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Ubuntu 14.04 has 7zip installed but does not compress anything no change in file compression 4.2 mb still comes out 4.2mb?

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    Not all file types can be reasonably compressed. Some formats like e.g. mp3 or jpg already include compression, so putting them in a compressed archive doesn't really achieve any more difference in size. Can you add some more details like the file type of your original files?
    – Byte Commander
    Sep 14 '17 at 19:53
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    If you need help you need to provide more details. What file(s) did you compress and post the command you ran and size of the files.
    – Panther
    Sep 14 '17 at 20:50
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File compression doesn't equal file resize. Your 7zip is working just fine.

Compression is an simplifying algorithm, for example, we can compress:

eeeezzz 

to look like

4e3z

(we saved some characters - very simple example)

As you can see, this is not always possible + the bigger file, the bigger difference, your file is very small to feel it.

If you want, you can read a bit here about this topic: http://www.makeuseof.com/tag/how-does-file-compression-work/

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As others already stated the files you tried may not be compressible much any more, but you may still play around with e.g. the -mx option which sets the level of compression. See the “ultra settings” example in the man 7z:

7z a -t7z -m0=lzma -mx=9 -mfb=64 -md=32m -ms=on archive.7z dir1
 adds all files from directory "dir1" to archive archive.7z using "ultra settings"

Possible values for -mx are 0, 1, 3, 5 (default), 7 and 9, where 9 is the “ultra” level.

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