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Using: Droid Bionic 4.04 without Root.
I can use the adb to pull a specific apk file to my PC, for example,
adb pull /data/app/com.google.android.gm-1.apk C:\my-hacks
which pull gmail's apk file. But when I try to copy the entire /data/app directory with
adb pull /data/app C:\myhacks
the command line reads
pull: building file list...
0 files pulled. 0 files skipped.
I don't understand why I have permission to pull specific files from /data/ but not everything.
It's a "permission and privacy" thing. While the .apk does not have any privacy concerns involved (and thus can give read permission to everyone), the data do, and thus the directory is only readable by owner, group, and root (not by "other", which is what you would need -- "other" has only eXecute permissions).
ls -l / | grep data
drwxrwx--x 25 system system 4096 Jan 28 17:51 data
Which means: Specifying the file directly, you can pull it (as it has the required file permissions, and the directory permits you to cd into due to the set eXecute bit). Specifying the directory instead fails, as this would require to change (cd) into the directory (which works) and build a list of files contained -- which is impossible due to the missing read permission on the directory entry itself.
Izzy's answer is good and well explained. Building on that, if you are root, you can open root explorer, navigate to /data/ and change the permissions on the app folder.
Simply long press the app, tap Permissions, and next to other, check the appropriate box(es).
