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How to List Open Files in Linux | lsof Command

In the world of Linux, understanding and managing open files is crucial for system administrators and users alike. The Linux operating system provides a powerful utility called lsof (List Open Files) that allows users to gain insights into the files currently open on their system. In this article, we will delve into the intricacies of the lsof command, exploring its syntax, options, and practical use cases.

Linux/Unix considers everything as a file and maintains a folder. So "Files or a File" is very important in Linux/Unix. While working in a Linux/Unix system there might be several files and folders that are being used, some of them would be visible and some not. 

lsof command stands for List Open Files. This command provides a list of files that are opened. Basically, it gives the information to find out the files which are opened by which process. With one go it lists out all open files in the output console. It can be combined with 'grep' command and can be used to do advanced searching and listing. 

These files can include:

• Regular files (e.g., text or binary files)
• Directories
• Block special files (e.g., devices)
• Character special files
• Pipes
• Sockets (both UNIX and internet)
• Network connections (TCP and UDP)

With lsof, you can find out which processes are accessing which files, which is extremely useful in a range of scenarios, such as:

• Diagnosing resource usage issues
• Debugging applications that open multiple files
• Monitoring network connections
• Managing file locks and permissions

The basic syntax of the `lsof` command is as follows:

lsof [option]

here,

• lsof: The command itself, used to list open files.
• [options]: Various flags that allow you to customize the behavior of lsof, filtering results by user, process ID, file type, and more.

List all open files: This command lists out all the files that are opened by any process in the system. 

lsof

Here, you observe there are details of files which are opened. Process Id, the user associated with the process, FD(file descriptor), size of the file all together gives detailed information about the file opened by the command, process ID, user, its size etc. 

• FD represents as File descriptor.
• cwd : Current working directory.
• txt : Text file.
• mem : Memory file.
• mmap : Memory mapped device.

List all files opened by a user: There are several users of a system and each user have different requirements and accordingly they use files and devices. To find a list of files that are opened by a specific user this command is useful. 

Syntax: 

lsof -u username

Replace "username" with the desired username. This command provides a list of files opened by the specified user.

In the figure given above with the command lsof -u ubuntu lists out all the files opened by ubuntu user. Along with that we can see the type of file here and they are: 

• DIR: Directory
• REG: Regular file
• CHR: Character special file

With the help of this command you can list out all the files opened by all the process and all the user. But when we want to find the list of files that are opened by all users except a particular user then we can use: 

Syntax: 

lsof -u ^root

In the given figure we can observe there are no files that are opened by the root user.

This command can list out all the files opened by a particular process. '-c' followed by process names can find out all the files that are opened by that particular process that is named in the command. 

Syntax: 

lsof -c Mysql

Here, you can observe that the files and their description opened by Mysql process. Another example is the files that are opened by the apache process: 

Each file is associated with some process ID. There can be many files that are opened by a particular process. By using lsof -p process ID, files opened by a particular process can be checked. 

Syntax: 

lsof -p process ID

As the above-given figure command lists out the files opened by a particular process ID. In the same way, you can use below command option to find out the list of files which are not opened by a particular process ID. 

Syntax: 

lsof -p ^process ID

There is a large number of process running in a system and they have files opened for its usage. There may be many child processes of a process and this process can also be termed as the parent process. To find out the list of files opened by parent process Id lsof command is used with the option -R. 

Syntax: 

lsof -R

It lists out the files which are opened by a particular directory. There are files as well as the directory in a system. So there can be several files opened by a directory as well as the regular file. 

Syntax: 

lsof -D directory path

Our Pc/system can be connected through various networks which helps in a variety of purpose. As we know that in Linux everything is a file, so we can even check the files that are opened by some network connections in the system. 

Syntax: 

lsof -i

Here in the figure, we can see the files opened by the TCP network. In the same way, we can check for UDP etc.

Note: To know more in details about the lsof command you can see the manual page as follows:  

man lsof

In this article, we’ve explored the power of the lsof command in Linux, learning how to list open files, processes, and network connections. The ability to monitor open files is essential for troubleshooting, security, and system performance management. By understanding how to use lsof with its various options, you can gain valuable insights into the behavior of your Linux system and optimize its resource usage.

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