CODEWITHSUNDEEP
unixgzip
Alby
Alby

Reputation: 5742

what is the difference between using "--stdout" and not using it in gzip?

The below is the code that makes me curious about --stdout of the gzip. Using the diff command on the outputs from two different commands tells me the two files are different, but the manual check says otherwise. What's going on? 

$echo "test" > tmpx
$cat tmpx | gzip > tmpx1.gz
$cat tmpx | gzip --stdout > tmpx2.gz

$diff tmpx1.gz tmpx2.gz
Binary files tmpx1.gz and tmpx2.gz differ

$zcat tmpx1.gz
test

$zcat tmpx2.gz
test

$cat tmpx1.gz
a▒U+I-.▒▒5▒; 

$cat tmpx2.gz
?▒U+I-.▒▒5▒;

Upvotes: 1

Views: 349

Answers (2)

Mark Adler
Mark Adler

Reputation: 112219

To add to Terrence's correct answer, there is no difference, period. When you pipe input to gzip, it automatically sends its output to stdout, whether or not you specify --stdout.

As Terrence noted, the difference is simply in the time stamp. If you did cat tmpx | gzip > tmpx1.gz followed by cat tmpx | gzip > tmpx2.gz at least two seconds later, then you would see that the resulting files will differ due to the time stamp.

Upvotes: 1

Tea Curran
Tea Curran

Reputation: 2983

There is no difference between the compressed data.

What you are seeing is a difference in bytes 4-8 of the file. This contains a unix timestamp of when the gzip file was created.

Upvotes: 2

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