You can find the first article of the series here: Crazy DevOps interview questions.
Question 1:
Suppose you run the following commands:
# cd /tmp # mkdir a # cd a # mkdir b # cd b # ln /tmp/a a
… what is the result?
At this point one may point out that the hardlink being defined may basically create a circular reference, which is a correct answer on its own. It’s not complete, though: how would the operating system (the file system) handle such command, anyway?
A command line guru may simply dismiss the question saying that hardlinks are not allowed for directories and that’s about it. Another guru may point out that we’re missing the -d parameter to ln and the command will fail before anything else considered. Correct, but still not the complete answer expected by the interviewer.
The complete answer must point out that:
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Not all file systems disallow directory hardlinks (most do). The notable exception is HFS+ (Apple OS/X).
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The hard links are, by definition, multiple directory entries pointing to a single inode. There is a “hardlink counter” field within the inode. Deleting a hard link will not delete the file unless that counter is 1.
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Directory hard links are not by definition dangerous to be disallowed by default. The major problem with them is the circular reference situation described above. This can be solved by using graph theory but such implementation is both cpu and memory intensive.
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The decision to disallow hard links for directories was taken with this computation cost in mind. Such computation cost grows with the file system size.
I agree to you that a comprehensive answer is usually expected in an interview setting by a company within the “Big Four” technology companies.
Every large deployment has the odd Windows node; well, not every deployment, as many places have Windows-only environments. The reason is quite simple: most closed-source SDKs do not run on anything but Windows, so one needs to write a Windows service to access those functionalities. Fortunately for command-line gurus, Microsoft has created an extraordinary toolset, the PowerShell.
When coding something in PowerShell, one must remember 2 things:
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The script files should end in .ps1 rather than .bat (and one must usually override a security setting to get the system to run them);
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Most tasks are one-liners.
Coming from Linux scripting some things may look odd (e.g. it is not possible to directly move directories, one must copy them recursively and then delete the source) but in the end the job gets done. I have put below a few basic tasks that one may at some point need to perform.
1. Getting a zip file from some http server and unzip it:
$source_file = "http://internal.repo/package/module.zip" $tmp_folder = "C\Temp" $dest_folder = "C:\Program Files\Internal\Application" $source_file_name = [io.path]::GetFileNameWithoutExtension($source_file) $dest_zip = "$tmp_folder\$source_file_name" New-Item -ItemType Directory -Force -Path $temp_folder Invoke-WebRequest $source_file -OutFile $dest_zip New-Item -ItemType Directory -Force -Path $dest_folder Add-Type -AssemblyName System.IO.Compression.FileSystem [System.IO.Compression.ZipFile]::ExtractToDirectory($dest_zip, $dest_folder) Remove-Item -Path "$dest_zip" -Force ### No -Force for the temp folder Remove-Item -Path "$tmp_folder"
Who likes interviews? Me neither. Well, depends…
If you get any of the following questions during an interview then either the interviewer did read this one or he’s getting info from the same sources as I am. Either way, let’s get one step forward.
Question 1:
Suppose you log into a node and find the following situation:
# ls -la /bin/chmod rw-r--r-- 1 root root 56032 ian 14 2015 /bin/chmod
Is it fixable?
… yes. Most of the time. Let’s remember how the executables are being started on Linux: with a system loader, ld-linux-something. Let’s check:
# ldd /bin/chmod linux-vdso.so.1 => (0x00007ffdf27fc000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb11a650000) /lib64/ld-linux-x86-64.so.2 (0x00007fb11aa15000)
OK, got it:
# /lib64/ld-linux-x86-64.so.2 /bin/chmod +x /bin/chmod
Fun, isn’t it? The follow up question obviously is “what if the loader’s rights are unproperly set” or something similar. The answer to that is that not all the filesystem issues are fixable with easy commands. One may even have to mount the file system on a different installation (e.g. from a Live CD or attach the virtual storage to another, “good” node) and fix things up from there.