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I'm using media info, to get some xml information about movie:

mediainfo --Output=XML Krtek a buldozer-jdvwqZUEbhc.mkv | xmlstarlet format

which output is:

<?xml version="1.0" encoding="UTF-8"?>
<MediaInfo xmlns="https://mediaarea.net/mediainfo" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://mediaarea.net/mediainfo https://mediaarea.net/mediainfo/mediainfo_2_0.xsd" version="2.0">
 <creatingLibrary version="18.03" url="https://mediaarea.net/MediaInfo">MediaInfoLib</creatingLibrary>
 <media ref="Krtek a buldozer-jdvwqZUEbhc.mkv">
 <track type="General">
 <UniqueID>101120522676894244607292274887483611459</UniqueID>
 <VideoCount>1</VideoCount>
 <AudioCount>1</AudioCount>
 <FileExtension>mkv</FileExtension>
 <Format>Matroska</Format>
 <Format_Version>4</Format_Version>
 <FileSize>60132643</FileSize>
 <Duration>374.101</Duration>
 <OverallBitRate>1285912</OverallBitRate>
 <FrameRate>25.000</FrameRate>
 <FrameCount>9352</FrameCount>
 <IsStreamable>Yes</IsStreamable>
 <File_Modified_Date>UTC 2018-10-15 07:09:29</File_Modified_Date>
 <File_Modified_Date_Local>2018-10-15 09:09:29</File_Modified_Date_Local>
 <Encoded_Application>Lavf57.71.100</Encoded_Application>
 <Encoded_Library>Lavf57.71.100</Encoded_Library>
 <extra>
 <ErrorDetectionType>Per level 1</ErrorDetectionType>
 </extra>
 </track>
 <track type="Video">
 <StreamOrder>0</StreamOrder>
 <ID>1</ID>
 <UniqueID>1</UniqueID>
 <Format>AVC</Format>
 <Format_Profile>High</Format_Profile>
 <Format_Level>4</Format_Level>
 <Format_Settings_CABAC>Yes</Format_Settings_CABAC>
 <Format_Settings_RefFrames>3</Format_Settings_RefFrames>
 <CodecID>V_MPEG4/ISO/AVC</CodecID>
 <Duration>374.080000000</Duration>
 <Width>1920</Width>
 <Height>1080</Height>
 <Stored_Height>1088</Stored_Height>
 <Sampled_Width>1920</Sampled_Width>
 <Sampled_Height>1080</Sampled_Height>
 <PixelAspectRatio>1.000</PixelAspectRatio>
 <DisplayAspectRatio>1.778</DisplayAspectRatio>
 <FrameRate_Mode>CFR</FrameRate_Mode>
 <FrameRate_Mode_Original>VFR</FrameRate_Mode_Original>
 <FrameRate>25.000</FrameRate>
 <FrameCount>9352</FrameCount>
 <ColorSpace>YUV</ColorSpace>
 <ChromaSubsampling>4:2:0</ChromaSubsampling>
 <BitDepth>8</BitDepth>
 <ScanType>Progressive</ScanType>
 <Delay>0.000</Delay>
 <Default>Yes</Default>
 <Forced>No</Forced>
 <colour_range>Limited</colour_range>
 <colour_description_present>Yes</colour_description_present>
 <colour_primaries>BT.709</colour_primaries>
 <transfer_characteristics>BT.709</transfer_characteristics>
 <matrix_coefficients>BT.709</matrix_coefficients>
 </track>
 <track type="Audio">
 <StreamOrder>1</StreamOrder>
 <ID>2</ID>
 <UniqueID>2</UniqueID>
 <Format>Opus</Format>
 <CodecID>A_OPUS</CodecID>
 <Duration>374.101000000</Duration>
 <Channels>2</Channels>
 <ChannelPositions>Front: L R</ChannelPositions>
 <SamplingRate>48000</SamplingRate>
 <SamplingCount>17956848</SamplingCount>
 <BitDepth>32</BitDepth>
 <Compression_Mode>Lossy</Compression_Mode>
 <Delay>0.000</Delay>
 <Delay_Source>Container</Delay_Source>
 <Language>en</Language>
 <Default>Yes</Default>
 <Forced>No</Forced>
 </track>
 </media>
</MediaInfo>

now say that I want to get all IDs:

... | xmlstarlet sel -t -v "//ID"

and nothing is printed. What? Why? Well it turned out, that if i remove all parameters from tag on second line, the same selection command will work. Now I undestand, that xmlstarlet (probably) works just fine, I'm just missing some magic flag or syntax, so that it can process xmls with defined namespaces. Can someone advice?

You need to use the namespace with -N option, and use it in the query like <namespace>:<xpath>:

... | xmlstarlet sel -N n="https://mediaarea.net/mediainfo" -t -v "//n:ID" 

From the help page:

-N <name>=<value>

- predefine namespaces (name without 'xmlns:')

ex: xsql=urn:oracle-xsql

Multiple -N options are allowed.