I'm getting paranoid and would like to know anyone's views, experience in detecting malware, keylogers, and so on.

How can I scan files for malicious code?

TIA

Plug the following in to your favourite search engine:

rkhunter
ClamAV
AIDE

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The first search result is this thread

Don't be paranoid, just be safe and sensible. Use the likes of uBlock Origin and NoScript on your browser, backed up by a sandbox/resettable virtual machine if you're going to insist on browsing dodgy sites, don't open unknown attachments/documents or run unknown executables, exercise good judgement as to what you allow to run as root, occasionally run a malware checker (clamTK, Sophos AV, rkhunter, etc.). Make regular system backups and restore to a previous clean one (or even do a wipe & fresh install) if you suspect that you may have contracted some malware. It's all common sense and it should become second nature.

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It may be a little complicated, but if you have a file you're suspicious of and want to verify that it hasn't been replaced with a trojaned copy you can always perform a stat of the file to look at inode numbers. Generally, files installed together (as part of the same package or packages installed concurrently, which you can figure out by looking at your install logs) will take inodes next to each other. So a file which is part of Package A might have 11554730, while a file which is also part of Package A = 11554731. If files of an important system package aren't lining up, you might want to investigate further.

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this is a very nice gotcha, which makes me wonder is there any security scanner software that does such kind of filesystem checks?