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))
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file flags:
-  R/W/RW ..... read/write/read-write
-  CR ......... create
-  AP ......... append
-  TR ......... truncate
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-s protocols
-  TCP, UDP
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--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
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--s states (UDP)
-  Unbound, Idle
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Examples

File flags

Show open files with file flags for process:

lsof +fg -p <pid>
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Open TCP connections

Show open tcp connections for $USER:

lsof -a -u $USER -i TCP
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Note: -a ands the results. If -a is not given all open files matching -$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
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Open connection to specific port

Show open connections to port :1234 for $USER:

lsof -a -u $USER -i :1234
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IPv4 TCP connections in `ESTABLISHED` state

lsof -i 4TCP -s TCP:ESTABLISHED
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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
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- -

Notes