How to use the Fuser command in Linux system

What is Fuser Command?

The fuser command is a very clever Unix utility for finding processes that are using a certain file, directory, or socket. It also provides information about the user who owns the process and the access type. . The fuser tool displays the process ID (PID) of each process that is using a specified file or file system.

Install

If your simplified version runs fuser, the following message will be displayed:

-bash: fuser: command not found

Please execute the following command to install:

[winbert@winbert-server ~]$ sudo yum -y install psmisc

How to use fuser command?

The man command can be used to view the help manual for any command, but the best way to learn new things, especially Linux commands, is by reading real examples and constantly typing commands into the terminal. Run the following command in the terminal to get information about the usage options of the fuser utility.

[winbert@winbert-server ~]$ fuser
No process specification given
Usage: fuser [-fMuvw] [-a|-s] [-4|-6] [-c|-m|-n SPACE] [-k [-i] [-SIGNAL]] NAME...
  fuser -l
  fuser -V
Show which processes use the named files, sockets, or filesystems.

 -a, --all display unused files too
 -i, --interactive ask before killing (ignored without -k)
 -k, --kill kill processes accessing the named file
 -l, --list-signals list available signal names
 -m, --mount show all processes using the named filesystems or block device
 -M, --ismountpoint fulfill request only if NAME is a mount point
 -n,--namespace SPACE search in this name space (file, udp, or tcp)
 -s, --silent silent operation
 -SIGNAL send this signal instead of SIGKILL
 -u, --user display user IDs
 -v, --verbose verbose output
 -w,--writeonly kill only processes with write access
 -V, --version display version information
 -4, --ipv4 search IPv4 sockets only
 -6, --ipv6 search IPv6 sockets only
 - reset options

 udp/tcp names: [local_port][,[rmt_host][,[rmt_port]]]

How to view the processes using a directory

The fuser program can be used with the -v option, which runs the tool in verbose mode. The verbose option is used to produce detailed output on the computer screen, so the user can see in real time what the utility is doing.

[winbert@winbert-server ~]$ fuser -v .
      USER PID ACCESS COMMAND
/home/winbert: winbert 1435 ..c.. bash

The output above shows that fuser provides information about USER , PID , ACCESS , and COMMAND when run in verbose mode. The c character under ACCESS indicates the access type, meaning "current directory". There are many access types, such as e (running executable), r (root directory), f (open file. f is omitted in the default display mode), F (open file for writing. F is omitted in the default display mode), and m (mmap file or shared library).

See what process is using your tcp or udp socket?

Sometimes you need to find processes using TCP and UDP sockets. To find these processes, use the -n option. The -n option is used to select the corresponding namespace.

[root@huidukongjian-h4 docker]# fuser -v -n tcp 80
      USER PID ACCESS COMMAND
80/tcp: root 27411 F....docker-proxy

By default, fuser will look in both IPv6 and IPv4 sockets, but the default can be changed using -4 and -6 options. The -4 option represents IPv4 , and the -6 option represents IPv6 . Note that fuser only outputs the PID to stdout, everything else is sent to stderr.

The result of fuser -v -n tcp 80 command shows that the process ID of the process using docker is 27411, and the command used to start the process is docker-proxy . A process ID (PID) can be used in many ways, one of which is process termination. When used with a PID, the kill command kills the process based on that process ID. fuser can also be used to kill processes accessing specific files. In the following command, -k option is used to kill the process that is using tcp listener running on port 123 . To ensure that the user does not kill the wrong process, use the -i option to ask the user for confirmation before terminating the process.

fuser -k 123/tcp

Use the fuser -k command with the -i option to ask for user confirmation before terminating the process. The user can answer with y for "yes" or N to not kill the process.

fuser -i -k 123/tcp
123/tcp: 12216
Kill process 12216 ? (y/N)
Use The -6 Option To Look For IPv6 Sockets.

The following command uses fuser in verbose mode and attempts to find an IPv6 socket running on port 123.

fuser -v -n tcp -6 123

Find the process that occupies a certain file system

The -m option can be used with the fuser command to find processes accessing files on the file system. This option requires a file name as an input argument. The -m option is very useful, especially when used to discover processes that are accessing the file system and identify processes to kill.

The following command displays all processes accessing the file system where "example.txt" is located. Look carefully at how the -m option works with fuser .

[root@huidukongjian-h4 docker]# fuser -v -m data/v2/config.json 
      USER PID ACCESS COMMAND
/root/docker-v2/data/v2/config.json:
      root kernel mount /
      root 1 .rce.systemd
      root 2 .rc.. kthreadd
      root 3.rc..rcu_gp
      root 4 .rc.. rcu_par_gp
      root 6 .rc.. kworker/0:0H-kbl
      root 8 .rc.. mm_percpu_wq
      root 9 .rc.. ksoftirqd/0
      root 10 .rc.. rcu_sched
      root 11 .rc.. migration/0
      root 12 .rc.. watchdog/0
      root 13 .rc.. cpuhp/0
      root 16 .rc.. netns
      root 17 .rc.. kauditd
      root 18 .rc.. khungtaskd
      root 19 .rc..oom_reaper
      root 20 .rc..writeback
      root 21 .rc.. kcompactd0
      root 22.rc..ksmd
      root 23.rc..khugepaged
      root 24 .rc.. crypto
      root 25 .rc..kintegrityd
      root 26.rc..kblockd
      root 27 .rc.. tpm_dev_wq
      root 28 .rc..md
      root 29 .rc.. edac-poller
      root 30 .rc.. watchdogd
      root 42 .rc.. kswapd0
      root 93 .rc.. kthrotld
      root 94 .rc.. acpi_thermal_pm
      root 95 .rc.. kmpath_rdacd
      root 96 .rc.. kaluad
      root 97 .rc.. ipv6_addrconf
      root 98 .rc.. kstrp
      root 326 .rc.. scsi_eh_0
      root 327 .rc.. scsi_tmf_0
      root 329 .rc.. kworker/0:1H-kbl
      root 361 .rc.. ata_sff
      root 363 .rc.. scsi_eh_1
      root 365 .rc..scsi_tmf_1
      root 366 .rc.. scsi_eh_2
      root 367 .rc..scsi_tmf_2
      root 387 .rc..xfsalloc
      root 390 .rc.. xfs_mru_cache
      root 391 .rc.. xfs-buf/vda1
      root 394 .rc.. xfs-data/vda1
      root 395 .rc.. xfs-conv/vda1
      root 396 .rc.. xfs-cil/vda1
      root 397 .rc.. xfs-reclaim/vda
      root 398 .rc.. xfs-log/vda1
      root 399 .rc.. xfs-eofblocks/v
      root 400 .rc.. xfsaild/vda1
      root 486 .rce. systemd-journal
      rpc 541 .rce. rpcbind
      root 543 Frce.auditd
      root 545 .rce. sedispatch
      root 558 .rc.. rpciod
      root 559 .rc.. kworker/u3:0
      root 561 .rc..xprtiod
      root 582 Frce.sssd
      polkitd 585 .rce. polkitd
      root 589 .rce.rngd
      dbus 593 frce. dbus-daemon
      chrony 612 .rce. chronyd
      root 652 Frce.sssd_be
      root 668 Frce.sssd_nss
      root 671 .rc.. ttm_swap
      root 672 .rc..nfit
      root 675 frce. systemd-logind
      root 683 Frce.gssproxy
      root 740 frce.NetworkManager
      root 743 Frce. tuned
      root 814 frce. systemd-udevd
      root 889 frce.sshd
      root 890 Frce. rsyslogd
      root 895 frce.agetty
      root 898 frce. crond
      root 899 frce.agetty
      root 21821 .rc.. kworker/u2:0-flu
      root 25475 frce.sshd
      root 25480 .rce.systemd
      root 25485 frce. (sd-pam
      root 25491 frce.sshd
      root 25492 frce.bash
      root 25705 Frce.containerd
      root 25706 Frce.dockerd
      root 26375 .rc.. kworker/u2:1-eve
      root 27251 Fr.e. containerd-shim
      root 27267 F...m v2
      root 27273 Fr.e. containerd-shim
      root 27295 ....m sh
      root 27400 .rce.docker-proxy
      root 27411 .rce.docker-proxy
      root 27416 Fr.e. containerd-shim
      root 27432 ....m sh
      root 27478 ....m sh
      root 27479 F...m nginx
      root 27480 ....m sleep
      (unknown) 27481 F...m nginx
      root 27561 ....m sleep
      root 27705 .rc.. kworker/0:0-xfs-
      root 27765 .rc.. kworker/0:1-xfs-
      root 27836 .rc.. kworker/0:2-even
      root 27860 frce.sshd
      root 27883 frce.sshd
      sshd 27884 frce.sshd

fuser can also be used to send specific instructions to a process. When used with the -k option, fuser command sends KILL instruction to the process. There are many commands that can be sent to a running process. The -l option helps in finding a list of commands that can be used with fuser.

[root@huidukongjian-h4 docker]# fuser -l
HUP INT QUIT ILL TRAP ABRT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM STKFLT
CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR SYS

Summarize

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