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?
Answer
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 [email protected]
Show connections based on the host and the port using @host:port
lsof [email protected]: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)