[General Questions and Answers] cilyrik:
The problem is that there is no mmcblk0 nor mmcblk1 on my device. Or maybe they were made invisible or somehow unlisted for ls.

$ adb -s XXXXXXXX shell 'ls -shlap /dev/block'
total 0
0 drwxr-xr-x 5 root root 3.1K 1970-04-15 15:06 ./
0 drwxr-xr-x 11 root root 5.9K 1970-04-15 15:06 ../
0 lrwxrwxrwx 1 root root 37 1970-04-15 15:06 bootdevice -> /dev/block/platform/soc/1d84000.ufshc
0 drwxr-xr-x 2 root root 2.4K 1970-04-15 15:06 by-name/
0 brw------- 1 root root 7, 0 1970-04-15 15:06 loop0
0 brw------- 1 root root 7, 8 1970-04-15 15:06 loop1
0 brw------- 1 root root 7, 80 1970-04-15 15:06 loop10
0 brw------- 1 root root 7, 88 1970-04-15 15:06 loop11
0 brw------- 1 root root 7, 96 1970-04-15 15:06 loop12
0 brw------- 1 root root 7, 104 1970-04-15 15:06 loop13
0 brw------- 1 root root 7, 112 1970-04-15 15:06 loop14
0 brw------- 1 root root 7, 120 1970-04-15 15:06 loop15
0 brw------- 1 root root 7, 16 1970-04-15 15:06 loop2
0 brw------- 1 root root 7, 24 1970-04-15 15:06 loop3
0 brw------- 1 root root 7, 32 1970-04-15 15:06 loop4
0 brw------- 1 root root 7, 40 1970-04-15 15:06 loop5
0 brw------- 1 root root 7, 48 1970-04-15 15:06 loop6
0 brw------- 1 root root 7, 56 1970-04-15 15:06 loop7
0 brw------- 1 root root 7, 64 1970-04-15 15:06 loop8
0 brw------- 1 root root 7, 72 1970-04-15 15:06 loop9
0 drwxr-xr-x 3 root root 60 1970-04-15 15:06 platform/
0 brw------- 1 root root 1, 0 1970-04-15 15:06 ram0
0 brw------- 1 root root 1, 1 1970-04-15 15:06 ram1
0 brw------- 1 root root 1, 10 1970-04-15 15:06 ram10
0 brw------- 1 root root 1, 11 1970-04-15 15:06 ram11
0 brw------- 1 root root 1, 12 1970-04-15 15:06 ram12
0 brw------- 1 root root 1, 13 1970-04-15 15:06 ram13
0 brw------- 1 root root 1, 14 1970-04-15 15:06 ram14
0 brw------- 1 root root 1, 15 1970-04-15 15:06 ram15
0 brw------- 1 root root 1, 2 1970-04-15 15:06 ram2
0 brw------- 1 root root 1, 3 1970-04-15 15:06 ram3
0 brw------- 1 root root 1, 4 1970-04-15 15:06 ram4
0 brw------- 1 root root 1, 5 1970-04-15 15:06 ram5
0 brw------- 1 root root 1, 6 1970-04-15 15:06 ram6
0 brw------- 1 root root 1, 7 1970-04-15 15:06 ram7
0 brw------- 1 root root 1, 8 1970-04-15 15:06 ram8
0 brw------- 1 root root 1, 9 1970-04-15 15:06 ram9
0 brw------- 1 root root 8, 0 1970-04-15 15:06 sda
0 brw------- 1 root root 8, 1 1970-04-15 15:06 sda1
0 brw------- 1 root root 8, 10 1970-04-15 15:06 sda10
0 brw------- 1 root root 8, 11 1970-04-15 15:06 sda11
0 brw------- 1 root root 8, 12 1970-04-15 15:06 sda12
0 brw------- 1 root root 8, 13 1970-04-15 15:06 sda13
0 brw------- 1 root root 8, 14 1970-04-15 15:06 sda14
0 brw------- 1 root root 8, 15 1970-04-15 15:06 sda15
0 brw------- 1 root root 259, 0 1970-04-15 15:06 sda16
0 brw------- 1 root root 259, 1 1970-04-15 15:06 sda17
0 brw------- 1 root root 259, 2 1970-04-15 15:06 sda18
0 brw------- 1 root root 259, 3 1970-04-15 15:06 sda19
0 brw------- 1 root root 8, 2 1970-04-15 15:06 sda2
0 brw------- 1 root root 259, 4 1970-04-15 15:06 sda20
0 brw------- 1 root root 259, 5 1970-04-15 15:06 sda21
0 brw------- 1 root root 259, 6 1970-04-15 15:06 sda22
0 brw------- 1 root root 259, 7 1970-04-15 15:06 sda23
0 brw------- 1 root root 8, 3 1970-04-15 15:09 sda3
0 brw------- 1 root root 8, 4 1970-04-15 15:06 sda4
0 brw------- 1 root root 8, 5 1970-04-15 15:06 sda5
0 brw------- 1 root root 8, 6 1970-04-15 15:06 sda6
0 brw------- 1 root root 8, 7 1970-04-15 15:06 sda7
0 brw------- 1 root root 8, 8 1970-04-15 15:06 sda8
0 brw------- 1 root root 8, 9 1970-04-15 15:06 sda9
0 brw------- 1 root root 8, 16 1970-04-15 15:06 sdb
0 brw------- 1 root root 8, 17 1970-04-15 15:06 sdb1
0 brw------- 1 root root 8, 18 1970-04-15 15:06 sdb2
0 brw------- 1 root root 8, 32 1970-04-15 15:06 sdc
0 brw------- 1 root root 8, 33 1970-04-15 15:06 sdc1
0 brw------- 1 root root 8, 34 1970-04-15 15:06 sdc2
0 brw------- 1 root root 8, 48 1970-04-15 15:06 sdd
0 brw------- 1 root root 8, 49 1970-04-15 15:06 sdd1
0 brw------- 1 root root 8, 50 1970-04-15 15:06 sdd2
0 brw------- 1 root root 8, 51 1970-04-15 15:06 sdd3
0 brw------- 1 root root 8, 52 1970-04-15 15:06 sdd4
0 brw------- 1 root root 8, 64 1970-04-15 15:06 sde
0 brw------- 1 root root 8, 65 1970-04-15 15:06 sde1
0 brw------- 1 root root 8, 74 1970-04-15 15:06 sde10
0 brw------- 1 root root 8, 75 1970-04-15 15:06 sde11
0 brw------- 1 root root 8, 76 1970-04-15 15:06 sde12
0 brw------- 1 root root 8, 77 1970-04-15 15:06 sde13
0 brw------- 1 root root 8, 78 1970-04-15 15:06 sde14
0 brw------- 1 root root 8, 79 1970-04-15 15:06 sde15
0 brw------- 1 root root 259, 8 1970-04-15 15:06 sde16
0 brw------- 1 root root 259, 9 1970-04-15 15:06 sde17
0 brw------- 1 root root 259, 10 1970-04-15 15:06 sde18
0 brw------- 1 root root 259, 11 1970-04-15 15:06 sde19
0 brw------- 1 root root 8, 66 1970-04-15 15:06 sde2
0 brw------- 1 root root 259, 12 1970-04-15 15:06 sde20
0 brw------- 1 root root 259, 13 1970-04-15 15:06 sde21
0 brw------- 1 root root 259, 14 1970-04-15 15:06 sde22
0 brw------- 1 root root 259, 15 1970-04-15 15:06 sde23
0 brw------- 1 root root 259, 16 1970-04-15 15:06 sde24
0 brw------- 1 root root 259, 17 1970-04-15 15:06 sde25
0 brw------- 1 root root 259, 18 1970-04-15 15:06 sde26
0 brw------- 1 root root 259, 19 1970-04-15 15:06 sde27
0 brw------- 1 root root 259, 20 1970-04-15 15:06 sde28
0 brw------- 1 root root 259, 21 1970-04-15 15:06 sde29
0 brw------- 1 root root 8, 67 1970-04-15 15:06 sde3
0 brw------- 1 root root 259, 22 1970-04-15 15:06 sde30
0 brw------- 1 root root 259, 23 1970-04-15 15:06 sde31
0 brw------- 1 root root 259, 24 1970-04-15 15:06 sde32
0 brw------- 1 root root 259, 25 1970-04-15 15:06 sde33
0 brw------- 1 root root 259, 26 1970-04-15 15:06 sde34
0 brw------- 1 root root 259, 27 1970-04-15 15:06 sde35
0 brw------- 1 root root 259, 28 1970-04-15 15:06 sde36
0 brw------- 1 root root 259, 29 1970-04-15 15:06 sde37
0 brw------- 1 root root 259, 30 1970-04-15 15:06 sde38
0 brw------- 1 root root 259, 31 1970-04-15 15:06 sde39
0 brw------- 1 root root 8, 68 1970-04-15 15:06 sde4
0 brw------- 1 root root 259, 32 1970-04-15 15:06 sde40
0 brw------- 1 root root 259, 33 1970-04-15 15:06 sde41
0 brw------- 1 root root 259, 34 1970-04-15 15:06 sde42
0 brw------- 1 root root 259, 35 1970-04-15 15:06 sde43
0 brw------- 1 root root 259, 36 1970-04-15 15:06 sde44
0 brw------- 1 root root 259, 37 1970-04-15 15:06 sde45
0 brw------- 1 root root 259, 38 1970-04-15 15:06 sde46
0 brw------- 1 root root 259, 39 1970-04-15 15:06 sde47
0 brw------- 1 root root 259, 40 1970-04-15 15:06 sde48
0 brw------- 1 root root 259, 41 1970-04-15 15:06 sde49
0 brw------- 1 root root 8, 69 1970-04-15 15:06 sde5
0 brw------- 1 root root 259, 42 1970-04-15 15:06 sde50
0 brw------- 1 root root 259, 43 1970-04-15 15:06 sde51
0 brw------- 1 root root 259, 44 1970-04-15 15:06 sde52
0 brw------- 1 root root 259, 45 1970-04-15 15:06 sde53
0 brw------- 1 root root 259, 46 1970-04-15 15:06 sde54
0 brw------- 1 root root 259, 47 1970-04-15 15:06 sde55
0 brw------- 1 root root 259, 48 1970-04-15 15:06 sde56
0 brw------- 1 root root 259, 49 1970-04-15 15:06 sde57
0 brw------- 1 root root 259, 50 1970-04-15 15:06 sde58
0 brw------- 1 root root 259, 51 1970-04-15 15:06 sde59
0 brw------- 1 root root 8, 70 1970-04-15 15:06 sde6
0 brw------- 1 root root 259, 52 1970-04-15 15:06 sde60
0 brw------- 1 root root 259, 53 1970-04-15 15:06 sde61
0 brw------- 1 root root 259, 54 1970-04-15 15:06 sde62
0 brw------- 1 root root 259, 55 1970-04-15 15:06 sde63
0 brw------- 1 root root 259, 56 1970-04-15 15:06 sde64
0 brw------- 1 root root 259, 57 1970-04-15 15:06 sde65
0 brw------- 1 root root 259, 58 1970-04-15 15:06 sde66
0 brw------- 1 root root 259, 59 1970-04-15 15:06 sde67
0 brw------- 1 root root 259, 60 1970-04-15 15:06 sde68
0 brw------- 1 root root 259, 61 1970-04-15 15:06 sde69
0 brw------- 1 root root 8, 71 1970-04-15 15:06 sde7
0 brw------- 1 root root 259, 62 1970-04-15 15:06 sde70
0 brw------- 1 root root 259, 63 1970-04-15 15:06 sde71
0 brw------- 1 root root 259, 64 1970-04-15 15:06 sde72
0 brw------- 1 root root 259, 65 1970-04-15 15:06 sde73
0 brw------- 1 root root 259, 66 1970-04-15 15:06 sde74
0 brw------- 1 root root 259, 67 1970-04-15 15:06 sde75
0 brw------- 1 root root 259, 68 1970-04-15 15:06 sde76
0 brw------- 1 root root 8, 72 1970-04-15 15:06 sde8
0 brw------- 1 root root 8, 73 1970-04-15 15:06 sde9
0 brw------- 1 root root 8, 80 1970-04-15 15:06 sdf
0 brw------- 1 root root 8, 81 1970-04-15 15:06 sdf1
0 brw------- 1 root root 8, 82 1970-04-15 15:06 sdf2
0 brw------- 1 root root 8, 83 1970-04-15 15:06 sdf3
0 brw------- 1 root root 8, 84 1970-04-15 15:06 sdf4
0 brw------- 1 root root 8, 85 1970-04-15 15:06 sdf5
0 brw------- 1 root root 8, 86 1970-04-15 15:06 sdf6
0 brw------- 1 root root 8, 87 1970-04-15 15:06 sdf7
0 brw------- 1 root root 8, 88 1970-04-15 15:06 sdf8
0 brw------- 1 root root 8, 89 1970-04-15 15:06 sdf9
0 drwxr-xr-x 2 root root 40 1970-04-15 15:06 volmgr/
0 brw------- 1 root root 254, 0 1970-04-15 15:06 zram0

The command below produces an empty 71 byte archive.

$ adb -s XXXXXXXX exec-out 'cat /dev/block/mmcblk0 | gzip' > backup/partitions/mmcblk0.tar.gz
$ ls -shlap backup/partitions/mmcblk0.tar.gz
4.0K -rw-rw-r-- 1 tanhok tanhok 71 Aug 5 18:26 backup/partitions/mmcblk0.tar.gz