4 ms·

"symlink is just a file…" A file for which you exclusively use lstat() & readlink() instead of all the other usual functions in the standard library you use fo

by mzs 7y ago

"symlink is just a file…" A file for which you exclusively use lstat() & readlink() instead of all the other usual functions in the standard library you use for regular files. What about named pipes? Or sockets? Or whiteout? Are they too just files since they have a filename?

bigiain 7y ago
"Please wait: backing up /dev/urandom - 0% of NaN" (To be fair, anyone who tries to backup /dev deserves whatever they get...)
- mzs 7y ago
  $ echo /dev/urandom | cpio -o | xxd -a 1 block 00000000: 3037 3037 3037 3737 3737 3737 3030 3030 0707077777770000 00000010: 3031 3032 3036 3636 3030 3030 3030 3030 0102066600000000 00000020: 3030 3030 3030 3030 3031 3737 3737 3737 0000000001777777 00000030: 3133 3436 3430 3530 3035 3730 3030 3031 1346405005700001 00000040: 3530 3030 3030 3030 3030 3030 2f64 6576 500000000000/dev 00000050: 2f75 7261 6e64 6f6d 0030 3730 3730 3730 /urandom.0707070 00000060: 3030 3030 3030 3030 3030 3030 3030 3030 0000000000000000 00000070: 3030 3030 3030 3030 3030 3030 3030 3030 0000000000000000 00000080: 3030 3130 3030 3030 3030 3030 3030 3030 0010000000000000 00000090: 3030 3030 3030 3030 3133 3030 3030 3030 0000000013000000 000000a0: 3030 3030 3054 5241 494c 4552 2121 2100 00000TRAILER!!!. 000000b0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ * 000001f0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ $ echo $? 0
  - ktpsns 7y ago
    How does this work? Looks like a bug in xxd for me (o.O)
    - mzs 7y ago
      Nope xxd is fine, read the man page. $ PAGER='col -bx' man 5 cpio | awk '$1 == "mode" { print; while (getline) print }' | head mode The mode specifies both the regular permissions and the file type. It consists of several bit fields as follows: 0170000 This masks the file type bits. 0140000 File type value for sockets. 0120000 File type value for symbolic links. For symbolic links, the link body is stored as file data. 0100000 File type value for regular files. 0060000 File type value for block special devices. 0040000 File type value for directories. 0020000 File type value for character special devices. $ echo /dev/urandom | cpio -o | dd bs=6 skip=3 count=1 | od -tc 1 block 1+0 records in 1+0 records out 6 bytes transferred in 0.000025 secs (239675 bytes/sec) 0000000 0 2 0 6 6 6 0000006
  - jolmg 7y ago
    I chuckled at the "TRAILER!!!". I wonder what other file formats have silly things like this. From `man 5 cpio`: > The end of the archive is indicated by a special record with the pathname “TRAILER!!!”. This makes me wonder if this means that the cpio format can't reliably be used for files with the path "TRAILER!!!". Why would the format even need a special record to indicate the end of the archive? Is there any reason why one wouldn't be able to rely on the end of the file to indicate the end of the archive?
    - jolmg 7y ago
      Seems cpio really can't deal with "TRAILER!!!" files: $ cd $(mktemp -d) $ echo foo > 'TRAILER!!!' $ echo bar > barfile $ echo 'TRAILER!!!' | cpio -o | xxd -a 1 block 00000000: c771 2a00 31cc a481 e803 e803 0100 0000 .q*.1........... 00000010: 025d ef8c 0b00 0000 0400 5452 4149 4c45 .]........TRAILE 00000020: 5221 2121 0000 666f 6f0a c771 0000 0000 R!!!..foo..q.... 00000030: 0000 0000 0000 0100 0000 0000 0000 0b00 ................ 00000040: 0000 0000 5452 4149 4c45 5221 2121 0000 ....TRAILER!!!.. 00000050: 0000 0000 0000 0000 0000 0000 0000 0000 ................ * 000001f0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ $ echo 'TRAILER!!!' | cpio -o | cpio -t 1 block 1 block $ echo barfile | cpio -o | cpio -t 1 block barfile 1 block $ echo barfile | cpio -o | cpio -i --to-stdout barfile 1 block bar 1 block $ echo 'TRAILER!!!' | cpio -o | cpio -i --to-stdout 'TRAILER!!!' 1 block 1 block
      - mzs 7y ago
        tar has two blocks of zeros at the end. GNU tar uses entries where other tar implementations will overwrite previous files on extraction, AIX tar uses special four character names for binary blobs (plus xattrs as names, both after the affected entry), Solaris cpio uses another mode bit for sparse files, ACLs, and xattrs (I actually really like this, it extracts as a simple to parse text file[0] on other implementations), and most pax commands on Linux can't read/write PAX archive files. FWIW I make use of TRAILER!!! to let me know in a pipeline that everything worked (imagine a command earlier in the pipe fails early and not every entry is processed but ends on a boundary), but here the two pages of zeros that tar uses makes more sense (about the only thing tar does better than cpio). 0: https://www.mail-archive.com/opensolaris-arc@mail.opensolaris.org/msg01795.html https://www.mail-archive.com/opensolaris-arc@mail.opensolari...

by mzs 7y ago

bigiain 7y ago

"Please wait: backing up /dev/urandom - 0% of NaN" (To be fair, anyone who tries to backup /dev deserves whatever they get...)

mzs 7y ago
$ echo /dev/urandom | cpio -o | xxd -a 1 block 00000000: 3037 3037 3037 3737 3737 3737 3030 3030 0707077777770000 00000010: 3031 3032 3036 3636 3030 3030 3030 3030 0102066600000000 00000020: 3030 3030 3030 3030 3031 3737 3737 3737 0000000001777777 00000030: 3133 3436 3430 3530 3035 3730 3030 3031 1346405005700001 00000040: 3530 3030 3030 3030 3030 3030 2f64 6576 500000000000/dev 00000050: 2f75 7261 6e64 6f6d 0030 3730 3730 3730 /urandom.0707070 00000060: 3030 3030 3030 3030 3030 3030 3030 3030 0000000000000000 00000070: 3030 3030 3030 3030 3030 3030 3030 3030 0000000000000000 00000080: 3030 3130 3030 3030 3030 3030 3030 3030 0010000000000000 00000090: 3030 3030 3030 3030 3133 3030 3030 3030 0000000013000000 000000a0: 3030 3030 3054 5241 494c 4552 2121 2100 00000TRAILER!!!. 000000b0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ * 000001f0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ $ echo $? 0
- ktpsns 7y ago
  How does this work? Looks like a bug in xxd for me (o.O)
  - mzs 7y ago
    Nope xxd is fine, read the man page. $ PAGER='col -bx' man 5 cpio | awk '$1 == "mode" { print; while (getline) print }' | head mode The mode specifies both the regular permissions and the file type. It consists of several bit fields as follows: 0170000 This masks the file type bits. 0140000 File type value for sockets. 0120000 File type value for symbolic links. For symbolic links, the link body is stored as file data. 0100000 File type value for regular files. 0060000 File type value for block special devices. 0040000 File type value for directories. 0020000 File type value for character special devices. $ echo /dev/urandom | cpio -o | dd bs=6 skip=3 count=1 | od -tc 1 block 1+0 records in 1+0 records out 6 bytes transferred in 0.000025 secs (239675 bytes/sec) 0000000 0 2 0 6 6 6 0000006
- jolmg 7y ago
  I chuckled at the "TRAILER!!!". I wonder what other file formats have silly things like this. From `man 5 cpio`: > The end of the archive is indicated by a special record with the pathname “TRAILER!!!”. This makes me wonder if this means that the cpio format can't reliably be used for files with the path "TRAILER!!!". Why would the format even need a special record to indicate the end of the archive? Is there any reason why one wouldn't be able to rely on the end of the file to indicate the end of the archive?
  - jolmg 7y ago
    Seems cpio really can't deal with "TRAILER!!!" files: $ cd $(mktemp -d) $ echo foo > 'TRAILER!!!' $ echo bar > barfile $ echo 'TRAILER!!!' | cpio -o | xxd -a 1 block 00000000: c771 2a00 31cc a481 e803 e803 0100 0000 .q*.1........... 00000010: 025d ef8c 0b00 0000 0400 5452 4149 4c45 .]........TRAILE 00000020: 5221 2121 0000 666f 6f0a c771 0000 0000 R!!!..foo..q.... 00000030: 0000 0000 0000 0100 0000 0000 0000 0b00 ................ 00000040: 0000 0000 5452 4149 4c45 5221 2121 0000 ....TRAILER!!!.. 00000050: 0000 0000 0000 0000 0000 0000 0000 0000 ................ * 000001f0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ $ echo 'TRAILER!!!' | cpio -o | cpio -t 1 block 1 block $ echo barfile | cpio -o | cpio -t 1 block barfile 1 block $ echo barfile | cpio -o | cpio -i --to-stdout barfile 1 block bar 1 block $ echo 'TRAILER!!!' | cpio -o | cpio -i --to-stdout 'TRAILER!!!' 1 block 1 block
    - mzs 7y ago
      tar has two blocks of zeros at the end. GNU tar uses entries where other tar implementations will overwrite previous files on extraction, AIX tar uses special four character names for binary blobs (plus xattrs as names, both after the affected entry), Solaris cpio uses another mode bit for sparse files, ACLs, and xattrs (I actually really like this, it extracts as a simple to parse text file[0] on other implementations), and most pax commands on Linux can't read/write PAX archive files. FWIW I make use of TRAILER!!! to let me know in a pipeline that everything worked (imagine a command earlier in the pipe fails early and not every entry is processed but ends on a boundary), but here the two pages of zeros that tar uses makes more sense (about the only thing tar does better than cpio). 0: https://www.mail-archive.com/opensolaris-arc@mail.opensolaris.org/msg01795.html https://www.mail-archive.com/opensolaris-arc@mail.opensolari...