file permissions - How to read the mode field of git-ls-tree's output

Question

$ git ls-tree fb3a8bdd0ce
100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c    .gitignore
100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d    .mailmap
100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3    COPYING
040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745    Documentation
100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200    GIT-VERSION-GEN
100644 blob 289b046a443c0647624607d471289b2c7dcd470b    INSTALL
100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1    Makefile
100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52    README
...

I know the last 3 oct digits are file mode, but what are the first 3 digits for? I can't find it out in git user's manual.

Answer 1

From the Git index-format.txt file, regarding the mode:

32-bit mode, split into (high to low bits)

    4-bit object type
      valid values in binary are 1000 (regular file), 1010 (symbolic link)
      and 1110 (gitlink)

    3-bit unused

    9-bit unix permission. Only 0755 and 0644 are valid for regular files.
    Symbolic links and gitlinks have value 0 in this field.

Also, a directory object type (binary 0100) and group-writeable (0664 permissions) regular file are allowed as indicated by the fsck.c fsck_tree method. The regular non-executable group-writeable file is a non-standard mode that was supported in earlier versions of Git.

This makes valid modes (as binary and octal):

• 0100000000000000 (040000): Directory
• 1000000110100100 (100644): Regular non-executable file
• 1000000110110100 (100664): Regular non-executable group-writeable file
• 1000000111101101 (100755): Regular executable file
• 1010000000000000 (120000): Symbolic link
• 1110000000000000 (160000): Gitlink