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}'
  • 1
    Have you compared the output when you give sha1sum a filename? – Jeff Schaller Dec 10 '18 at 1:23
  • 1
    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

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.

  • 1
    The - actually represents stdin. From man md5sum With no FILE, or when FILE is -, read standard input. – Isaac Dec 10 '18 at 8:14


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

 $ echo test | sha1sum
 4e1243bd22c66e76c2ba9eddc1f91394e57f9f83  -

 $ echo -n test | sha1sum
 a94a8fe5ccb19ba61c4c0873d391e987982fbbd3  -


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.


A couple of simpler solutions on other languages are:

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

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 

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

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

 import hashlib
     m = hashlib.sha1()

 import sys

 for line in sys.stdin:

 print m.hexdigest()

Use it:

 $ echo "test" | python sha1.py

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