5 UNIX / Linux fuser Command Examples

What is fuser?
5 fuser examples
Syntax and Options
Related Commands

What is fuser?

The fuser command in linux is a useful tool through which we can identify the process(es) which are using a particular file. The file in question could be a normal file, a directory, an executable etc.

5 fuser Examples

1. Check processes using the current directory

In this example, we will run the fuser command to see which processes are using the current directory.

$ fuser .
.:                    1562c  1620c  1626c  1627c  1629c  1630c  1631c  1632c  1633c  1655c  1697c  1777c  1824c  1852c  1886c  2380c  2464c

So we see above that a lot of processes are using the current directory (as its the home directory). The numeric value(s) in the output above output represent the PID of the processes using the directory. The alphabet ‘c’ signifies ‘current directory’. Complete list of alphabets that can be part of fuser output include :

c : current directory
e : executable being run
f : open file. f is omitted in default display mode
F : open file for writing. F is omitted in default display mode
r : root directory
m : mmap’ed file or shared library

2. For verbose output use -v

If we wish to have a detailed output then ‘-v’ option can be used. We repeat the same example as above but this time with -v option :

$ fuser -v .
                     USER        PID ACCESS COMMAND
.:                   himanshu   1562 ..c.. gnome-session
                     himanshu   1620 ..c.. metacity
                     himanshu   1626 ..c.. polkit-gnome-au
                     himanshu   1627 ..c.. nm-applet
                     himanshu   1629 ..c.. gnome-power-man
                     himanshu   1630 ..c.. mintUpdate
                     himanshu   1631 ..c.. nautilus
                     himanshu   1632 ..c.. gnome-panel
                     himanshu   1633 ..c.. bluetooth-apple
                     himanshu   1655 ..c.. python
                     himanshu   1697 ..c.. syndaemon
                     himanshu   1777 ..c.. python
                     himanshu   1824 ..c.. gnome-terminal
                     himanshu   1852 ..c.. firefox
                     himanshu   1886 ..c.. plugin-containe
                     himanshu   2380 ..c.. bash
                     himanshu   2464 ..c.. vlc

3. Check processes using an executable

Lets consider example of an executable here and see how fuser identifies the processes using an executable. I opted for running fuser on firefox executable which is running on my system :

$ ps -aef | grep firefox
himanshu  1852     1  7 06:57 ?        00:06:06 /usr/lib/firefox-11.0/firefox
himanshu  1886  1852 14 06:58 ?        00:12:10 /usr/lib/firefox-11.0/plugin-container /usr/lib/flashplugin-installer/libflashplayer.so -greomni /usr/lib/firefox-11.0/omni.ja 1852 true plugin
himanshu  2551  2380  0 08:24 pts/1    00:00:00 grep --colour=auto firefox

So the first entry in the output above gives the path to firefox executable running in the system. I’ll use the same path in the following fuser command

$ fuser /usr/lib/firefox-11.0/firefox
/usr/lib/firefox-11.0/firefox:  1852e

So we see that the output is the PID of the process (which can be cross checked against the output of the ‘ps’ command above). The post fix ‘e’ signifies that the file is an executable.

4. To output process owner use -u

An extra information regrading the user name of the process owner can also be appended to each PID using the -u option.

$ fuser -uv .
                     USER        PID ACCESS COMMAND
.:                   himanshu   2380 ..c.. (himanshu)bash
                     himanshu   2760 ..c.. (himanshu)bash
                     himanshu   2855 ..c.. (himanshu)gcalctool

5. Display information about all files on command line

We can specify multiple files as input to fuser but by default it provides information on the first file it encounters in the input. If we wish to have information on all the files we supplied as input then -a option can be used.

$ fuser  ./Desktop/ ./Downloads/
./Desktop/:           1892c  1894c
$ fuser -a ./Desktop/ ./Downloads/
./Desktop/:           1892c  1894c
./Downloads/:

In the example above, I tried two commands. One without -a option and one with -a option and we can see the difference. The command with -a option displayed information on all the files listed as input to fuser.

Syntax and Options

fuser [-fuv] [-a|-s] [-4|-6] [-c|-m|-n space ] [-k [-i] [-M] [-SIGNAL ] ] name ...
fuser -l
fuser -V
Short Option Long Option Option Description
-a –all Show all files specified on the command line. By default, only files that are accessed by at least one process are shown.
-c Same as -m option, used for POSIX compatibility.
-f Silently ignored, used for POSIX compatibility.
-k –kill Kill processes accessing the file. Unless changed with -SIGNAL, SIGKILL is sent. An fuser process never kills itself, but may kill other fuser processes. The effective user ID of the process executing fuser is set to its real user ID before attempting to kill.
-i –interactive Ask the user for confirmation before killing a process. This option is silently ignored if -k is not present too
-m NAME –mount NAME NAME specifies a file on a mounted file system or a block device that is mounted. All processes accessing files on that file system are listed. If a directory file is specified, it is automatically changed to NAME/. to use any file system that might be mounted on that directory.
-M –ismountpoint Request will be fulfilled only if NAME specifies a mountpoint. This is an invaluable seatbelt which prevents you from killing the machine if NAME happens to not be a filesystem.
-v –verbose Verbose mode. Processes are shown in a ps-like style. The fields PID, USER and COMMAND are similar to ps. ACCESS shows how the process accesses the file. Verbose mode will also show when a particular file is being access as a mount point, knfs export or swap file. In this case kernel is shown instead of the PID.
-SIGNAL Use the specified signal instead of SIGKILL when killing processes. Signals can be specified either by name (e.g. -HUP) or by number (e.g. -1). This option is silently ignored if the -k option is not used.

Related Commands

kill
killall
lsof
pkill
ps

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