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More reproducible tarball doc

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* doc/tar.texi (PAX keywords): Improve discussion of how
to generate reproducible tarballs.
This commit is contained in:
Paul Eggert
2021-12-13 12:42:11 -08:00
parent ac119c80e4
commit b846956c60
1 changed files with 24 additions and 17 deletions
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doc/tar.texi
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@@ -10423,6 +10423,9 @@ the following forms:
When used with one of archive-creation commands,
this option instructs @command{tar} to omit from extended header records
that it produces any keywords matching the string @var{pattern}.
If the pattern contains shell metacharacters like @samp{*}, it should
be quoted to prevent the shell from expanding the pattern before
@command{tar} sees it.
When used in extract or list mode, this option instructs tar
to ignore any keywords matching the given @var{pattern} in the extended
@@ -10431,7 +10434,7 @@ matching notation described in @acronym{POSIX 1003.2}, 3.13
(@pxref{wildcards}). For example:
@smallexample
--pax-option delete=security.*
--pax-option 'delete=security.*'
@end smallexample
would suppress security-related information.
@@ -10560,11 +10563,9 @@ For example, to set all modification times to the current date, you
use the following option:
@smallexample
--pax-option='mtime:=@{now@}'
--pax-option 'mtime:=@{now@}'
@end smallexample
Note quoting of the option's argument.
@cindex archives, binary equivalent
@cindex binary equivalent archives, creating
As another example, here is the option that ensures that any two
@@ -10572,7 +10573,7 @@ archives created using it, will be binary equivalent if they have the
same contents:
@smallexample
--pax-option=atime:=0
--pax-option delete=atime
@end smallexample
@noindent
@@ -10581,27 +10582,33 @@ from them, you will also need to eliminate changes due to ctime, as
shown in examples below:
@smallexample
--pax-option=atime:=0,ctime:=0
--pax-option 'delete=[ac]time'
@end smallexample
@noindent
or
Normally @command{tar} saves an mtime value with subsecond resolution
in an extended header for any file with a timestamp that is not on a
one-second boundary. This is in addition to the traditional mtime
timestamp in the header block, which can represent integer timestamps
in the 1970-01-01 00:00:00 through 2242-03-16 12:56:31 @sc{utc}. If
this traditional timestamp suffices and you do not want subsecond
timestamp resolution, you can use:
@smallexample
--pax-option=atime:=0,delete=ctime
--pax-option 'delete=[acm]time'
@end smallexample
Notice, that if you create an archive in POSIX format (@pxref{posix})
and the environment variable @env{POSIXLY_CORRECT} is set, then the
two archives created using the same options on the same set of files
will not be byte-to-byte equivalent even with the above option. This
is because the posix default for extended header names includes the
PID of the tar process, which is different at each run. To produce
byte-to-byte equivalent archives in this case, either unset
@env{POSIXLY_CORRECT}, or use the following option:
If the environment variable @env{POSIXLY_CORRECT} is set, two POSIX
archives created using the same options on the same set of files might
not be byte-to-byte equivalent even with the above options. This is
because the POSIX default for extended header names includes
@command{tar}'s process @acronym{ID}, which typically differs at each
run. To produce byte-to-byte equivalent archives in this case, either
unset @env{POSIXLY_CORRECT}, or use the following option, which can be
combined with the above options:
@smallexample
---pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0,ctime:=0
--pax-option exthdr.name=%d/PaxHeaders/%f
@end smallexample
@node Checksumming