One of the most important tasks in Linux system administration is process management, which involves several operations such as monitoring processes, sending signals to processes, and setting process priorities on the system.
There are many Linux tools/utilities for monitoring and handling processes, such as top, ps, pgrep, kill, killall, nice, etc.
This article will explore how to use a powerful and resource-rich Linux utility – <span>fuser</span> to find processes.
1. What is fuser in Linux?
<span>fuser</span> is a simple yet powerful command-line utility designed to locate processes based on the specific files, directories, or sockets they are accessing. In short, it helps system users identify which processes are using specific files or sockets.
The basic syntax for using <span>fuser</span> is:
fuser [options] [file|socket]
fuser [options] -signal [file|socket]
fuser -l
2. Finding which process is accessing a directory
Running <span>fuser</span> without any options will display the PIDs of the processes currently accessing your working directory.
[root@oracledb ~]# fuser .
/root: 3187c 3196c 3197c 3239c 3243c 3248c 3344c 3349c 3351c 3381c 3441c 3445c 3447c 3449c 3461c 3466c 3475c 3480c 3482c 3485c 3494c 3500c 3508c 3513c 3522c 3530c 3532c 3533c 3534c 3535c 3536c 3541c 3544c 3546c 3550c 3567c 3569c 3571c 3573c 3575c 3576c 3577c 3580c 3597c 3639c 3642c 3650c 3656c 3657c 3660c 3666c 3668c 3670c 3672c 3690c 3734c 3741c 3748c 3755c 3767c 3796c 3859c 3883c 17098c
[root@oracledb ~]#
Alternatively
[root@oracledb ~]# fuser /home/oracle
/home/oracle: 4618c
[root@oracledb ~]#
3. Finding running processes in a directory – Detailed output
To obtain more detailed and clearer output, enable the <span>-v</span> or <span>--verbose</span> option, as shown below. In the output, <span>fuser</span> will print the name of the current directory followed by columns for process owner (USER), process ID (PID), access type (ACCESS), and command (COMMAND), as shown in the figure below.
[root@oracledb ~]# fuser -v .
用户 进程号 权限 命令
/root: root 3187 ..c.. gnome-session-b
root 3196 ..c.. dbus-launch
root 3197 ..c.. dbus-daemon
root 3239 ..c.. imsettings-daem
root 3243 ..c.. gvfsd
root 3248 ..c.. gvfsd-fuse
root 3344 ..c.. at-spi-bus-laun
root 3349 ..c.. dbus-daemon
root 3351 ..c.. at-spi2-registr
root 3381 ..c.. gnome-shell
root 3441 ..c.. ibus-daemon
root 3445 ..c.. ibus-dconf
root 3447 ..c.. ibus-x11
root 3449 ..c.. ibus-portal
root 3461 ..c.. xdg-permission-
root 3466 ..c.. gnome-shell-cal
root 3475 ..c.. evolution-sourc
root 3480 ..c.. mission-control
root 3482 ..c.. gvfs-udisks2-vo
root 3485 ..c.. goa-daemon
……
In the access type column, you will see the access types represented by the following letters:
- c – Current directory.
- e – Executable file currently running.
- f – Open file, but in the output
<span><span>f</span></span>is omitted. - F – File opened for writing,
<span><span>F</span></span>is also omitted in the output. - r – Root directory.
- m – Memory-mapped file or shared library.
4. Finding which process is accessing a file or filesystem
To determine which processes are accessing your <span>~/.bashrc</span> file, run:
fuser -v -m .bashrc
[root@oracledb ~]# fuser -v -m .bashrc
用户 进程号 权限 命令
/root/.bashrc: root kernel mount /
root 1 .rce. systemd
root 2 .rc.. kthreadd
root 4 .rc.. kworker/0:0H
root 6 .rc.. ksoftirqd/0
root 7 .rc.. migration/0
root 8 .rc.. rcu_bh
root 9 .rc.. rcu_sched
root 10 .rc.. lru-add-drain
root 11 .rc.. watchdog/0
root 12 .rc.. watchdog/1
root 13 .rc.. migration/1
root 14 .rc.. ksoftirqd/1
root 16 .rc.. kworker/1:0H
root 17 .rc.. watchdog/2
root 18 .rc.. migration/2
root 19 .rc.. ksoftirqd/2
root 21 .rc.. kworker/2:0H
root 22 .rc.. watchdog/3
root 23 .rc.. migration/3
root 24 .rc.. ksoftirqd/3
root 26 .rc.. kworker/3:0H
root 27 .rc.. watchdog/4
root 28 .rc.. migration/4
……
<span>-m NAME</span> or <span>--mount NAME</span> option shows all processes accessing the given file or directory. If a directory is passed as NAME, it automatically appends a <span>/</span> to reference the filesystem mounted on that directory.
5. Finding which process is using a specific port
Another practical use case is identifying which process is using a specific network port, which is particularly useful for debugging service conflicts.
fuser 1539/tcp
Or
fuser -v 1539/tcp
[root@oracledb ~]# fuser 1539/tcp
1539/tcp: 4618
[root@oracledb ~]# fuser -v 1539/tcp
用户 进程号 权限 命令
1539/tcp: oracle 4618 F.... tnslsnr
[root@oracledb ~]#
As can be seen, it displays the PID of the process using TCP port 1539, and adding <span>-v</span> provides detailed output.
6. How to use fuser to terminate and send signals to processes
To terminate all processes accessing a file or socket, use the <span>-k</span> or <span>--kill</span> option.
fuser -k .
To interactively terminate processes (the system will ask you to confirm whether to terminate the processes accessing the file or socket), use the <span>-i</span> or <span>--interactive</span> option.
fuser -ki .
Both of the above commands will terminate all processes accessing the current directory; unless the <span>-signal</span> option is used, the default signal sent to the processes is SIGKILL.
7. Listing all available signals in Linux
We can list all signals using the <span>-l</span> or <span>--list-signals</span> option, as shown below.
fuser --list-signals
[root@oracledb ~]# fuser --list-signals
HUP INT QUIT ILL TRAP ABRT IOT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM
STKFLT CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH IO PWR SYS
UNUSED
[root@oracledb ~]#
8. Sending specific signals to processes
Thus, you can send signals to processes as shown in the next command, where SIGNAL is any of the output signals listed above.
fuser -k -SIGNAL
For example, to send a HUP (hang up) signal to the process accessing <span>/boot</span>:
fuser -k -HUP /boot
9. fuser Manual Page
For advanced usage and more details, refer to the <span>fuser</span> manual page.
man fuser
<span>fuser</span> command may not be the first tool that comes to mind for process management, but it is a hidden gem for any Linux user or system administrator. It is excellent for finding which processes are using specific files, directories, or ports – and gives us the power to deal with them directly.
