lsof(8)

lsof
+  -r <s> ..... repeatedly execute command ervery <s> seconds
+  -a ......... AND slection filters instead ORing (OR: default)
+  -p <pid> ... filter by <pid>
+  +fg ........ show file flags for file descripros
+  -n ......... don't convert network addr to hostnames
+  -P ......... don't convert network port to service names
+  -i <@h[:p]>. show connections to h (hostname|ip addr) with optional port p
+  -s <p:s> ... in conjunction with '-i' filter for protocol <p> in state <s>
+  -U ......... show unix domain sockets ('@' indicates abstract sock name, see unix(7))
+

file flags:
+  R/W/RW ..... read/write/read-write
+  CR ......... create
+  AP ......... append
+  TR ......... truncate
+

-s protocols
+  TCP, UDP
+
+-s states (TCP)
+  CLOSED, IDLE, BOUND, LISTEN, ESTABLISHED, SYN_SENT, SYN_RCDV, ESTABLISHED,
+  CLOSE_WAIT, FIN_WAIT1, CLOSING, LAST_ACK, FIN_WAIT_2, TIME_WAIT
+
+-s states (UDP)
+  Unbound, Idle
+

Examples

File flags

Show open files with file flags for process:

lsof +fg -p <pid>
+

Open TCP connections

Show open tcp connections for $USER:

lsof -a -u $USER -i TCP
+

Note: -a ands the results. If -a is not given all open files matching +$USER and all tcp connections are listed (ored).

Open connection to specific host

Show open connections to localhost for $USER:

lsof -a -u $USER -i @localhost
+

Open connection to specific port

Show open connections to port :1234 for $USER:

lsof -a -u $USER -i :1234
+

IPv4 TCP connections in `ESTABLISHED` state

lsof -i 4TCP -s TCP:ESTABLISHED
+

List open files in a mounted directory.

This may help to find which processes keep devices busy when trying to unmount +and the device is currently busy.

# Assuming /proc is a mount point.
+lsof /proc
+

+ +

Notes