Read binary stdout data from adb shell?
Is it possible to read binary stdout from an adb shell command? For example, all examples of how to use screencap include two steps:
adb shell screencap -p /sdcard/foo.png
adb pull /sdcard/foo.png
However, the service supports writing to stdout. You can for instance, do the following:
adb shell "screencap -p > /sdcard/foo2.png"
adb pull /sdcard/foo2.png
And this works equally well. But, what about reading the output across ADB? What I want to do is the following:
adb shell screencap -p > foo3.png
And avoid the intermediate write to the SD card. This generates something that looks like a PNG file (running strings foo3.png generates something with an IHDR, IEND, etc.) and is approximately the same size, but the file is corrupted as far as image readers are concerned.
I have also attempted to do this using ddmlib in java and the results are the same. I would be happy to use any library necessary. My goal is to reduce total time to get the capture. On my device, using the two-command solution, it takes about 3 seconds to get the image. Using ddmlib and capturing stdout takes less than 900ms, but it doesn't work!
Is it possible to do this?
EDIT: Here is the hexdump of two files. The first one, screen.png came from stdout and is corrupted. The second one, xscreen is from the two-command solution and works. The images should be visually identical.
$ hexdump -C screen.png | head
00000000 89 50 4e 47 0d 0d 0a 1a 0d 0a 00 00 00 0d 49 48 |.PNG..........IH|
00000010 44 52 00 00 02 d0 00 00 05 00 08 06 00 00 00 6e |DR.............n|
00000020 ce 65 3d 00 00 00 04 73 42 49 54 08 08 08 08 7c |.e=....sBIT....||
00000030 08 64 88 00 00 20 00 49 44 41 54 78 9c ec bd 79 |.d... .IDATx...y|
00000040 9c 1d 55 9d f7 ff 3e 55 75 f7 de b7 74 77 d2 d9 |..U...>Uu...tw..|
00000050 bb b3 27 10 48 42 16 c0 20 01 86 5d 14 04 11 dc |..'.HB.. ..]....|
00000060 78 44 9d c7 d1 d1 11 78 70 7e 23 33 8e 1b 38 33 |xD.....xp~#3..83|
00000070 ea 2c 8c 8e 0d 0a 08 a8 23 2a 0e 10 82 ac c1 40 |.,......#*.....@|
00000080 12 02 81 24 64 ef ec 5b ef fb 5d 6b 3b bf 3f ea |...$d..[..]k;.?.|
00000090 de db dd 49 27 e9 ee 74 77 3a e3 79 bf 5e 37 e7 |...I'..tw:.y.^7.|
$ hexdump -C xscreen.png | head
00000000 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52 |.PNG........IHDR|
00000010 00 00 02 d0 00 00 05 00 08 06 00 00 00 6e ce 65 |.............n.e|
00000020 3d 00 00 00 04 73 42 49 54 08 08 08 08 7c 08 64 |=....sBIT....|.d|
00000030 88 00 00 20 00 49 44 41 54 78 9c ec 9d 77 98 1c |... .IDATx...w..|
00000040 c5 99 ff 3f d5 dd 93 37 27 69 57 5a e5 55 4e 08 |...?...7'iWZ.UN.|
00000050 24 a1 00 58 18 04 26 08 8c 01 83 31 38 c0 19 9f |$..X..&....18...|
00000060 ef 7c c6 3e 1f 70 f8 7e 67 ee 71 e2 b0 ef ce f6 |.|.>.p.~g.q.....|
00000070 f9 ec 73 04 1b 1c 31 60 23 84 30 22 88 a0 40 10 |..s...1`#.0"..@.|
00000080 08 65 69 95 d3 4a 9b c3 c4 4e f5 fb a3 67 66 77 |.ei..J...N...gfw|
00000090 a5 95 b4 bb da a4 73 7d 9e 67 55 f3 ed 50 5d dd |......s}.gU..P].|
Just at quick glance it seems like a couple of extra 0x0d (13) bytes get added. Carriage return?? Does that ring any bells? Is it mixing in some blank lines?
Answer
Unlike adb shell the adb exec-out command doesn't use pty which mangles the binary output. So you can do
adb exec-out screencap -p > test.png
https://android.googlesource.com/platform/system/core/+/5d9d434efadf1c535c7fea634d5306e18c68ef1f
Note that if you are using this technique for a command that produces output on STDERR, you should redirect it to /dev/null, otherwise adb will include STDERR in its STDOUT corrupting your output. For example, if you are trying to backup and compress a directory:
adb exec-out "tar -zcf - /system 2>/dev/null" > system.tar.gz