lsof stands for List Open Files. It can help us find which process is using a file at a given point in time. The reason lsof is so useful in Unix/Linux systems is that sockets and devices are treated the same way as files (Pretty much everything is considered a file in Unix/Linux).

Running lsof without any arguments will list all open files in the system. If you have a lot of processes working with a lot of files, prepare to wait. The output looks somethins like this:

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$ sudo lsof
COMMAND     PID   TID            USER   FD      TYPE             DEVICE  SIZE/OFF       NODE NAME
systemd       1                  root  cwd       DIR              253,1      4096          2 /
systemd       1                  root  rtd       DIR              253,1      4096          2 /
systemd       1                  root  txt       REG              253,1   1577264    5374284 /lib/systemd/systemd
systemd       1                  root  mem       REG              253,1     18976    5375835 /lib/x86_64-linux-gnu/libuuid.so.1.3.0
...

This command works better if executed by root or using sudo. If you execute as any other user, you might only be able to see files owned by that user.

You can see a few things in the output:

  • COMMAND - The unix command associated with the process. This field might be truncated
  • PID - ID of the process using the file
  • TID - ID of the thread using the file
  • USER - User that owns the process
  • FD - Usually this is a number representing a file a descriptor, but there are also some special values (they can be found in man lsof). A file descriptor can be followed by r, w or u to represent read, write and read-write modes
  • TYPE - Because pretty much everything is considered a file, lsof will list all kinds of things. This field helps identify exactly what is this thing (file, directory, socket, etc.)
  • DEVICE - Identifier for the device
  • SIZE/OFF - Depending on the type of file, this will be the size of the file or offset
  • NODE - This varies depending on the type of file, but it can be an inode number for a regular file
  • NAME - Name of the file, device, stream, etc

Find who is using the network

The most common use I have for lsof is finding which process is using a port I’m trying to use. I wrote an article a while ago explaining how to do this with ss (netstat), but it’s good to know how to do this with lsof too, in case ss is not available in the machine.

To see all the network connections we can use:

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lsof -i

To find who is using port 4000, we can use:

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lsof -i :4000

It is also possible to filter by the protocol, but I haven’t had a use case for this:

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lsof -i tcp
lsof -i udp

Find information about a program

If you have a process ID, you can find all files opened by that process using this command:

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lsof -p 950

If you are not sure about the process ID, but you know the command that was used, you can use:

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lsof -c jekyll

The command doesn’t have to be an exact match. The output will include anything that starts with jekyll. If more flexibility is needed, the argument can be surrounded by slashes to search using a regular expression (/<regex>/):

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lsof -c /.*kyll/

Specific files

If you know the file you are interested in, you can find all processes using that file with this command:

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lsof /var/log/syslog

If you want to list all processes that are using anything inside a directory:

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lsof +D /home/adrian/

What a user is doing

If you want to know what a specific user is doing:

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lsof +u adrian

Conclusion

lsof is a very powerful command with many options that I didn’t mention in this article. If you need lsof to do something not mentioned here, you should take a look at the man pages: man lsof.

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