I'm running this command:

strace -f -e trace=desc ./my-program

(So you see I'm running the -f parameter)

Which lets me see the stdout/stderr write commands of the parent process:

[pid    10] 07:36:46.668931 write(2, "..\n"..., 454 <unfinished ...>

<stdout of ..>

<stdout other output - but I don't see the write commands - so probably from a child process>

[pid    10] 07:36:46.669684 write(2, "My final output\n", 24 <unfinished ...>

<stdout of My final output>

What I want to see is the other write commands.

This is running in a docker container being executed with:

docker run --privileged=true my-label/my-container

My question is: How to strace all write calls (to stdout/stderr) from all child processes inside a docker container?

EDIT: Note that this application starts and stops in about 2 seconds. strace can follow its application cycle. But tools that expect it to hang around like a server process won't work. (Perhaps sysdig can be used for this - but I haven't seen an example of this model yet).

  • you may also consider sysdig which I believe has container support – thrig Jan 27 '18 at 15:46
  • thanks @thrig - I think that sysdig won't run against applications (ones that start and stop quickly) - only against server process that hang around perpetually. (Correct me if I'm wrong). – hawkeye Jan 28 '18 at 10:53
  • how would the kernel (where sysdig runs) not see a "quick" process? – thrig Jan 28 '18 at 16:12

It turns out strace truncates string output - you have to explicitly tell it that you want more than the first n (10?) string chars. You do this with -s 800.

strace -s 800 -ff  ./myprogram

You can also get all the write commands by asking strace explicitly with -e write.

strace -s 800 -ff -e write  ./myprogram
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