For a Linux user, fuser is an efficacious utility. It stands for file user and as it suggests, it tells us the processes which are using the file. In this article we will study when and how to use Linux fuser command through practical scenarios and examples. Since in Linux everything is a file, we shall see later in the article, how it is used for directories and sockets too.
To get familiar with Linux fuser command syntax, here is how it is normally used:
$fuser [option(s)] <name-of-file>
To elaborate on the detailed usage, here is a snapshot taken from its help information:
$ fuser -h Usage: fuser [-fMuv] [-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]]]
All the usage options are self explanatory from the help output, however we shall illustrate more through examples. However, note some of the options like ‘-k’ which is used to kill a process and ‘-SIGNAL’ which is used to send a signal. Therefore fuser command can be used to kill a process and send signals also.
Linux fuser command examples
Some practical scenarios
To answer the question – where shall we use this ‘fuser’ utility, I am listing a few example scenarios:
- You are about to unmount a file system, and you get device busy as an error. To analyse, what could be causing the issue, a plausible reason could be a file being used. And hence, fuser command will help resolve the problem by listing all processes, if any using the file system. Not only file system, Linux fuser command will help determine the process/user accessing any socket or even a device.
- In a multi-threaded environment, if a thread somewhere closes a file, which might be used by another thread later on, introduces a bug. Such bugs can be easily caught using Linux fuser command.
- The fuser command can also be used to force delete a file (that is already being used), by killing all processes accessing the file. However, it depends on the user’s requirement and circumstances to force delete a file. Generally, it is not a recommended action though.
Now lets understand the usage of this command through some examples.
Example 1- Process accessing a file
To illustrate the usage of Linux fuser command, we need a process which is in running state and using a file. As a C programmer, it is pretty convenient to write and run an executable that opens a file and retain it for some time. Following is the C source, where we open an existing file on disk in read-only mode, and sleep for some time, following which we close the file. The file would be in-use for the entire time since it has been opened and till it has been closed.
#include <stdio.h>
#include <unistd.h>
int main()
{
FILE *fp = NULL;
fp = fopen("test.txt", "r");
sleep(120);
fclose(fp);
return 0;
}
Further lets compile and get the executable running to have a process running and using the file ‘test.txt’ for a while.
$ gcc fusertest.c -Wall -o fusertest $./fusertest &
Let us now quickly check the current running process to know the PID of our newly triggered process.
$ps PID TTY TIME CMD 3036 pts/1 00:00:00 fusertest 3037 pts/1 00:00:00 ps 10077 pts/1 00:00:02 bash 18628 pts/1 00:03:39 gedit
Note the first line of the ‘ps’ command output. Our executable name is the command and the PID is 3036.
Moving ahead, to use the Linux fuser command, let us now see and verify what all processes that are using the file ‘test.txt’. Please make sure, the user runs ‘fuser’ utility within 120 seconds i.e. 2 minutes of running the executable. In case, one wants more time, raise the sleep time as per the convenience.
Here is how we run the ‘fuser’ utility for ‘test.txt’.
$ fuser -v test.txt
USER PID ACCESS COMMAND
test.txt: rupali 3036 f.... fusertest
First of all, have a look at the PID, it equals ‘3036’ and name of the command i.e. ‘fusertest’ in the output. It is the same as our triggered executable, verifying our demonstration to illustrate what ‘fuser’ does.
To understand more on the output, under ‘ACCESS’ column it has something like
f....
This means, the type of access is a file open. The various types of access are:
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 - mmaped file or shared library.
Note that, we use ‘-v’ option to get a verbose output. However, if we are interested in just the PID, one can omit the ‘-v’ option while using ‘fuser’.
$ fuser test.txt test.txt: 3036
Hence, we have now seen and learnt how we can determine all the processes accessing a particular file.
Example 2 – Process accessing a directory
Since a directory is also a file in Linux, we can use ‘fuser’ to identify processes accessing a particular directory. As an example, let us experiment using fuser command with our current directory. First we run our executable again, and then the utility fuser.
$ ./fusertest &
[2] 3345
$ fuser -v ./
USER PID ACCESS COMMAND
./: rupali 3345 ..c.. fusertest
rupali 10077 ..c.. bash
The output displays for the current directory, which is being accessed by two processes, both by the same user ‘rupali’ but of different PID’s i.e. 3345, the fusertest executable and 10077, bash. The type of access in both cases is
..c..
which means the current directory. Hence, this implies that the directory which we gave as input to the ‘fuser’ utility is accessed as the current directory in two of these processes.
Example 3 – Process accessing a file system
The fuser command provides us with a ‘-m’ option, this option requires a file name to passed as input argument. All processes accessing files on the files system (of the input file) is displayed in output.
For example, try
$ fuser -m -v test.txt
This will list all the processes, which are accessing files present on the file system on which ‘test.txt’ resides. In my case, I get a huge list, here is a snapshot of it:
USER PID ACCESS COMMAND
test.txt: root kernel mount /
rupali 1760 .rce. nxnode
rupali 1784 Frce. nxagent
rupali 1824 frce. nxnode
rupali 1834 Frce. ck-launch-sessi
rupali 1950 Frce. x-session-manag
rupali 1953 .rce. dbus-launch
rupali 1954 .rce. dbus-daemon
rupali 24763 .rce. mission-control
rupali 24768 Frce. bonobo-activati
rupali 24781 Frce. wnck-applet
rupali 24783 Frce. trashapplet
rupali 24786 Frce. indicator-apple
rupali 24788 Frce. notification-ar
rupali 24790 .rce. gvfsd-burn
rupali 24801 Frce. gvfsd-metadata
rupali 26243 Frce. nautilus
rupali 26244 Frce. bluetooth-apple
rupali 26245 Frce. nm-applet
rupali 26256 Frce. vino-server
rupali 26257 Frce. evolution-alarm
rupali 26258 Frce. tint2
rupali 26263 Frce. gnome-power-man
rupali 26264 Frce. polkit-gnome-au
rupali 26277 Frce. notify-osd
rupali 26280 .rce. mission-control
rupali 26290 Frce. gnome-screensav
rupali 26292 .rce. gvfsd-trash
rupali 26293 Frce. gdu-notificatio
rupali 26300 .rce. gvfsd-burn
rupali 26306 .rce. dconf-service
rupali 26318 Frce. sh
rupali 26319 Frce. unity-window-de
rupali 26322 frce. bamfdaemon
rupali 26333 Frce. gvfsd-metadata
rupali 26336 Frce. unity-panel-ser
rupali 26349 Frce. indicator-sessi
rupali 26351 Frce. indicator-me-se
rupali 26356 Frce. indicator-datet
rupali 26359 .rce. indicator-sound
rupali 26380 Frce. geoclue-master
rupali 26388 .rce. indicator-appli
rupali 26390 .rce. indicator-messa
rupali 26402 Frce. applet.py
rupali 26412 Frce. ubuntuone-syncd
rupali 26436 Frce. update-notifier
rupali 26452 Frce. update-manager
rupali 26476 Frce. unity-applicati
rupali 26496 .rce. unity-files-dae
rupali 32356 .rce. bash
We need to know the processes accessing a file system, in situations where we need to unmount a filesystem, and are not able to due to some process accessing it. The Linux fuser command will come to rescue in such cases, to identify and kill those processes.
Example 4 – Process accessing a network port
While working on or resolving network issues, it is very useful to know what all ports are busy and are being used by which processes. The ‘fuser’ utility also assists in knowing the same for tcp and udp ports. As an example, here is how we get to know processes using a tcp port:
$ fuser -v -n tcp 22
It may give no output in your case depending upon any process using tcp port 22 or not. One can know the active ports on a Linux system through the netstat command. Here is the netstat command along with a sample output snippet on my system.
$ netstat -antu Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State … … … tcp 9 0 127.0.0.1:5008 127.0.0.1:40713 ESTABLISHED tcp 7570 0 192.168.1.72:49844 69.59.197.29:443 CLOSE_WAIT … … …
So from the ‘netstat’ output above, we see TCP ports 5008 and 49844 being used by some process. What are those processes, let us run following command,
$ fuser -n tcp 5008 5008/tcp: 1784 $ fuser -n tcp 49844 49844/tcp: 18647
The output identifies the PID of processes, however again if we would like to see a detailed output, ‘-v’ option is the key,
$ fuser -v -n tcp 49844
USER PID ACCESS COMMAND
49844/tcp: rupali 18647 F.... firefox
Just see how easily it was known that Firefox was using that port.
Example 5 – To kill processes
Not only fuser command comes in handy to identify the processes, it can also be used to kill the identified processes if need be. The processes identified can be killed by using ‘-k’ option.
Here is a demonstration, which first runs a process accessing a file, and then list the running processes and finally killing the processes which are using the file. In the end, we again check the list of running processes to verify if the relevant process has been killed.
$ ./fusertest &
[2] 3544
$ ps
PID TTY TIME CMD
3544 pts/1 00:00:00 fusertest
3545 pts/1 00:00:00 ps
10077 pts/1 00:00:03 bash
18628 pts/1 00:03:40 gedit
$ fuser -v -k test.txt
USER PID ACCESS COMMAND
test.txt: rupali 3544 F.... fusertest
[2]+ Killed ./fusertest
$ ps
PID TTY TIME CMD
3547 pts/1 00:00:00 ps
10077 pts/1 00:00:03 bash
18628 pts/1 00:03:40 gedit
It is worth mentioning that ‘fuser’ does not ask for any confirmation before killing a process. The command given is the command executed. In case, there are a number of processes using this file, all the processes would be killed.
However, we do have an option to kill the processes interactively. This means the user is prompted at run time to confirm whether a particular action is to be taken or not. The option is ‘-i’. Here is the sample usage:
$ fuser -v -k -i test.txt
USER PID ACCESS COMMAND
test.txt: rupali 3548 F.... fusertest
Kill process 3548 ? (y/N) y
[2]+ Killed ./fusertest
Note that line (in bold) above. The fuser command asked the user whether to kill process 3548 or not.
A sample usage, where we have more than one process to be killed:
$ fuser -v -k -i ./
USER PID ACCESS COMMAND
./: rupali 3553 ..c.. fusertest
rupali 10077 ..c.. bash
Kill process 3553 ? (y/N) y
[2]+ Killed ./fusertest
Kill process 10077 ? (y/N) n
This way multiple actions can be done interactively.
Example 6 – Sending signals to processes
To add to the bucket of whole lot of capabilities, ‘fuser’ can also be used to send specific signals to processes. When we kill the process through ‘-k’ option, we are internally sending a ‘KILL’ signal to the process. First of all, what are the kinds of signals which can be sent to a running process. Here is how we get the list:
$ fuser -l 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 UNUSE
To send a signal, ‘fuser’ offers ‘-SIGNAL’ option, along with ‘-k’. The difference is, here now since what kind of signal to be sent is mentioned, so the mentioned signal will be sent to the concerned processes. In case, no signal is mentioned, by default ‘KILL’ signal is sent.
$ fuser -v -ki -SIGSTOP test.txt
USER PID ACCESS COMMAND
/home/rupali/aprograms/test.txt:
rupali 24140 f.... fusertest
Kill process 24140 ? (y/N) y
[4]+ Stopped ./fusertest
With the stop signal, the process is now in the stopped state. To confirm, run the following Linux ps command
$ ps aux | grep fusertest rupali 24140 0.0 0.0 2136 284 pts/2 T 15:00 0:00 ./fusertest rupali 24974 0.0 0.0 4388 828 pts/2 S+ 15:09 0:00 grep --color=auto fusertest
The process state is specified by the 8th column i.e. ‘T’, which signifies the state code for a stopped process.
Conclusion
This article helps to understand more about where and how to use the ‘fuser’ utility. However, generally it is a great tool under the umbrella of many such tools to provide information about processes and file system. In any eco-system, it is never just one tool which will do the wonders in finding and resolving an issue. If you note, in many of the examples above, I’ve used ‘ps’ command and ‘netstat’ command. There are many more linux utilities which can be used along with each other to develop big software and resolve even bigger issues. Therefore, in the end, I would say, since now we know yet another powerful Linux tool, we are one step closer to working on Linux efficiently.
NOTE : To learn more command line tools, read our articles on Linux netcat and strace commands
