Ny way to tell exactly what files a command is accessing

filesresources

I'm working on a piece of software that requires me to know what files and resources any certain launched process are accessing.

I'm not planning on attempting to track what every single script, application, and daemon is accessing, just a certain process provided by the user.

Is there any way to do this in Python (or any other language for that matter)? I'm going to do some research of my own, I just figured I'd ask here in case there are knowledgeable users out there who know about this sort of thing and can provide a bit more explanation.

Best Answer

You can trace the system calls that a program makes. This is the usual method to find out what files it accesses. The tool to do this is called truss in many Unix systems, dtruss on OSX, strace on Linux. I'll describe Linux usage here; check the manual on other systems.

The simplest form is

strace myprogram arg1 arg2

This prints a log of all the system calls made by myprogram. (Example.) To save the log in a file, use the option -o. To also log calls made by subprocesses, use the option -f. To select which system calls are logged, use the option -e. See the manual for details of what you can use as an argument to -e. For example, the following invocation logs file-related system calls (opening and closing, directory listing, etc.) except read and write.

strace -e'file,!read,!write' -o /tmp/myprogram.log -f myprogram arg1 arg2

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Linux – way to limit overall CPU consumption

I don't know that limiting CPU to the whole system is something that's possible without a lot of hacking, but you can easily limit the amount of CPU used by a single process using cpulimit

The only way I can think of you being able to use this effectively is writing a wrapper script (can't really call it a script, it's so small) for the applications which you know are resource hogs. Say for example, you find google-chrome uses a lot of CPU, you could replace the google-chrome binary in your path with something like:

#! /bin/bash
cpulimit --limit 70 /usr/bin/google-chrome-bin

I haven't tested this so take it with a grain of salt. From cpulimit's website, it seems like you might be able to set rules for cpu limits on different applications. I'm not sure, you'd have to take a look.

Shell Command – Why No Command to Create Files

I would say because it's hardly ever necessary to create an empty file that you won't fill with content immediately on the command line or in shell scripting.

There is absolutely no benefit in creating a file first and then using I/O redirection to write to the file if you can do so in one step.

In those cases where you really want to create an empty file and leave it I'd argue that > "${file}" could not be briefer and more elegant.

TL;DR: It doesn't exist because creating empty files most often has no use, and in cases it has there are already a myriad of options available to achieve this goal.

On a side note, using touch only works if the file does not exist whereas options using redirection will always truncate the file, even if it exists (so technically, those solutions are not identical). > foo is the preferred method since it saves a fork and echo -n should be avoided in general since it's highly unportable.

Best Answer

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Linux – way to limit overall CPU consumption

Shell Command – Why No Command to Create Files

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