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Is it possible to have SHA sums print without the - appended to the end?

$ echo test | sha1sum 
4e1243bd22c66e76c2ba9eddc1f91394e57f9f83  -  <--- this "-" dash/hyphen

I know we can use awk and other command line tools, but can it be done without using another tool?

$ echo test | sha1sum | awk '{print $1}'
4e1243bd22c66e76c2ba9eddc1f91394e57f9f83
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    Have you compared the output when you give sha1sum a filename? – Jeff Schaller Dec 10 '18 at 1:23
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    Also, man sha1sum implies (not explicitly) when - is used. – Sparhawk Dec 10 '18 at 1:24
  • Oh, I see. The - means the sum was created using stdin? Is it possible to omit the - using only sha1sum? – user325067 Dec 10 '18 at 1:25
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This is not possible without another tool or without editing the actual sha1sum script/binary. When sha1sum is fed a file it prints the filename after the sum. When sha1sum is not fed a file or is used with a pipe. It puts the - there as a placeholder to indicate that the input was not a file.

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    The - actually represents stdin. From man md5sum With no FILE, or when FILE is -, read standard input. – Isaac Dec 10 '18 at 8:14
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Newline

Understand that the echo is adding a newline at the end which changes the hash:

 $ echo test | sha1sum
 4e1243bd22c66e76c2ba9eddc1f91394e57f9f83  -

 $ echo -n test | sha1sum
 a94a8fe5ccb19ba61c4c0873d391e987982fbbd3  -

Answer

But no, there is no way to make sha1sum to print only the hash. The reason is that the line is actually an encoded string. The line could start with a \ and the second space (yes, there are two) between the hash and the filename could become a * to indicate a binary hash (useful in DOS):

 $ echo "hello" >'a\file'
 $ md5sum -b 'a\file'
 \b1946ac92492d2347c6235b4d2611184 *a\\file

So, no, it is not a good idea to try to parse that output without understanding the above.


Alternatives

A couple of simpler solutions on other languages are:

 $ echo "test" | perl -le 'use Digest::SHA qw(sha1_hex); print sha1_hex(<>);'
 4e1243bd22c66e76c2ba9eddc1f91394e57f9f83

Or (for longer input, less memory used), write to a file (lets call it sha1.perl):

 use Digest::SHA qw(sha1_hex);
 $state = Digest::SHA->new(sha1);
 for (<>) { $state->add($_) }
 print $state->hexdigest, "\n";

Execute it:

 $ echo "test" | perl sha1.perl 
 4e1243bd22c66e76c2ba9eddc1f91394e57f9f83

 $ echo "test" | php -r '$f = fgets(STDIN);  echo sha1($f),"\n";'
 4e1243bd22c66e76c2ba9eddc1f91394e57f9f83

Write this to a file (lets call it sha1.py):

 import hashlib
     m = hashlib.sha1()

 import sys

 for line in sys.stdin:
     m.update(line)

 print m.hexdigest()

Use it:

 $ echo "test" | python sha1.py
 4e1243bd22c66e76c2ba9eddc1f91394e57f9f83

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