Fuser

fuser

Identify processes using files or sockets, optionally killing the process that is accessing the file.

Syntax:

fuser [-a|-s|-c] [-4|-6] [-n space ] [-k [-i] [-signal ] ] [-muvf] name
fuser -l
fuser -V

Options:

-a

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 processes accessing the specified file.
If not specified with -signal, SIGKILL is sent.
An fuser process will never kill itself, but it can 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

Ask for confirmation before killing a process. This is ignored unless -k is also used.

-l

List all known signal names.

-m name

Specifies a file or file system that is mounted.
All processes accessing files on that file system will be listed.
If a directory is specified, it is automatically treated as name/.

-n space

Select a different namespace:

• file (default) for files.
• udp for local UDP ports.
• tcp for local TCP ports.
  You can specify a shortcut like 80/tcp for specific ports.

-s

Silent operation. The options -u and -v are ignored in this mode.
The -a option cannot be used with -s.

-signal

Specify a signal to use instead of SIGKILL when killing processes (e.g., -HUP or signal number).
This is ignored unless used with -k.

-u

Append the username of the process owner to each PID.

-v

Verbose mode.
Processes are shown in a ps-like format with fields such as PID, USER, COMMAND, ACCESS.
If the access is by the kernel (e.g., a swap file), kernel is shown instead of the PID.

-V

Display version information.

-4

Search only for IPv4 sockets.
This must not be used with -6 and only affects tcp and udp namespaces.

-6

Search only for IPv6 sockets.
This must not be used with -4 and only affects tcp and udp namespaces.

-

Reset all options and revert the signal back to SIGKILL.

Output:

fuser outputs a list of PIDs for processes that are using the specified files or file systems. The type of access is indicated by a letter after the file name:

• c - Current directory.
• e - Executable being run.
• f - Open file.
• F - Open file for writing.
• r - Root directory.
• m - Memory-mapped file or shared library.

If multiple processes access the same file, only one entry will be shown for each access type. If a file is specified multiple times, some entries may be omitted.

Examples:

1. Kill all processes accessing the /home directory:

$ fuser -km /home

2. List the process (or processes) locking a file:

$ fuser -f /path/to/file

3. Check if no process is using /dev/ttyS1 before running a command:

$ if fuser -s /dev/ttyS1; then :; else something; fi

4. Show processes using the local TELNET port (assuming port 23):

$ fuser 23/tcp

5. Show processes accessing a specific file and display verbose output:

$ fuser -v /path/to/file

Restrictions:

• Privileges: fuser requires root privileges to display information about processes owned by other users. Without elevated privileges, information may be incomplete.
• Kernel access: Processes accessing kernel resources may only be shown with the -v option.
• UDP and TCP: fuser works with both IPv6 and IPv4 for udp and tcp namespaces, but the address fields for TCP and UDP can only accept IPv4 addresses.

Files:

• Location of the proc file system: /proc