lsof is an increadibly powerful command-line utility for unix systems. It lists open files, displaying information about them. And since most everything is a file on unix systems, lsof can give sysadmins a ton of useful diagnostic data.
What are some of the most common and useful ways of using lsof, and which command-line switches are used for that?
To show all networking related to a given port:
lsof -iTCP -i :port
lsof -i :22
To show connections to a specific host, use @host
lsof -i@192.168.1.5
Show connections based on the host and the port using @host:port
lsof -i@192.168.1.5:22
grepping for LISTEN shows what ports your system is waiting for connections on:
lsof -i| grep LISTEN
Show what a given user has open using -u:
lsof -u daniel
See what files and network connections a command is using with -c
lsof -c syslog-ng
The -p switch lets you see what a given process ID has open, which is good for learning more about unknown processes:
lsof -p 10075
The -t option returns just a PID
lsof -t -c Mail
Using the -t and -c options together you can HUP processes
kill -HUP $(lsof -t -c sshd)
You can also use the -t with -u to kill everything a user has open
kill -9 $(lsof -t -u daniel)
lsof -i :port
will tell you what programs are listening on a specific port.
lsof -i will provide a list of open network sockets. The -n option will prevent DNS lookups, which is useful when your network connection is slow or unreliable.
lsof +D /some/directory
Will display recursively all the files opened in a directory. +d for just the top-level.
This is useful when you have high wait% for IO, correlated to use on a particular FS and want to see which processes are chewing up your io.
See what files a running application or daemon has open:
lsof -p pid
Where pid is the process ID of the application or daemon.
lsof +f -- /mountpoint
lists the processes using files on the mount mounted at /mountpoint. Particularly useful for finding which process(es) are using a mounted USB stick or CD/DVD.
