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✏️ Editing: 00QUICKSTART

A Quick Start for Lsof

1.  Introduction
================

Agreed, the lsof man page is dense and lsof has a plethora of
  options.  There are examples, but the manual page format buries
  them at the end.  How does one get started with lsof?

This file is an attempt to answer that question.  It plunges
  immediately into examples of lsof use to solve problems that
  involve looking at the open files of Unix processes.

Contents

1.  Introduction
	    2.  Finding Uses of a Specific Open File
	    3.  Finding Open Files Filling a File System
		a.  Finding an Unlinked Open File
	    4.  Finding Processes Blocking Umount
	    5.  Finding Listening Sockets
	    6.  Finding a Particular Network Connection
	    7.  Identifying a Netstat Connection
	    8.  Finding Files Open to a Named Command
	    9.  Deciphering the Remote Login Trail
		a.  The Fundamentals
		b.  The idrlogin.perl[5] Scripts
	    10. Watching an Ftp or Rcp Transfer
	    11. Listing Open NFS Files
	    12. Listing Files Open by a Specific Login
		a.  Ignoring a Specific Login
	    13. Listing Files Open to a Specific Process Group
	    14. When Lsof Seems to Hang
		a.  Kernel lstat(), readlink(), and stat() Blockages
		b.  Problems with /dev or /devices
		c.  Host and Service Name Lookup Hangs
		d.  UID to Login Name Conversion Delays
	    15. Output for Other Programs
	    16. The Lsof Exit Code and Shell Scripts
	    17. Strange messages in the NAME column

Options

A.  Selection Options
	    B.  Output Options
	    C.  Precautionary Options
	    D.  Miscellaneous Lsof Options

2.  Finding Uses of a Specific Open File
========================================

Often you're interested in knowing who is using a specific file.
  You know the path to it and you want lsof to tell you the processes
  that have open references to it.

Simple -- execute lsof and give it the path name of the file of
  interest -- e.g.,

$ lsof /etc/passwd

Caveat: this only works if lsof has permission to get the status
  (via stat(2)) of the file at the named path.  Unless the lsof
  process has enough authority  -- e.g., it is being run with a
  real User ID (UID) of root -- this AIX example won't work:

Further caveat: this use of lsof will fail if the stat(2) kernel
  syscall returns different file parameters -- particularly device
  and inode numbers -- than lsof finds in kernel node structures.
  This condition is rare and is usually documented in the 00FAQ
  file of the lsof distribution.

$ lsof /etc/security/passwd
  lsof: status error on /etc/security/passwd: Permission denied

3.  Finding Open Files Filling a File System
============================================

Oh! Oh!  /tmp is filling and ls doesn't show that any large files
  are being created.  Can lsof help?

Maybe.  If there's a process that is writing to a file that has
  been unlinked, lsof may be able to discover the process for you.
  You ask it to list all open files on the file system where /tmp
  is located.

Sometimes /tmp is a file system by itself.  In that case,

$ lsof /tmp

is the appropriate command.  If, however, /tmp is part of another
  file system, typically /, then you may have to ask lsof to list
  all files open on the containing file system and locate the
  offending file and its process by inspection -- e.g.,

$ lsof / | more
  or
    $ lsof / | grep ...

Caveat: there must be a file open to a for the lsof search to
  succeed.  Sometimes the kernel may cause a file reference to
  persist, even where there's no file open to a process.  (Can you
  say kernel bug?  Maybe.)  In any event, lsof won't be able to
  help in this case.

a.  Finding an Unlinked Open File
  =================================

A pesky variant of a file that is filling a file system is an
  unlinked file to which some process is still writing.  When a
  process opens a file and then unlinks it, the file's resources
  remain in use by the process, but the file's directory entries
  are removed.  Hence, even when you know the directory where the
  file once resided, you can't detect it with ls.

This can be an administrative problem when the unlinked file is
  large, and the process that holds it open continues to write to
  it.  Only when the process closes the file will its resources,
  particularly disk space, be released.

Lsof can help you find unlinked files on local disks.  It has an
  option, +L, that will list the link counts of open files.  That
  helps because an unlinked file on a local disk has a zero link
  count.  Note: this is NOT true for NFS files, accessed from a
  remote server.

You could use the option to list all files and look for a zero
  link count in the NLINK column -- e.g.,

$lsof +L
    COMMAND   PID USER   FD  TYPE DEVICE SIZE/OFF NLINK  NODE NAME
    ...
    less    25366  abe  txt  VREG    6,0    40960     1 76319 /usr/...
    ...
  > less    25366  abe    3r VREG    6,0    17360     0 98768 / (/dev/sd0a)

Better yet, you can specify an upper bound to the +L option, and
  lsof will select only files that have a link count less than the
  upper bound.  For example:

$ lsof +L1
    COMMAND   PID USER   FD   TYPE DEVICE SIZE/OFF NLINK  NODE NAME
    less    25366  abe    3r  VREG    6,0    17360     0 98768 / (/dev/sd0a)

You can use lsof's -a (AND) option to narrow the link count search
  to a particular file system.  For example, to look for zero link
  counts on the /home file system, use:

$ lsof -a +L1 /home

CAUTION: lsof can't always report link counts for all file types
  -- e.g., it may not report them for FIFOs, pipes, or sockets.
  Remember also that link counts for NFS files on an NFS client
  host don't behave as do link counts for files on local disks.

4.  Finding Processes Blocking Umount
=====================================

When you need to unmount a file system with the umount command,
  you may find the operation blocked by a process that has a file
  open on the file systems.  Lsof may be able to help you find the
  process.  In response to:

$ lsof <file_system_name>

Lsof will display all open files on the named file system.  It
  will also set its exit code zero when it finds some open files
  and non-zero when it doesn't, making this type of lsof call
  useful in shell scripts.  (See section 16.)

Consult the output of the df command for file system names.

See the caveat in the preceding section about file references
  that persist in the kernel without open file traces.  That
  situation may hamper lsof's ability to help with umount, too.

5.  Finding Listening Sockets
=============================

Sooner or later you may wonder if someone has installed a network
  server that you don't know about.  Lsof can list for you all the
  network socket files open on your machine with:

$ lsof -i

The -i option without further qualification lists all open Internet
  socket files.  You can add network names or addresses, protocol
  names, and service names or port numbers to the -i option to
  refine the search.  (See the next section.)

6.  Finding a Particular Network Connection
===========================================

When you know the source or destination of a network connection
  whose open files and process you'd like to identify, the -i option
  may help.

If, for example, you want to know what process has a connection
  open to or from the Internet host named aaa.bbb.ccc, you can ask
  lsof to search for it with:

$ lsof -i@aaa.bbb.ccc

If you're interested in a particular protocol -- TCP or UDP --
  and a specific port number or service name, you can add those
  discriminators to the -i information:

$ lsof -iTCP@aaa.bbb.ccc:ftp-data

If you're interested in a particular IP version -- IPv4 or IPv6
  -- and your UNIX dialect supports both (It does if "IPv[46]"
  appears in the lsof -h output.), you can add the '4' or '6'
  selector immediately after -i:

$ lsof -i4
  $ lsof -i6

7.  Identifying a Netstat Connection
====================================

How do I identify the process that has a network connection
  described in netstat output?  For example, if netstat says:

Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
  tcp        0      0  vic.1023               ipscgate.login         ESTABLISHED

What process is connected to service name ``login'' on ipscgate?

Use lsof's -i option:

$lsof -iTCP@ipscgate:login
  COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF  INODE NAME
  rlogin    25023      abe    3u  inet 0x10144168      0t184    TCP lsof.itap.purdue.edu:1023->ipscgate.cc.purdue.edu:login
  ...

There's another way.  Notice the 0x10144168 in the DEVICE column
  of the lsof output?  That's the protocol control block (PCB)
  address.  Many netstat applications will display it when given
  the -A option:

$ netstat -A
  PCB      Proto Recv-Q Send-Q  Local Address      Foreign Address    (state)
  10144168 tcp        0      0  vic.1023           ipscgate.login     ESTABLISHED
  ...

Using the PCB address, lsof, and grep, you can find the process this
  way, too:

$ lsof -i | grep 10144168
  rlogin    25023      abe    3u  inet 0x10144168      0t184    TCP lsof.itap.purdue.edu:1023->ipscgate.cc.purdue.edu:login
  ...

If the file is a UNIX socket and netstat reveals and adress for it,
  like this Solaris 11 example:

$ netstat -a -f unix
  Active UNIX domain sockets
  Address  Type          Vnode     Conn  Local Addr      Remote Addr
  ffffff0084253b68 stream-ord 0000000 0000000

Using lsof's -U option and its output piped to a grep on the address
  yields:

$ lsof -U | grep ffffff0084253b68
  squid 1638 nobody 12u unix 18,98 0t10 9437188 /devices/pseudo/tl@0:ticots->0xffffff0084253b68 stream-ord

8.  Finding Files Open to a Named Command
=========================================

When you want to look at the files open to a particular command,
  you can look up the PID of the process running the command and
  use lsof's -p option to specify it.

$ lsof -p <PID>

However, there's a quicker way, using lsof's -c option, provided
  you don't mind seeing output for every process running the named
  command.

$ lsof -c <first_characters_of_command_name_that_interest_you>

The lsof -c option is useful when you want to see how many instances
  of a given command are executing and what their open files are.
  One useful example is for the sendmail command.

$ lsof -c sendmail

9.  Deciphering the Remote Login Trail
======================================

If the network connection you're interested in tracing has been
  initiated externally and is connected to an rlogind, sshd, or
  telnetd process, asking lsof to identify that process might not
  give a wholly satisfying answer.  The report may be that the
  connection exists, but to a process owned by root.

a.  The Fundamentals
  ====================

How do you get from there to the login name really using the
    connection?  You have to know a little about how real and pseudo
    ttys are paired in your system, and then use several lsof probes
    to identify the login.

This example comes from a Solaris 2.4 system, named klaatu.cc.
    I've logged on to it via rlogin from lsof.itap.  The first lsof
    probe,

$ lsof -i@lsof.itap

yields (among other things):

COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF  INODE NAME
    in.rlogin  7362     root    0u  inet 0xfc0193b0      0t242    TCP klaatu.cc.purdue.edu:login->lsof.itap.purdue.edu:1023
    ...

This confirms that a connection exists.  A second lsof probe
    shows:

$ lsof -p7362
    COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF  INODE NAME
    ...
    in.rlogin  7362     root    0u  inet 0xfc0193b0      0t242    TCP klaatu.cc.purdue.edu:login->lsof.itap.purdue.edu:1023
    ...
    in.rlogin  7362     root    3u  VCHR    23,   0       0t66  52928 /devices/pseudo/clone@0:ptmx->pckt->ptm

7362 is the Process ID (PID) of the in.rlogin process, discovered
    in the first lsof probe.  (I've abbreviated the output to simplify
    the example.)  Now comes a need to understand Solaris pseudo-ttys.
    The key indicator is in the DEVICE column for FD 3, the major/minor
    device number of 23,0.  This translates to /dev/pts/0, so a third
    lsof probe,

$ lsof /dev/pts/0
    COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF  INODE NAME
    ksh        7364      abe    0u  VCHR    24,   0     0t2410  53410 /dev/pts/../../devices/pseudo/pts@0:0

shows in part that login abe has a ksh process on /dev/pts/0.
    (The NAME that lsof shows is not /dev/pts/0 but the full expansion
    of the symbolic link that lsof finds at /dev/pts/0.)

Here's a second example, done on an HP-UX 9.01 host named ghg.ecn.
    Again, I've logged on to it from lsof.itap, so I start with:

$ lsof -i@lsof.itap
    COMMAND     PID     USER   FD   TYPE       DEVICE   SIZE/OFF  INODE NAME
    rlogind   10214     root    0u  inet   0x041d5f00     0t1536    TCP ghg.ecn.purdue.edu:login->lsof.itap.purdue.edu:1023
    ...

Then,

$ lsof -p10214
    COMMAND     PID     USER   FD   TYPE       DEVICE   SIZE/OFF  INODE NAME
    ...
    rlogind   10214     root    0u  inet   0x041d5f00     0t2005    TCP ghg.ecn.purdue.edu:login->lsof.itap.purdue.edu:1023
    ...
    rlogind   10214     root    3u  VCHR  16,0x000030     0t2037  24642 /dev/ptym/ptys0

Here the key is the NAME /dev/ptym/ptys0.  In HP-UX 9.01 tty and
    pseudo tty devices are paired with the names like /dev/ptym/ptys0
    and /dev/pty/ttys0, so the following lsof probe is the final step.

$ lsof /dev/pty/ttys0
    COMMAND     PID     USER   FD   TYPE       DEVICE   SIZE/OFF  INODE NAME
    ksh       10215      abe    0u  VCHR  17,0x000030     0t3399  22607 /dev/pty/ttys0
    ...

Here's a third example for an AIX 4.1.4 system.  I've used telnet
    to connect to it from lsof.itap.purdue.edu.  I start with:

$ lsof -i@lsof.itap.purdue.edu
    COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF      INODE NAME
    ...
    telnetd   15616     root    0u  inet 0x05a93400     0t5156        TCP cloud.cc.purdue.edu:telnet->lsof.itap.purdue.edu:3369

Then I look at the telnetd process:

$ lsof -p15616
    COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF      INODE NAME
    ...
    telnetd   15616     root    0u  inet 0x05a93400     0t5641        TCP cloud.cc.purdue.edu:telnet->lsof.itap.purdue.edu:3369
    ...
    telnetd   15616     root    3u  VCHR    25,   0     0t5493        103 /dev/ptc/0

Here the key is /dev/ptc/0.  In AIX it's paired with /dev/pts/0.
    The last probe for that shows:

$ lsof /dev/pts/0
    COMMAND     PID     USER   FD   TYPE     DEVICE   SIZE/OFF      INODE NAME
    ...
    ksh       16642      abe    0u  VCHR    26,   0     0t6461        360 /dev/pts/0

b.  The idrlogin.perl[5] Scripts
  ================================

There's another, perhaps easier way, to go about the job of
    tracing a network connection.  The lsof distribution contains
    two Perl scripts, idrlogin.perl (Perl 4) and idrlogin.perl5
    (Perl 5), that use lsof field output to display values for
    shells that are parented by rlogind, sshd, or telnetd, or
    connected directly to TCP sockets.  The lsof test suite contains
    a C library that can be adapted for use with C programs that
    need to call lsof and process its field output.

The two Perl scripts use the lsof -R option; it causes the
    paRent process ID (PPID) to be listed in the lsof output.  The
    scripts identify all shell processes -- e.g., ones whose command
    names end in ``sh'' -- and determine if: 1) the ultimate ancestor
    process before a PID greater than 2 (e.g., init's PID is 1) is
    rlogind, sshd, or telnetd; or 2) the shell process has open
    TCP socket files.

Here's an example of output from idlogin.perl on a Solaris 2.4
    system:

centurion: 1 = cd src/lsof4/scripts
    centurion: 2 = ./idrlogin.perl
    Login    Shell       PID Via           PID TTY        From
    oboyle   ksh       12640 in.telnetd  12638 pts/5      opal.cc.purdue.edu
    icdtest  ksh       15158 in.rlogind  15155 pts/6      localhost
    sh       csh       18207 in.rlogind  18205 pts/1      babylon5.cc.purdue.edu
    root     csh       18242 in.rlogind  18205 pts/1      babylon5.cc.purdue.edu
    trouble  ksh       19208 in.rlogind  18205 pts/1      babylon5.cc.purdue.edu
    abe      ksh       21334 in.rlogind  21332 pts/2      lsof.itap.purdue.edu

The scripts assume that its parent directory contains an
    executable lsof.  If you decide to use one of the scripts, you
    may want to customize it for your local lsof and perl paths.

Note that processes executing as remote shells are also
    identified.

Here's another example from a UnixWare 7.1.0 system.

tweeker: 1 = cd src/lsof4/scripts
    tweeker: 9 = ./idrlogin.perl
    Login    Shell       PID Via           PID TTY        From
    abe      ksh        9438 in.telnetd   9436 pts/3      lsof.itap.purdue.edu

10. Watching an Ftp or Rcp Transfer
===================================

The nature of the Internet being one of unpredictable performance
  at times, occasionally you want to know if a file transfer, being
  done by ftp or rcp, is making any progress.

To use lsof for watching a file transfer, you need to know the
  PID of the file transfer process.  You can use ps to find that.
  Then use lsof,

$ lsof -p<PID>

to examine the files open to the transfer process.  Usually the
  ftp files or interest are at file descriptors 9 and 10 or 10 and
  11; for rcp, 3 and 4.  They describe the network socket file and
  the local data file.

If you want to watch only those file descriptors as the file
  transfer progresses, try these lsof forms (for ftp in the example):

$ lsof -p<PID> -ad9,10 -r
  or
    $ lsof -p<PID> -ad10,11 -r

Some options need explaining:

-p<PID>	specifies that lsof is to restrict its attention
		to the process whose ID is <PID>.  You can specify
		a set of PIDs by separating them with commas.

$ lsof -p 1234,5678,9012

-a		specifies that lsof is to AND its tests together.
		The two tests that are specified are tests on the
		PID and tests on file descriptions (``d9,10'').

d9,10	specifies that lsof is to test only file descriptors
		9 and 10.  Note that the `-' is absent, since ``-a''
		is a unary option and can be followed immediately
		by another lsof option.

-r          tells lsof to list the requested open file information,
		sleep for a default 15 seconds, then list the open
		file information again.  You can specify a different
		time (in seconds) after -r and override the default.
		Lsof issues a short line of equal signs between
		each set of output to distinguish it.

For an rcp transfer, the above example becomes:

$ lsof -p<PID> -ad3,4 -r

11. Listing Open NFS Files
==========================

Lsof will list all files open on remote file systems, supported
  by an NFS server.  Just use:

$ lsof -N

Note, however, that when run on an NFS server, lsof will not list
  files open to the server from one of its clients.  That's because
  lsof can only examine the processes running on the machine where
  it is called -- i.e., on the NFS server.

If you run lsof on the NFS client, using the -N option, it will
  list files open by processes on the client that are on remote
  NFS file systems.

12. Listing Files Open by a Specific Login
==========================================

If you're interested in knowing what files the processes owned
  by a particular login name have open, lsof can help.

$ lsof -u<login>
  or
    $ lsof -u<User ID number>

You can specify either the login name or the UID associated with
  it.  You can specify multiple login names and UID numbers, mixed
  together, by separating them with commas.

$ lsof -u548,abe

On the subject of login names and UIDs, it's worth noting that
  lsof can be told to report either.  By default it reports login
  names; the -l option switches reporting to UIDs.  You might want
  to use -l if login name lookup is slow for some reason.

a.  Ignoring a Specific Login
  =============================

The -u option can also be used to direct lsof to ignore a
    specific login name or UID, or a list of them.  Simply prefix
    the login names or UIDs with a `^' character, as you might do
    in a regular expression.  The `^' prefix is useful, for example,
    when you want to have lsof ignore the files open to system
    processes, owned by the root (UID 0) login.  Try:

$ lsof -u ^root
    or
      $ lsof -u ^0

13. Listing Files Open to a Specific Process Group
==================================================

There's a Unix collection of processes called a process group.
  The name indicates that the processes of the group have a common
  association and are grouped so that a signal sent to one (e.g.,
  a keyboard kill stroke) is delivered to all.

This causes Unix to create a two element process group:

$ lsof | less

You can use lsof to look at the open files of all members of a
  process group, if you know the process group ID number.  Assuming
  that it is 12717 for the above example, this lsof command:

$ lsof -g12717 -adcwd

would produce on a Solaris 8 system:

$ lsof -g12717 -adcwd
  COMMAND   PID  PGID USER  FD TYPE DEVICE SIZE/OFF    NODE NAME
  sshd    11369 12717 root cwd VDIR    0,2      189 1449175 /tmp (swap)
  sshd    12717 12717 root cwd VDIR  136,0     1024       2 /

The ``-g12717'' option specifies the process group ID of interest;
  the ``-adcwd'' option specifies that options are to be ANDed and
  that lsof should limit file output to information about current
  working directory (``cwd'') files.

14. When Lsof Seems to Hang
===========================

On occasion when you run lsof it seems to hang and produce no
  output.  This may result from system conditions beyond the control
  of lsof.  Lsof has a number of options that may allow you to
  bypass the blockage.

a.  Kernel lstat(), readlink(), and stat() Blockages
  ====================================================

Lsof uses the kernel (system) calls lstat(), readlink(), and
    stat() to locate mounted file system information.  When a file
    system has been mounted from an NFS server and that server is
    temporarily unavailable, the calls lsof uses may block in the
    kernel.

Lsof will announce that it is being blocked with warning messages
    (unless they have been suppressed by the lsof builder), but
    only after a default waiting period of fifteen seconds has
    expired for each file system whose server is unavailable.  If
    you have a number of such file systems, the total wait may be
    unacceptably long.

You can do two things to shorten your suffering: 1) reduce the
    wait time with the -S option; or 2) tell lsof to avoid the
    kernel calls that might block by specifying the -b option.

$ lsof -S 5
    or
      $ lsof -b

Avoiding the kernel calls that might block may result in the
    lack of some information that lsof needs to know about mounted
    file systems.  Thus, when you use -b, lsof warns that it might
    lack important information.

The warnings that result from using -b (unless suppressed by
    the lsof builder) can themselves be annoying.  You can suppress
    them by adding the -w option.  (Of course, if you do, you won't
    know what warning messages lsof might have issued.)

$ lsof -bw

Note: if the lsof builder suppressed warning message issuance,
    you don't need to use -w to suppress them.  You can tell what
    the default state of message warning issuance is by looking at
    the -h (help) output.  If it says ``-w enable warnings'' then
    warnings are disabled by default; ``-w disable warnings'', they
    are enabled by default.

b.  Problems with /dev or /devices
  ==================================

Lsof scans the /dev or /devices branch of your file system to
    obtain information about your system's devices.  (The scan isn't
    necessary when a device cache file exists.)

Sometimes that scan can take a very long time, especially if
    you have a large number of devices, and if your kernel is
    relatively slow to process the stat() system call on device
    nodes.  You can't do anything about the stat() system call
    speed.

However, you can make sure that lsof is allowed to use its
    device cache file feature.  When lsof can use a device cache
    file, it retains information it gleans via the stat() calls
    on /dev or /devices in a separate file for later, faster
    access.

The device cache file feature is described in the lsof man
    page.  See the DEVICE CACHE FILE, LSOF PERMISSIONS THAT AFFECT
    DEVICE CACHE FILE ACCESS, DEVICE CACHE FILE PATH FROM THE -D
    OPTION, DEVICE CACHE PATH FROM AN ENVIRONMENT VARIABLE,
    SYSTEM-WIDE DEVICE CACHE PATH, PERSONAL DEVICE CACHE PATH
    (DEFAULT), and MODIFIED PERSONAL DEVICE CACHE PATH sections.

There is also a separate file in the lsof distribution, named
    00DCACHE, that describes the device cache file in detail,
    including information about possible security problems.

One final observation: don't overlook the possibility that your
    /dev or /devices tree might be damaged.  See if

$ ls -R /dev
    or
      $ ls -R /devices

completes or hangs.  If it hangs, then lsof will probably hang,
    too, and you should try to discover why ls hangs.

c.  Host and Service Name Lookup Hangs
    ======================================

Lsof can hang up when it tries to convert an Internet dot-form
    address to a host name, or a port number to a service name.  Both
    hangs are caused by the lookup functions of your system.

An independent check for both types of hangs can be made with
    the netstat program.  Run it without arguments.  If it hangs,
    then it is probably having lookup difficulties.  When you run
    it with -n it shouldn't hang and should report network and port
    numbers instead of names.

Lsof has two options that serve the same purpose as netstat's
    -n option.  The lsof -n option tells it to avoid host name
    lookups; and -P, service name lookups.  Try those options when
    you suspect lsof may be hanging because of lookup problems.

$ lsof -n
    or
      $ lsof -P
    or
      $ lsof -nP

d.  UID to Login Name Conversion Delays
    =======================================

By default lsof converts User IDentification (UID) numbers to
    login names when it produces output.  That conversion process
    may sometimes hang because of system problems or interlocks.

You can tell lsof to skip the lookup with the -l option; it
    will then report UIDs in the USER column.

$ lsof -l

15. Output for Other Programs
=============================

The -F option allows you to specify that lsof should describe
  open files with a special form of output, called field output,
  that can be parsed easily by a subsequent program.  The lsof
  distribution comes with sample AWK, Perl 4, and Perl 5 scripts
  that post-process field output.  The lsof test suite has a C
  library that could be adapted for use by C programs that want to
  process lsof field output from an in-bound pipe.

The lsof manual page describes field output in detail in its
  OUTPUT FOR OTHER PROGRAMS section.  A quick look at a sample
  script in the scripts/ subdirectory of the lsof distribution will
  also give you an idea how field output works.

The most important thing about field output is that it is relatively
  homogeneous across Unix dialects.  Thus, if you write a script
  to post-process field output for AIX, it probably will work for
  HP-UX, Solaris, and Ultrix as well.

16. The Lsof Exit Code and Shell Scripts
========================================

When lsof exits successfully it returns an exit code based on
  the result of its search for specified files.  (If no files were
  specified, then the successful exit code is 0 (zero).)

If lsof was asked to search for specific files, including any
  files on specified file systems, it returns an exit code of 0
  (zero) if it found all the specified files and at least one file
  on each specified file system.  Otherwise it returns a 1 (one).

If lsof detects an error and makes an unsuccessful exit, it
  returns an exit code of 1 (one).

You can use the exit code in a shell script to search for files
  on a file system and take action based on the result -- e.g.,

#!/bin/sh
    lsof <file_system_name> > /dev/null 2>&1
    if test $? -eq 0
    then
      echo "<file_system_name> has some users."
    else
      echo "<file_system_name> may have no users."
    fi

17. Strange messages in the NAME column
=======================================

When lsof encounters problems analyzing a particular file, it may
  put a message in the file's NAME column.  Many of those messages
  are explained in the 00FAQ file of the lsof distribution.

So consult 00FAQ first if you encounter a NAME column message you
  don't understand.  (00FAQ is a possible source of information
  about other unfamiliar things in lsof output, too.)
  
  If you can't find help in 00FAQ, you can use grep to look in the
  lsof source files for the message -- e.g.,

$ cd .../lsof_4.76_src
    $ grep "can't identify protocol" *.[ch]

The code associated with the message will usually make clear the
  reason for the message.

If you have an lsof source tree that has been processed by the
  lsof Configure script, you need grep only there.  If, however,
  your source tree hasn't been processed by Configure, you may
  have to look in the top-level lsof source directory and in the
  dialects sub-directory for the UNIX dialect you are using - e.g.,

$ cd .../lsof_4.76_src
    $ grep "can't identify protocol" *.[ch]
    $ cd dialects/Linux
    $ grep "can't identify protocol" *.[ch]

In rare cases you may have to look in the lsof library, too --
  e.g.,

$ cd .../lsof_4.76_src
    $ grep "can't identify protocol" *.[ch]
    $ cd dialects/Linux
    $ grep "can't identify protocol" *.[ch]
    $ cd ../../lib
    $ grep "can't identify protocol" *.[ch]

Options
=======

The following appendices describe the lsof options in detail.

A.  Selection Options
====================

Lsof has a rich set of options for selecting the files to be
  displayed.  These include:

-a	tells lsof to AND the set of selection options that
		are specified.  Normally lsof ORs them.
		
		For example, if you specify the -p<PID> and -u<UID>
		options, lsof will display all files for the
		specified PID or for the specified UID.

By adding -a, you specify that the listed files
		should be limited to PIDs owned by the specified
		UIDs -- i.e., they match the PIDs *and* the UIDs.

$ lsof -p1234 -au 5678

-c	specifies that lsof should list files belonging
		to processes having the associated command name.

Hint: if you want to select files based on more than
		one command name, use multiple -c<name> specifications.

$ lsof -clsof -cksh

-d      tells lsof to select by the associated file descriptor
		(FD) set.  An FD set is a comma-separated list of
		numbers and the names lsof normally displays in
		its FD column:  cwd, Lnn, ltx, <number>, etc.  See
		the OUTPUT section of the lsof man page for the
		complete list of possible file descriptors.  Example:

$ lsof -dcwd,0,1,2

-g      tells lsof to select by the associated process
		group ID (PGID) set.  The PGID set is a comma-separated
		list of PGID numbers.  When -g is specified, it also
		enables the display of PGID numbers.

Note: when -g isn't followed by a PGID set, it
		simply selects the listing of PGID for all processes.
		Examples:

$ lsof -g
		    $ lsof -g1234,5678

-i	tells lsof to display Internet socket files.  If no
		protocol/address/port specification follows -i,
		lsof lists all Internet socket files.

If a specification follows -i, lsof lists only the
		socket files whose Internet addresses match the
		specification.

Hint: multiple addresses may be specified with
		multiple -i options.  Examples:

$ lsof -iTCP
		    $ lsof -i@lsof.itap.purdue.edu:sendmail

-N	selects the listing of files mounted on NFS devices.

-U	selects the listing of socket files in the Unix
		domain.

B.  Output Options
==================

Lsof has these options to control its output format:

-F	produce output that can be parsed by a subsequent
		program.

-g	print process group (PGID) IDs.

-l	list UID numbers instead of login names.

-n	list network numbers instead of host names.

-o	always list file offset.

-P	list port numbers instead of port service names.

-s	always list file size.

C.  Precautionary Options
=========================

Lsof uses system functions that can block or take a long time,
  depending on the health of the Unix dialect supporting it.  These
  include:

-b	directs lsof to avoid system functions -- e.g.,
		lstat(2), readlink(2), stat(2) -- that might block
		in the kernel.  See the BLOCKS AND TIMEOUTS
		section of the lsof man page.

You might want to use this option when you have
		a mount from an NFS server that is not responding.

-C	tells lsof to ignore the kernel's name cache.  As
		a precaution this option will have little effect on
		lsof performance, but might be useful if the kernel's
		name cache is scrambled.  (I've never seen that
		happen.)

-D	might be used to direct lsof to ignore an existing
		device cache file and generate a new one from /dev
		(and /devices).  This might be useful if you have
		doubts about the integrity of an existing device
		cache file.

-l      tells lsof to list UID numbers instead of login
		names -- this is useful when UID to login name
		conversion is slow or inoperative.

-n	tells lsof to avoid converting Internet addresses
		to host numbers.  This might be useful when your
		host name lookup (e.g., DNS) is inoperative.

-O      tells lsof to avoid its strategy of forking to
		perform potentially blocking kernel operations.
		While the forking allows lsof to detect that a
		block has occurred (and possibly break it), the
		fork operation is a costly one.  Use the -O option
		with care, lest your lsof be blocked.

-P      directs lsof to list port numbers instead of trying
		to convert them to port service names.  This might
		be useful if port to service name lookups (e.g.,
		via NIS) are slow or failing.

-S      can be used to change the lstat/readlink/stat
		timeout interval that governs how long lsof waits
		for response from the kernel.  This might be useful
		when an NFS server is slow or unresponsive.  When
		lsof times out of a kernel function, it may have
		less information to display.  Example:

$ lsof -S2

-w	tells lsof to avoid issuing warning messages, if
		they are enabled by default, or enable them if they
		are disabled by default.  Check the -h (help) output
		to determine their status.  If it says ``-w enable
		warnings'', then warning messages are disabled by
		default; ``-w disable warnings'', they are enabled
		by default.

This may be a useful option, for example, when you
		specify -b, if warning messages are enabled, because
		it will suppress the warning messages lsof issues
		about avoiding functions that might block in the
		kernel.

D.  Miscellaneous Lsof Options
==============================

There are some lsof options that are hard to classify, including:

-?	these options select help output.
	-h

-F      selects field output.  Field output is a mode where
		lsof produces output that can be parsed easily by
		subsequent programs -- e.g., AWK or Perl scripts.
		See ``15. Output for Other Programs'' for more
		information.

-k	specifies an alternate kernel symbol file -- i.e.,
		where nlist() will get its information.  Example:

$ lsof -k/usr/crash/vmunix.1

-m	specifies an alternate kernel memory file from
		which lsof will read kernel structures in place
		of /dev/kmem or kvm_read().  Example:

$ lsof -m/usr/crash/vmcore.n

-r	tells lsof to repeat its scan every 15 seconds (the
		default when no associated value is specified).  A
		repeat time, different from the default, can follow
		-r.  Example:

$ lsof -r30

-v	displays information about the building of the
		lsof executable.

--      The double minus sign option may be used to
		signal the end of options.  It's particularly useful
		when arguments to the last option are optional and
		you want to supply a file path that could be confused
		for arguments to the last option.  Example:

$ lsof -g -- 1
		
		Where `1' is a file path, not PGID ID 1.

Vic Abell <abe@purdue.edu>
October 13, 2014

MR SKB