For a heavy-duty investigative or forensics tool, look no
further than lsof.
It comes bundled with pretty much every Linux distribution, and it’s an indispensable
program. The name of the tool stands for “list open files,” and
that’s exactly what it does, with an amazing amount of detail.

For instance, on a Web server that sends e-mail, has a POP3
and IMAP server, and has MySQL, mailman, etc., executing lsof provides more than 6,200 lines of data. Now, “open
files” is a bit of a misnomer as lsof will also display open TCP ports,
open UDP ports, open sockets, and so forth. For instance, you could display
listening TCP connections and their programs by using the netstat command. The output in
Listing A displays open TCP connections.

Now, use the lsof

# lsof -i|grep TCP

This command accomplishes the same thing, but you will see
much more data output. For instance, with netstat,
you see one listening httpd process, but with lsof, you’ll see every listening
httpd process. To demonstrate more fully, compare these commands:

# netstat -l --tcp -p|grep http


# lsof -i|grep http

With netstat, you’ll
see two lines of text—one for httpd listening to port 80 and the other for it
listening to port 443. With lsof,
you’ll see every httpd thread listening, which in my case, was 40 processes.

Of course, lsof is
useful for more than just tracking which processes are listening to which
ports. With it, you can see the programs that are using certain files or
filesystems, such as:

# lsof /home
bash    14650 joe     cwd    DIR    9,4 4096 50331821 /home/joe
screen  19868 joe     cwd    DIR    9,4 4096 50331821 /home/joe
bash    19869 joe     cwd    DIR    9,4 4096 50331821 /home/joe
ssh     19888 joe     cwd    DIR    9,4 4096 50331821 /home/joe

Compare this output to fuser:

# fuser -v /home   
                     USER        PID ACCESS COMMAND
/home                root     kernel mount  /home

As you can see, lsof
can provide a lot more detail than other tools such as fuser or netstat. When
investigating problems, be sure to have lsof
handy, as it can tell you things that no other program will.

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