule the shell script to run repetitively; in this case, the sixth field is the absolute pathname to the shell script.

5 Thus, to execute the /root/myscript shell script at 5:20 PM and 5:40 PM Monday to Friday regardless of the day of the month or month of the year, you could use the cron table depicted below: