Finding open files in linux

How to Check Open Files in Linux

You may have come across the saying, “Everything is a file in Linux.” Although this is not entirely true, it does hold a set of truths to it.

In Linux and Unix-like systems, everything is like a file. That means the resources in the Unix system get assigned a file descriptor, including storage devices, network sockets, processes, etc.

A file descriptor is a unique number that identifies a file and other input/output devices. It describes resources and how the kernel accesses them. Think of it as a gateway to the Kernel abstraction hardware resources.

Unfortunately, the concept of file descriptors is beyond the scope of this tutorial; consider the link provided below to get started on learning more:

That means that Unix and Unix-like systems such as Linux use such files heavily. As a Linux power user, seeing the open files and the process and users using them is incredibly useful.

This tutorial will focus on ways to view the files open and which process or user is responsible.

Pre-Requisites

Before we begin, ensure that you have:

If you have these, let us get started:

LSOF Utility

Created by Victor A Abell, List open files, or lsof for short, is a command-line utility that allows us to view the open files and the processes or users who opened them.

The lsof utility is available in major Linux distributions; however, you may find it not installed and thus may need to install manually.

How to Install lsof on Debian/Ubuntu

To install it on Debian, use the command:

sudo apt-get install lsof -y

How to Install on REHL/CentOS

To install on REHL and CentOS, use the command:

How to Install on Arch

On Arch, call the package manager using the command:

How to Install on Fedora

On Fedora, use the command:

Once you have the lsof utility installed and updated, we can begin using it.

Basic lsof Usage

To use the lsof tool, enter the command:

Once you execute the above command, lsof will dump a lot of information as shown below:

The above output shows all the files opened by the processes. The output has various columns, each representing specific information about the file.

• The COMMAND column – shows the name of the process that is using the file.
• PID – shows the Process Identifier of the process using the file.
• The TID – Shows the task ID (threads) of the process.
• TASKCMD – Represent the name of the task command.
• USER – The owner of the process.
• FD – Shows the file descriptor number. This is how processes use the file; the options available in this column output include:
• cwd – current working directory.
• mem – memory-mapped file
• pd – parent directory
• jld – jail directory
• ltx – shared library text
• rtd – root directory.
• txt – program code and data
• tr – kernel trace file.
• err – File descriptor information error
• mmp – Memory-mapped device.
• TYPE – Shows the type of node associated with the file, such as:
• Unix – for Unix domain socket.
• DIR – represents the directory
• REG – representing the regular file
• CHR – represents the special character file.
• LINK – symbolic link file
• BLK – Block special file
• INET – Internet domain socket
• FIFO – a named pipe (First In First Out file)
• PIPE – for pipes

• DEVICES – Shows the device numbers separated by commas in the order of special character file, block special, regular, directory, and NFS file.
• SIZE/OFF – shows the size of the file pr file offset in bytes.
• NODE – shows the node number of the local file, type for internet protocol type, etc.
• NAME – shows the name of the mount point and fs on which the file is located.

Note: Please Refer to the lsof Manual for detailed information on the columns.

How to Show Processes that Opened a File

Lsof provides us with options that help us filter the output to show only the processes that opened a specific file.

For example, to see the file that opened the file /bin/bash, use the command as:

This will give you an output as shown below:

COMMAND PID USER FD TYPE DEVICE SIZE / OFF NODE NAME

ksmtuned 1025 root txt REG 253 , 0 1150704 428303 / usr / bin / bash

bash 2968 centos txt REG 253 , 0 1150704 428303 / usr / bin / bash

bash 3075 centos txt REG 253 , 0 1150704 428303 / usr / bin / bash

How Show files Opened by a Specific User

We can also filter the output to show the files opened by a specific user. We do this by using the -u flag followed by the username as:

This will give you an output as shown below:

How to Show Files Opened by a Specific Process

Suppose we want to view all the files opened by a specific process? For this, we can use the PID of the process to filter the output.

For example, the below command shows the files opened by bash.

This will give you only the files opened by systemd as shown:

How to Show Files Opened in a Directory

To get the files opened in a specific directory, we can pass the +D option followed by the directory path.

For example, list open files in the /etc directory.

Below is the output for this:

How to Show Network Connection

Since everything in Linux is a file, we can get the network files such as TCP files or connections.

This will give you the TCP connections in the system.

You can also filter by the specific port using the command shown below:

This will give you the output as shown below:

How to Continuously Show Files

Lsof provides us with a mode to loop the output every few seconds. This allows you to monitor the files opened by a process or user continuously.

This option, however, requires you to terminate the process manually.

For example, the command below continuously monitors the files opened on port 22:

As you can see, in the third loop, lsof catches the established connection to the server on SSH.

Conclusion

Lsof is an incredibly useful utility. It allows you to monitor for critical files as well as monitor users and processes opening files. This can be incredibly useful when troubleshooting or looking for malicious attempts to the system.

As shown in this tutorial, using various examples and methods, you can combine the functionality provided by the lsof tool for custom monitoring.

Thank you for reading and sharing! I hope you learned something new!

About the author

John Otieno

My name is John and am a fellow geek like you. I am passionate about all things computers from Hardware, Operating systems to Programming. My dream is to share my knowledge with the world and help out fellow geeks. Follow my content by subscribing to LinuxHint mailing list

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Use of “lsof” Command to Find Open Files

“lsof” stands for List Open Files. It is a Linux utility for listing down all the open files of a system. This command can be combined with different parameters to modify its output as desired. You can see the details of all of its parameters and flags by seeing the help manual of the “lsof” command.

In today’s article, you will be able to learn the correct usage of the “lsof” command for finding all the open files in Linux Mint 20.

Use of “lsof” Command to Find the Open Files in Linux Mint 20

To learn the correct usage of the “lsof” command and use it to find the open files in Linux Mint 20, you can take a look at all the examples that have been shared below.

Example 1: List All the Open Files in Linux Mint 20
To list down all the open files in Linux Mint 20, you will have to execute the following command in the terminal:

This command will display a list of all the currently opened files on your Linux Mint 20 system, as shown in the image below:

Example 2: List All the Open Files Belonging to a Particular Directory in Linux Mint 20
You can also list down all the open files belonging to a particular directory in Linux Mint 20 by specifying the name of that directory with the “lsof” command in the following manner:

Here, you have to replace the directorypath with the actual path of the directory whose open files you want to list down. For example, we have replaced it with “/var/log/”.

This command will display a list of all the open files that belong to the “/var/log/” directory, as shown in the image below:

Example 3: List All the Open Files Belonging to a Particular User in Linux Mint 20
If you want to list down all the open files belonging to a particular user in Linux Mint 20, then you can do so by executing the “lsof” command in the following manner:

Here, you have to replace the username with the name of the user whose open files you want to list down. For example, we have replaced it with “kbuzdar”.

This command will display a list of all the open files that belong to the specified user, as shown in the image below:

Example 4: List All the Open Files Belonging to a Particular Internet Protocol in Linux Mint 20
You can also try to list down all the open files belonging to a particular Internet protocol in Linux Mint 20 by executing the “lsof” command in the following manner:

You can also replace “6” with “4” if you want to list down all the open files belonging to IPv4.

This command will display a list of all the open files belonging to the IPv6 protocol, as shown in the image below:

Example 5: List All the Open Files Belonging to a Particular File System in Linux Mint 20
If you want to list down all the open files belonging to a particular file system in Linux Mint 20, then you can do so by tweaking the “lsof” command in the following manner:

Here, you can replace “/proc” with any other file system of your choice as well.

This command will display a list of all the open files belonging to the “/proc” file system, as shown in the image below:

Conclusion

By going through all the examples that have been shown to you in this article, you will be able to learn the correct usage of the “lsof” command for finding all the open files in Linux Mint 20.

About the author

Karim Buzdar

Karim Buzdar holds a degree in telecommunication engineering and holds several sysadmin certifications. As an IT engineer and technical author, he writes for various web sites. He blogs at LinuxWays.

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