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opensnoop (Mac file open watcher)

© May 2019 Anthony Lawrence

One of the fun things to do with any new OS that runs bash is to hit a random letter and then TAB twice. As you probably know, that displays commands in your search path that begin with the letter(s) you typed. That's what led me to sandbox-exec yesterday, and it's what brought me to "opensnoop" today.

Opensnoop uses the powerful Dtrace facility, which does mean that you need to run it with "sudo", but if you can do that, it can watch any or all files you like for "open" events - including failed opens (which can be useful for troubleshooting and is far less wordy than poring through strace output).

By the way, there is a large pile of pre-made Dtrace scripts available on Leopard. An "apropos Dtrace" shows me all these:

bitesize.d(1m)           - analyse disk I/O size by process. Uses DTrace
cpuwalk.d(1m)            - Measure which CPUs a process runs on. Uses DTrace
creatbyproc.d(1m)        - snoop creat()s by process name. Uses DTrace
dappprof(1m)             - profile user and lib function usage. Uses DTrace
dapptrace(1m)            - trace user and library function usage. Uses DTrace
diskhits(1m)             - disk access by file offset. Uses DTrace
dispqlen.d(1m)           - dispatcher queue length by CPU. Uses DTrace
dtrace(1)                - generic front-end to the DTrace facility
dtruss(1m)               - process syscall details. Uses DTrace
errinfo(1m)              - print errno for syscall fails. Uses DTrace
execsnoop(1m)            - snoop new process execution. Uses DTrace
fddist(1m)               - file descriptor usage distributions. Uses DTrace
filebyproc.d(1m)         - snoop opens by process name. Uses DTrace
hotspot.d(1m)            - print disk event by location. Uses DTrace
httpdstat.d(1m)          - realtime httpd statistics. Uses DTrace
iofile.d(1m)             - I/O wait time by file and process. Uses DTrace
iofileb.d(1m)            - I/O bytes by file and process. Uses DTrace
iopattern(1m)            - print disk I/O pattern. Uses DTrace
iopending(1m)            - plot number of pending disk events. Uses DTrace
iosnoop(1m)              - snoop I/O events as they occur. Uses DTrace
iotop(1m)                - display top disk I/O events by process. Uses DTrace
kill.d(1m)               - snoop process signals as they occur. Uses DTrace
lastwords(1m)            - print syscalls before exit. Uses DTrace
loads.d(1m)              - print load averages. Uses DTrace
newproc.d(1m)            - snoop new processes. Uses DTrace
opensnoop(1m)            - snoop file opens as they occur. Uses DTrace
pathopens.d(1m)          - full pathnames opened ok count. Uses DTrace
pidpersec.d(1m)          - print new PIDs per sec. Uses DTrace
plockstat(1)             - front-end to DTrace to print statistics about POSIX 
mutexes and read/write locks
priclass.d(1m)           - priority distribution by scheduling class. Uses DTrace
pridist.d(1m)            - process priority distribution. Uses DTrace
procsystime(1m)          - analyse system call times. Uses DTrace
runocc.d(1m)             - run queue occupancy by CPU. Uses DTrace
rwbypid.d(1m)            - read/write calls by PID. Uses DTrace
rwbytype.d(1m)           - read/write bytes by vnode type. Uses DTrace
rwsnoop(1m)              - snoop read/write events. Uses DTrace
sampleproc(1m)           - sample processes on the CPUs. Uses DTrace
seeksize.d(1m)           - print disk event seek report. Uses DTrace
setuids.d(1m)            - snoop setuid calls as they occur. Uses DTrace
sigdist.d(1m)            - signal distribution by process. Uses DTrace
syscallbypid.d(1m)       - syscalls by process ID. Uses DTrace
syscallbyproc.d(1m)      - syscalls by process name. Uses DTrace
syscallbysysc.d(1m)      - syscalls by syscall. Uses DTrace
topsyscall(1m)           - top syscalls by syscall name. Uses DTrace
topsysproc(1m)           - top syscalls by process name. Uses DTrace
weblatency.d(1m)         - website latency statistics. Uses DTrace

I'll only look at "opensnoop" today, but obviously there's a lot of territory to be explored here..

The man page for "opensnoop" implies that you can use it to watch a specific file. You can, but it's not particularly smart about it. For example, the man page suggests:

opensnoop -f /etc/passwd

Well, there's a grand idea, right? But let's test it out, shall we? After typing that (with sudo, of course), I'll see this:

$ sudo opensnoop -f /etc/passwd
  UID    PID COMM          FD PATH 

Switching to another screen, I type "cat /etc/passwd", and when I return, I see:

  UID    PID COMM          FD PATH                 
  501  60638 cat            3 /etc/passwd 

Wonderful! Looks like I can easily watch any file I like, right? Well, no.. not exactly. If I instead do:

cd /etc
cat passwd

Opensnoop doesn't notice. Now, opensnoop is just a shell script that prepares a Dtrace script and runs it, so if you wanted to dig into the details of Dtrace (I'd like to, but not today), you could probably remedy this ignorance. Or you could only monitor the most likely usages (/etc/passwd, passwd, ./passwd) - after all, how likely is it that a program would open "/usr/../etc/passwd"?

Running just "opensnoop" (no arguments) watches everything. That can be an eye-opener.. there's a lot going on that you probably never thought about. Certain programs are more active than others: Parallels is constantly opening files even when I'm not using it, but even things you'd expect to sit quietly waiting for you to Alt-Tab will cause "opensnoop" to spit out some data now and then.. why did Preview suddenly need to open /.vol/234881026/9411827 just now? I sure don't know, but it is no wonder my battery life isn't so great when I leave all these other apps running and ready to Alt-Tab to..

You can also snoop a specific process by name or process id. "sudo opensnoop -n Preview" keeps an eye on that sneaky Preview app..

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