The Linux lsof command is a command line utility that is used to list the open files on a Linux system. An open file can be a regular file, a directory, a block special file, a character special file, an executing text reference, a library, a stream, a network file (Internet socket, NFS file, or UNIX domain socket), or a file descriptor.

The lsof command can be useful for a number of different purposes, including:

• Troubleshooting problems with open files or file descriptors
• Identifying processes that are using a particular file or network port
• Locating deleted files that are still open by a process
• Determining the files that a process has open

To use the lsof command, simply type lsof at the command prompt and press Enter. This will display a list of all open files on the system, along with the PID (process ID) of the process that has the file open, the user that owns the process, and the type and status of the file.

$ lsof
COMMAND     PID     USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
init         1       root  cwd    DIR   202,1     4096  2272 /
init         1       root  rtd    DIR   202,1     4096     2 /
init         1       root  txt    REG   202,1  1449600  7965 /sbin/init
...

The lsof command has a number of options that can be used to control the information that is displayed and the format of the output. For example, the -p option can be used to limit the output to files that are open by a specific process.

For example, to display a list of open files for process ID 1234, you could use the following command:

$ lsof -p 1234
COMMAND   PID     USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
init      1234    root  cwd    DIR   202,1     4096  2272 /
init      1234    root  rtd    DIR   202,1     4096     2 /
init      1234    root  txt    REG   202,1  1449600  7965 /sbin/init
...

The lsof command can also be used to display only files of a particular type, such as regular files, directories, or network files. To do this, you use the -t option followed by the type of file that you want to display.

The system, you could also use the following command:

$ lsof -t -i
COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
sshd     10671   root    3u  IPv4  20451      0t0  TCP *:ssh (LISTEN)
sshd     10671   root    4u  IPv6  20452      0t0  TCP *:ssh (LISTEN)
sshd     10673   root    3u  IPv4  20451      0t0  TCP *:ssh (LISTEN)
sshd     10673   root    4u  IPv6  20452      0t0  TCP *:ssh (LISTEN)
...

The lsof command can also be used to display files that are open by a specific user. To do this, you use the -u option followed by the username of the user.

For example, to display a list of open files for user john, you could use the following command:

$ lsof -u john
COMMAND    PID  USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
bash     12557 john  cwd    DIR  202,1     4096 143355 /home/john
bash     12557 john  rtd    DIR  202,1     4096      2 /
bash     12557 john  txt    REG  202,1  1472512 143354 /bin/bash
...

In conclusion, the lsof command is a useful tool for displaying information about open files on a Linux system. Whether you are troubleshooting a problem with open files, locating deleted files, or simply curious about the open files on your system, the lsof command can provide the information you need. With its many options and capabilities, the lsof command is a valuable tool for any Linux user or system administrator.

Christian Wells