The Linux lsof command

Lsof (list open files) is a tool that lists open files for the current system.

In a Linux environment, everything is in the form of files, with files that not only access regular data, but also access to network connectivity and hardware.

So, such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) sockets, the system assigns a file descriptor to the application in the background, regardless of the nature of the file, which provides a common interface for the interaction between the application and the underlying operating system. Because the application opens a descriptor list of files that provides a lot of information about the application itself, it is helpful to see the list through the Lsof tool for system monitoring and troubleshooting.

Output message Meaning

Enter lsof at the terminal to display the file opened by the system, because lsof needs to access core memory and various files, so it must be run as root to fully perform its functions.

The direct input lsof partial output is:

COMMAND PID USER FD TYPE DEVICE size/off NODE nameinit 1 root CWD          Dir 8,1 4096 2/init 1 root rtd DIR 8,1 4096        2/init 1 root txt REG 8,1 150584 654127/SBIN/INITUDEVD 415                Root 0u chr 1,3 0t0 6254/dev/nulludevd 415 root 1u CHR 1,3 0t0 6254/dev/nulludevd 415 root 2u CHR 1,3 0t0 6254/d EV/NULLUDEVD 690 Root Mem REG 8,1 51736 302589/lib/x86_64-linux-gnu/libnss_file S-2.13.SOSYSLOGD 1246 syslog 2w REG 8,1 10187 245418/var/log/auth.logsyslogd 124      6 syslog 3w REG 8,1 10118 245342/var/log/syslogdd 1271 root 0r  REG 0,3      0 4026532038/PROC/KMSGDD 1271 Root 1w FIFO 0,15 0t0 409/run/klogd/ KMSGDD 1271 Root 2u CHR 1,3 0t0 6254/dev/null ...

Each row displays an open file, and all files opened by all processes are displayed by default if you do not specify a condition.

The meaning of the lsof output column information is as follows:

COMMAND: Name of the process

PID: Process Identifier

USER: Process Owner

FD: File descriptor in which the application recognizes the file through a file descriptor. such as CWD, TXT, etc. type: file type, such as Dir, Reg, etc.

DEVICE: Specifies the name of the disk

Size: Sizes of files

Node: Index node (the identity of the file on disk)

Name: Open the exact name of the file

The file descriptor CWD value in the FD column represents the current working directory of the application, which is the directory that the application launches, unless it makes changes to the directory itself, the TXT type of file is the program code, such as the application binary itself or the shared library, as shown in the list above in the/sbin/init program.

The second value represents the application's file descriptor, which is an integer returned when the file is opened. As on the last line of file/dev/initctl, its file descriptor is 10. U means the file is open and in read/write mode, not read-only? or write-only (w) mode. Also, a capital W indicates that the application has a write lock on the entire file. This file descriptor is used to ensure that only one instance of the application can be opened at a time. When each application is initially opened, it has three file descriptors, from 0 to 2, representing standard input, output, and error streams, respectively. So most applications open files with FD starting from 3.

The Type column is more intuitive than the FD column. Files and directories are called REG and Dir, respectively. The CHR and BLK, respectively, represent characters and block devices, or UNIX, FIFO, and IPV4, respectively, representing the UNIX domain sockets, first in and Out (FIFO) queues, and Internet Protocol (IP) sockets.

Common parameters

The lsof syntax format is:
lsof [options] FileName

Lsof Abc.txt Show the process of opening file Abc.txt LSOF-C ABC show ABC process now open file Lsof-c-P 1234 List the process with process number 1234 Open File Lsof-g GID shows the process of attribution to GID lsof +d /usr/local/Display directory under the process open files Lsof +d/usr/local/, but the directory under the search directory, a long time lsof-d 4 shows the use of FD 4 process lsof-i to show the conditions of the process lsof-i[46] [PR otocol][@hostname |hostaddr][:service|port]-  ---IPV4 or IPV6 protocol--TCP or UDP  hostname-- > Internet Host name  hostaddr--IPv4 address  Service---/etc/service in service name (can be more than one)  Port--> ; Port number (can be more than one)

Lsof Use Instances

Find who is using the file system

When uninstalling a file system, the operation will typically fail if there are any open files in the file system. Then through lsof you can find out which processes are using the file system that is currently being uninstalled, as follows:
# lsof/gtes11/
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
Bash 4208 root cwd DIR 3,1 4096 2/gtes11/
Vim 4230 root cwd DIR 3,1 4096 2/gtes11/
In this example, user root is doing some work in its/GTES11 directory. One bash is the instance running, and its current directory is/GTES11, and the other is that Vim is editing the file under/GTES11. To successfully uninstall/GTES11, you should abort these processes after notifying the user to ensure that the situation is correct. This example illustrates the importance of the current working directory of the application because it retains the file resources and prevents the file system from being unloaded. This is why most daemons (background processes) change their directories to the root directory, or service-specific directories (such as/var/spool/mqueue in the SendMail example) to prevent the daemon from preventing the uninstallation of unrelated file systems.

Recovering deleted files

When a Linux computer is compromised, it is common for the log files to be deleted to conceal the attacker's traces. Administrative errors can also cause accidental deletion of important files, such as the active transaction log of the database is accidentally deleted when the old log is cleaned up. These files can sometimes be recovered by lsof.
When a process opens a file, it remains on disk as long as the process remains open for that file, even if it is deleted. This means that the process does not know that the file has been deleted, and it can still read and write to the file descriptor that was provided to it when the file was opened. In addition to this process, this file is not visible because its corresponding directory index node has been deleted.
In the/proc directory, it contains various files that reflect the kernel and the process tree. The/proc directory mounts an area that is mapped in memory, so these files and directories do not exist on disk, so when we read and write these files, we actually get the relevant information from memory. Most of the information related to lsof is stored in a directory named after the PID of the process, that is,/proc/1234 contains information about the process with PID 1234. There are various files in each process directory that allow the application to simply understand the process's memory space, file description list characters, symbolic links to files on disk, and other system information. The LSOF program uses this information and other information about the internal state of the kernel to produce its output. So lsof can display information such as the file descriptor of the process and the associated filename. That is, we can find information about the file by accessing the file descriptor of the process.
When a file in the system is accidentally deleted, as long as there are processes in the system that are accessing the file, we can recover the contents of the file from the/proc directory by lsof. If the/var/log/messages file is deleted due to misoperation, then the method to restore the/var/log/messages file is as follows:
First use lsof to see if there is currently a process open/var/logmessages file, as follows:
# lsof |grep/var/log/messages
RSYSLOGD 1283 root 2w REG 3,3 5381017 1773647/var/log/messages (Deleted)
From the above information you can see that the PID 1283 (syslogd) Open file has a file descriptor of 2. You can also see that/var/log/messages has been flagged for deletion. So we can view the corresponding information in/PROC/1283/FD/2 (each file descriptor for the process corresponding to a digitally named file under FD), as follows:
# head-n 10/PROC/1283/FD/2
4 13:50:15 holmes86 syslogd 1.4.1:restart.
4 13:50:15 holmes86 kernel:klogd 1.4.1, log Source =/proc/kmsg started.
4 13:50:15 holmes86 kernel:linux version 2.6.22.1-8 ([email protected]) (GCC version 4.2.0) #1 SMP Wed Jul 18 11:18:3 2 EDT 4 13:50:15 holmes86 kernel:bios-provided physical RAM Map:aug 4 13:50:15 holmes86 kernel:bios-e820:0000 000000000000-000000000009f000 (usable) 4 13:50:15 holmes86 kernel:bios-e820:000000000009f000-00000000000a0000 (r eserved) 4 13:50:15 holmes86 kernel:bios-e820:0000000000100000-000000001f7d3800 (usable) 4 13:50:15 holmes86 K ernel:bios-e820:000000001f7d3800-0000000020000000 (Reserved) 4 13:50:15 holmes86 kernel:bios-e820:00000000e00000 00-00000000f0007000 (Reserved) 4 13:50:15 holmes86 kernel:bios-e820:00000000f0008000-00000000f000c000 (Reserved)
As you can see from the information above, you can get the data you want to recover by looking at/PROC/8663/FD/15. If you can view the data through a file descriptor, you can use I/O redirection to copy it to a file, such as:
CAT/PROC/1283/FD/2 >/var/log/messages
This method of recovering deleted files is useful for many applications, especially log files and databases.

Utility Commands

Lsof ' which httpd '//That process is using Apache executable file LSOF/ETC/PASSWD//That process is taking up/ETC/PASSWDLSOF/DEV/HDA6//That process is taking up hda6lsof/dev/ CDROM//That process occupies the optical drive lsof-c sendmail//view file usage of SendMail process lsof-c courier-u ^zahn//show that those files are opened with a process that starts with courier, But does not belong to the user zahnlsof-p 30297//show those files by PID 30297 process Open lsof-d/TMP displays all the instance and files opened in the/tmp folder process. But the symbol file is not in the column lsof-u1000//view UID is 100 of the user's process file usage lsof-utony//view user Tony's process file usage lsof-u^tony// View file usage for processes that are not user Tony (^ is reversed) lsof-i//Show all open ports lsof-i:80//Show all open port 80 processes lsof-i-u//Show all open ports and UNIX domain files Lsof-i [ Email protected][url]www.akadia.com:123//Show those processes open the link to www.akadia.com UDP 123 (NTP) port lsof-i [email protected]:ftp-r Constantly check the current FTP connection situation (-r,lsof will always continue to execute until the interrupt signal is received, +R,LSOF will continue to execute until no file is displayed, the default is 15s refresh) lsof-i [email protected]:ftp-n//lsof-n Do not convert IP to hostname, the default is not to add the-n parameter

The Linux lsof command