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doc: Clarify octal bits in permissions

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* doc/perm.texi (Numeric Modes): Briefly explain octal.
Reorder description to make it more intuitive (Bug#29069).
This commit is contained in:
Paul Eggert
2018-04-03 08:40:34 -07:00
parent d1a754c827
commit a8a52e2f05
1 changed files with 45 additions and 41 deletions
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doc/perm.texi
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@@ -494,10 +494,47 @@ the file to all users.
As an
alternative to giving a symbolic mode, you can give an octal (base 8)
number that represents the mode.
This number is always interpreted in octal; you do not have to add a
leading @samp{0}, as you do in C. Mode @samp{0055} is the same as
mode @samp{55}. (However, modes of five digits or more, such as
@samp{00055}, are sometimes special. @xref{Directory Setuid and Setgid}.)
The permissions granted to the user,
to other users in the file's group,
and to other users not in the file's group each require three
bits: one bit for read, one for write, and one for execute/search permission.
These three bits are represented as one octal digit;
for example, if all three are present, the resulting 111 (in binary)
is represented as the digit 7 (in octal). The three special
mode bits also require one bit each, and they are as a group
represented as another octal digit. Here is how the bits are arranged,
starting with the highest valued bit:
@example
Value in Corresponding
Mode Mode Bit
Special mode bits:
4000 Set user ID on execution
2000 Set group ID on execution
1000 Restricted deletion flag or sticky bit
The file's owner:
400 Read
200 Write
100 Execute/search
Other users in the file's group:
40 Read
20 Write
10 Execute/search
Other users not in the file's group:
4 Read
2 Write
1 Execute/search
@end example
For example, numeric mode @samp{4751} corresponds to symbolic mode
@samp{u=srwx,g=rx,o=x}, and numeric mode @samp{664} corresponds to symbolic mode
@samp{ug=rw,o=r}. Numeric mode @samp{0} corresponds to symbolic mode
@samp{a=}.
A numeric mode is usually shorter than the corresponding symbolic
mode, but it is limited in that normally it cannot take into account the
@@ -507,43 +544,10 @@ to this general limitation. @xref{Directory Setuid and Setgid}.
Also, operator numeric modes can take previous file mode bits into
account. @xref{Operator Numeric Modes}.
The permissions granted to the user,
to other users in the file's group,
and to other users not in the file's group each require three
bits, which are represented as one octal digit. The three special
mode bits also require one bit each, and they are as a group
represented as another octal digit. Here is how the bits are arranged,
starting with the lowest valued bit:
@example
Value in Corresponding
Mode Mode Bit
Other users not in the file's group:
1 Execute/search
2 Write
4 Read
Other users in the file's group:
10 Execute/search
20 Write
40 Read
The file's owner:
100 Execute/search
200 Write
400 Read
Special mode bits:
1000 Restricted deletion flag or sticky bit
2000 Set group ID on execution
4000 Set user ID on execution
@end example
For example, numeric mode @samp{4755} corresponds to symbolic mode
@samp{u=rwxs,go=rx}, and numeric mode @samp{664} corresponds to symbolic mode
@samp{ug=rw,o=r}. Numeric mode @samp{0} corresponds to symbolic mode
@samp{a=}.
Numeric modes are always interpreted in octal; you do not have to add a
leading @samp{0}, as you do in C@. Mode @samp{0055} is the same as
mode @samp{55}. However, modes of five digits or more, such as
@samp{00055}, are sometimes special (@pxref{Directory Setuid and Setgid}).
@node Operator Numeric Modes
@section Operator Numeric Modes