List Open Files.
This is much more than it sounds like. It is similar to "fuser", but has much more power. I often use it to find processes using specific network ports:
bash-2.05a$ lsof -i:22 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME ssh 397 apl 3u inet 0x0279421c 0t0 TCP 10.0.0.14:49155->mine:ssh (ESTABLISHED) ssh 408 apl 3u inet 0x027944cc 0t0 TCP 10.0.0.14:49202->website:ssh (ESTABLISHED)
When invoked for a specific process, it will show very useful information:
bash-2.05a$ lsof -p 473 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME vi 473 apl cwd VDIR 14,5 24576 143413 /Users/apl vi 473 apl 0u VCHR 4,4 0t844 37283460 /dev/ttyp4 vi 473 apl 1u VCHR 4,4 0t844 37283460 /dev/ttyp4 vi 473 apl 2u VCHR 4,4 0t844 37283460 /dev/ttyp4 vi 473 apl 3r VREG 14,5 27 1401787 /Users/apl/t vi 473 apl 4u VREG 14,5 0 1440474 /private/var/tmp/vi.recover/vi.wqnCL1 vi 473 apl 5u VREG 14,5 0 1440475 / (/dev/disk0s5)
I can tell from the above that I'm editing /Users/apl/t and that it is 27 bytes long. I'm logged in on ttyp4, and am sitting in /Users/apl. I can see the actual file descriptors in use also. When troubleshooting, lsof can be very handy.
"lsof" is also useful when you need to know if some other process is "done" with a file you want to do something else with. For example, some background process scp's or ftp's a file from somewhere else. You can't process it, move it, look at it, whatever until it is complete, so use lsof (or fuser) to see if any process has the file open. Often that would be some loop like this:
cd /somewhere while : do for i in * do test -e $i || break lsof $i || dosomethingwith $i done sleep 3 done
But it doesn't stop there: amazingly enough, lsof can (sometimes) help you recover a deleted file. Conditions have to be right - specifically, some process needs to have the departed loved one open and you need an OS (like Linux) that has /proc. If all that is true, then there's a link in /proc that can be copied. How would you know where that link is? Well, lsof will tell you, of course. You'd just "lsof | grep missingfile" and lsof will tell you everything you need to know about any process that has the file open: process id and file descriptor (fd).
So let's say I'd just stupidly removed ~/IMPORTANT and instantly realized that, duh, it's IMPORTANT. What can I do? If I immediately do "lsof ~/IMPORTANT", I see I'm in luck:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME less 10971 root 4r REG 3,2 29 2785343 /root/IMPORTANT
I now know that there is a symbolic link in /proc/10971/fd/4:
# ls -l /proc/10971/fd/4 lr-x------ 1 root root 64 Nov 30 06:35 /proc/10971/fd/4 -> /root/IMPORTANT
So I can "cp /proc/10971/fd/4 ~/IMPORTANT" and all is well. How often is this going to save your bacon? Not very often, but it's worth a shot. Realistically, in most cases this won't help. Nothing will have your file open and you would need to take more drastic measures.
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