Learn GNU/Linux Commands (6): Link - ln, unlink

-

There are 2 types of links in GNU/Linux: soft links (symbolic links), and hard links.

Soft Links or Symbolic Links

Soft links, or symbolic links, are similar to shortcuts in Windows. A symbolic link is eventually a file that records the pathname, either relative or absolute, to another file or directory. Symbolic links are more often used by users.

Hard Links

Hard links are low-level links. When a hard link to a file (hard link to a directory is not allowed) is created, the file system records that the file can be found with one more address (which is under the parent directory of the hard link), and the occurrences of the file increase by 1. When the hard link is removed, the file system records that the file won't be found at where the hard link is, and the occurrences of the file decrease by 1. Unlike symbolic links, hard links can not be created across file systems. That is to say, hard links to files on another partition or disk, or even another computer are not allowed.

Make Link(s)

Links can be made with the "ln" command. "ln" makes hard links by default, to make symbolic links, use the option "-s" as "ln -s".

Symbolic links can also be made with "cp -s".

Make a link

ln TARGET LINK_NAME

Make (create) a link to TARGET with the name LINK_NAME. If LINK_NAME is omitted, make a link named TARGET under the current directory.

  • -s, --symbolic: make symbolic links instead of hard links (the default).

Notes

  • "ln -s" makes a symbolic link that stores the pathname of TARGET given exactly in the command. If the pathname is relative, when the link is moved to a new location, TARGET can not be accessed with the link anymore. So it's better to make symbolic links with absolute pathname. So instead of the command below: 
    [texpion@com ~]$ ln -s myfile mylink
    This is better: 
    [texpion@com myfolder]$ ln -s $PWD/myfile mylink
  • LINK_NAME is the name of a link. Don't use a pathname as LINK_NAME. For example, don't do this: 
    [texpion@com ~]$ ln -s myfile /myfolder/mylink
    The right way: 
    [texpion@com ~]$ cd /myfolder
[texpion@com myfolder]$ ln -s $HOME/myfile mylink
  • We introduced $PWD and $HOME in Part 2.

Make multiple links

ln TARGET... DIRECTORY # or
ln -t DIRECTORY TARGET...

Make (create) links to each TARGET in DIRECTORY.

Remove links

Links can be removed with the "rm" commands. The command below can also remove a link.

unlink FILE

This command can remove not only a link but also a file or a directory per call.

Copy Link(s)

By default, "cp" copies the content of the file that a link refers to under the name of the link, for example: 

[texpion@com ~]$ ln -s myfile mylink
[texpion@com ~]$ cp mylink ./myfolder

"cp" makes a file instead of a link under "myfolder" called "mylink", whose content is the same as "myfile".

To make a copy of a symbolic link itself, use "cp -d" or "cp -a" instead.

Related

Comments

Post a Comment

Popular

Intel 7th and 8th generation processor graphics driver for Windows 7/8/8.1

-
For Intel's 7th and 8th generation processors, the only Windows operating system that Intel supports is Windows 10 x64. There is no official driver for Windows 7, Windows 8, or Windows 8.1.But according to this YouTube video https://www.youtube.com/watch?v=7w9JofZQkxI, the Windows 7's Driver for the 6th gen processors can be installed for the 7th gen processors.According to 6th Generation Intel® Processor for U/Y-Platforms Datasheet, Volume 1 of 2,7th Generation Intel® Processor Families for S Platforms Datasheet, Volume 1 of 2,7th Generation Intel® Processor Families for U/Y Platforms and 8th Generation Intel® Processor Family for U Quad Core Platforms Datasheet, Volume 1 of 2,8th Generation Intel® Processor Family for S-Processor Platforms Datasheet, Volume 1 of 2, The graphics processors on not only Intel 6th gen mobile, but also all 7th and 8th gen processors are all based on Gen 9 LP (generation 9 Low Power) graphics core architecture.The graphics driver for 6th gen proce…
Read more

AV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Part II - Fidelity

-
Image
In this part, we tested the lossy compression capability. "Lossy" means that in order to reduce data size, the data is inexactly approximated and part of it is discarded. To measure the fidelity of the reconstruction (encoded video) of lossy compression codecs in our tests, we used PSNR (Peak signal-to-noise ratio).The greater the PSNR of two videos, the more similar they are. If the encoded video is lossless, the PSNR will be infinity when compared to the original.We tried to match the PSNR and compared the video stream size, the smaller the size, the better the codec. But it is hard to make the PSNR exactly the same. As mentioned earlier, as of June 2018, the AV1 is still experimental and we were not able to set its constant quality mode as we expected. And AV1 is extremely slow. So we did the AV1 in its vibrate bitrate mode first. We have found that the VP9 produces better results when encoded in its 2-pass constant quality mode (can be set with "-crf"), so we c…
Read more

AV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Part I - Lossless

-
Image
ConclusionIn terms of lossless compression, x264 and x265 took less time and produced smaller files. All video codecs produced smaller video files than LZMA/LZMA2, the general purpose compression algorithms.In our tests, AV1 takes approximately 160-190x more time than x264 to encode in their lossless mode. AV1 does have a lossless mode, but the output is not lossless at all.VP9 and AV1 can take advantage of multi-threading, but they can not load all CPU cores all the time as x264 and x265 do.NoteAt of June 2018, AV1 is still experimental.We used x264 as the AVC encoder.We used x265 as the HEVC encoder.We used pxz as the LZMA/LZMA2 compressor. LZMA is also the default algorithm of 7-Zip.The AV1 encoder ran with "--cpu-used=4" for "medium" speed, "--tile-columns=4" to enabling multi-threading. All encoders ran in their lossless mode, all other settings were as default.Test 1Source video
sintel_trailer_2k_480p24.y4m
© copyright Blender Foundation | durian.bl…
Read more

AV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Part IV - Decode

-
Image
In this part, we tested the decoding speed. Especially for video coding formats designed for video transmissions over the Internet, decoding speed could be much more important than encoding speed. We tried to encode in 2-pass vibrate bitrate mode (VBR) to get almost the same file sizes and counted the time needed for converting the encoded videos into raw video files. we decoded in RAM, because decoding is fast and the raw video could be very large. We also encoded in RAM this time, but the improvement of encoding speed, if any, wasn't very impressive.ConclusionAt of June 2018, the AV1 decoder can only decode in a single thread. Decoders for H.264, H265, and VP9 utilize multi-threads.We were not able to play videos encoded with AV1 in Chrome, Firefox Nightly, VLC, both on Windows and LINUX, though those are claimed to be able to.To decode the files encoded with AV1, the CPU time was approximately 3.5-6x longer than that to decode files encoded with x264. This means that it require…
Read more

OBS Studio settings for best quality (loseless) recording

-
Image
OBS - Open Broadcaster Software. Change Color Format to RGB. RGB is the color format in which computer hardwares talk, thus loseless. The lose in transformating to YUV is noticeable, especially for color gradients. H.264 video in RGB color format can be played by VLC and other FFmpeg-based players. Choose FFmpeg and select matroska as the container, libx264rgb as the video encoder, flac as the audio encoder.If your video editor does not support RGB format, select other color format and select libx264 as the video encoder.Decrease the keyframe interval if you have trouble playing the recorded video. For 1080p60fps videos, because the bitrate is so high, even though you set the keyframe interval to 60 (default is 250), you may still have trouble playing the video sometimes, That doesn't mean that the video is broken. If you don't care about playing the recorded video or you transcode it later anyway, you can set the keyframe interval to 600.Video Encoder Settings: preset=ultrafas…
Read more

AV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Part III - Quality

-
Image
Visual quality is somewhat subjective. However, Efforts are still being made to quantify it. There is a perceptual video quality assessment algorithm developed by Netflix called VMAF - Video Multi-Method Assessment Fusion.We tried to tune different CRF values to encode the video clips we used in previous parts. But unfortunately, we found that decreasing the CRF did not increase the VMAF score, it even dropped a little bit. We are not able to do a quality comparison at this moment.Anyway, a feature of x264 called Psy RDO caught our attention: the human eye doesn't just want the image to look similar to the original, it wants the image to have similar complexity. Therefore, we would rather see a somewhat distorted but still detailed block than a non-distorted but completely blurred block. The result is a bias towards a detailed and/or grainy output image, a bit like xvid except that its actual detail rather than ugly blocking.This preference is quite easy to understand, people wear…
Read more

Install VMAF on Fedora, CentOS/Red Hat (RHEL)

-
VMAF - Video Multi-Method Assessment Fusion.Tested on Fedora 28. For CentOS/Red Hat (RHEL), replace dnf with yum.The following commands install dependencies, download source files from github.com and compile VMAF. sudo dnf install gcc gcc-c++ gcc-gfortran freetype hdf5 git \ python2-tkinter python-numpy python-scipy python-matplotlib python-notebook python-pandas python-sympy python-nose python-scikit-learn python-scikit-image python-h5py git clone https://github.com/Netflix/vmaf.git cd vmaf git submodule update --init --recursive make echo export PYTHONPATH="$(pwd)/python/src:$(pwd)/sureal/python/src:$PYTHONPATH" >> ~/.bashrc source ~/.bashrc sudo make install # Optional Install On Other DistrosFor more info, or if you want to install it on Ubuntu or other distros, visit https://github.com/Netflix/vmaf/blob/df1d890eb6a6d0465d8e1b2a35d81fc60a1bd53a/README.md.RelatedAV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Part I - LosslessAV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Pa…
Read more