How to Use lsof Command in Linux to List Open Files

The good thing about Linux is that you can easily view and manage everything, from the boot process to the installation of software packages. Here we discuss how you can use the lsof command in Linux to view open files and the processes using them. Knowing how to view this can help you understand how the system works and even take the necessary actions for specific processes.

Lsof Command

To view open files and the users or processes responsible for them, we use the lsof utility. By default, lsof is pre-installed in most distributions.

However, if you do not have it installed, you can use the package manager to install it on your system.

Debian/Ubuntu

On Debian, run the command:

sudo apt-get install lsof

Arch/Manjaro

On Manjaro and other Arch-based distributions, use pacman by running the command:

sudo pacman -S lsof

CentOS/REHL/Fedora

For CentOS and the REHL family, you can use dnf:

sudo dnf install lsof

Use lsof Command to List Open Files for a Linux Process

Like most Linux commands, the lsof utility is incredibly simple to use. Start by typing the command lsof:

sudo lsof

Once you run the command above, lsof should return information about the open files in the system.

COMMAND PID TID TASKCMD USER   FD      TYPE             DEVICE SIZE/OFF            NODE NAME
init      1             root  cwd       DIR               8,48     4096               2 /
init      1             root  rtd       DIR               8,48     4096               2 /
init      1             root  txt       REG               0,19   632048 281474976743906 /init
init      1             root    0u      CHR                1,3      0t0           15362 /dev/null
init      1             root    1u      CHR                1,3      0t0           15362 /dev/null
init      1             root    2u      CHR                1,3      0t0           15362 /dev/null
init      1             root    3w      CHR               1,11      0t0           15367 /dev/kmsg
init      1             root    4u     sock                0,8      0t0           22689 protocol: AF_VSOCK
init      1             root    5r      REG                0,4        0      4026532185 mnt
init      1             root    6r      REG                0,4        0      4026532201 mnt
init      1             root    7r      DIR               8,48     4096             240 /home/cap
init      1             root    8u      DIR               8,48     4096               2 /
init      1             root    9u     sock                0,8      0t0           21853 protocol: AF_VSOCK

Note: if you have sudo privileges, run the command with sudo to avoid “permission denied” errors on specific files.

As shown in the output above, the lsof output has the following columns:

Column	Representation
Command	Shows the name of the process using the target file.
PID	The unique identifier for the process using the file.
TID	The column shows the thread identifier.
TASKCMD	The name of the task command.
USER	Username or UID of the user running the process.
FD	File descriptor of the file and modes.
TYPE	Node associated with the target file.
DEVICE	Device number separated by commas.
SIZE/OFF	File size in bytes of file offset size
NODE	Inode value of the local file. You can use the stat command to show `inode` information for the file.
NAME	Mount point of the file.

Now that you understand what the contents of the lsof command printout represent, let us use the command to filter for specific information.

How to Filter for Specific Process

To filter for only specific files opened by the specific process, we can use either the process name or the PID value.

For example, to show files used by the firefox process, we can use the command:

sudo lsof -c firefox

The command will show all the files opened by the firefox process.

To filter by process ID, we can use the -p option and pass the process ID. You can use the top command to get the process ID of the target process.

For example, to get the PID of the firefox process, we can use the command:

sudo ps aux | grep firefox

Once you have the PID of the target process, use lsof to show the open files:

sudo lsof -p 2121

The above command will print the files opened by the process with the PID specified.

How to Filter for a Specific User

To view only the files opened by a specific user, we can use the -u flag. For example, to filter for the Debian user, use the command:

sudo lsof -u debian

How to Filter for a Specific File

Suppose you only want to know the process and the user who opened a specific file. To do this, pass the name of the file to lsof:

sudo lsof /bin/sleep

The above will only filter for the specific file and return the related information, including the user, process ID, and more.

Wrapping Up

In this simple tutorial, we discussed how to query the system for information about open files using the lsof command in Linux. Here are some additional commands for you to list the content of a directory in the terminal.