lsof Seeks All Open Files

Last week's tip looked at fuser, which shows which process is accessing a given file. lsof is another tool for locating open files. What makes this especially useful is that in Linux, everything is treated as a file: pipes, directories, devices, inodes, sockets and so on. Tip of the Trade: From pipes to directories to sockets, lsof is able to locate all open files.

lsof (no options) will list all files opened by any processes currently running. To restrict this to processes owned by username, use lsof -u username. Here's some sample output:

COMMAND	  PID   USER   FD      TYPE DEVICE    SIZE      NODE NAME
sshd     2354 juliet  mem       REG  254,0   14880    105723 /lib/libcap.so.1.10
sshd     2354 juliet  DEL       REG    0,8         127123574 /dev/zero
bash     2363 juliet  cwd       DIR  254,4   20480   7274497 /home/juliet
bash     2363 juliet  txt       REG  254,0  769368      4126 /bin/bash
bash     2363 juliet  mem       REG  254,0   97928    105698 /lib/ld-2.3.6.so

The FD column shows file descriptor information, or identifies other types of file. Here, cwd indicates the current working directory, and txt indicates program text. The TYPE column has filetype info (REG indicates a regular file). The NODE column may be useful if you're trying to recover a deleted file. See the man page for a full explanation of the output.

lsof filename shows which processes have files of this name open. lsof +D /directory will show processes which have files in this directory open. You can use this if you're trying to unmount a filesystem but getting an 'in use' error, to find the processes using files on that FS and kill them as required.

lsof -c processname will show all processes beginning with processname that have files open; lsof +p PID does the same thing for a process ID. Using lsof -i will get you information about IP sockets. Check out the man page for more detail and for the many other available options.