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When I generate checksums for my files, I have to do a bit of extra work to get them formatted properly:

CHECKSUM="$(shasum "$file" | cut -b 1-40)"
echo -n "$CHECKSUM" > "$file.sha"

This is to ensure that in the file, I only have the checksum itself (hence cut) and that at the end of the file, there's no newline (hence echo -n).

Validating the checksums also requires a bit extra work:

CHECKSUM="$(shasum "$file" | cut -b 1-40)"
echo -n "$CHECKSUM" | diff - "$file.sha"

Is there an easier, more compact way of creating and validating checksums without storing the path? I prefer not to store the path in the checksum along with the file as it makes the files less portable IMO.

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1  
If you need the checksums for many files then it would be more efficient to run sha1sum over all files and make awk put each line in a single file: sha1sum * | awk ... –  Hauke Laging Apr 28 at 20:05
    
Would md5deep be helpful ? –  Iain Apr 28 at 20:21

1 Answer 1

up vote 4 down vote accepted

An obvious small improvement to

CHECKSUM="$(shasum "$file" | cut -b 1-40)"
echo -n "$CHECKSUM" > "$file.sha"

is:

shasum "$file" | cut -b 1-40 >"$file.sha"

The trailing newline by cut can be avoided with `awk:

shasum "$file" | awk -v ORS="" '{print $1}'

A simpler form of

CHECKSUM="$(shasum "$file" | cut -b 1-40)"
echo -n "$CHECKSUM" | diff - "$file.sha"

is:

if [ "$(shasum "$file" | cut -b 1-40)" = "$(<"$file.sha")" ]; then ...
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1  
shasum "$file" | cut -b 1-40 > "$file.sh" - Won't this append a newline at the end of the file by default? –  Naftuli Tzvi Kay Apr 28 at 19:33
    
@NaftuliTzviKay I guess you misunderstand the meaning of echo -n. That does not mean "strip off all trailing newlines". It means: "Do not add a newline" I.e. if there is a newline at the end of $CHECKSUM then both your and my solution write a trailing newline to the file. But who cares about trailing newlines when using $()? echo -n $CHECKSUM would strip off newlines, though. –  Hauke Laging Apr 28 at 19:36
    
My files end up being 41 bytes rather than the expected 40 bytes - trailing newline. –  Naftuli Tzvi Kay Apr 28 at 19:41
    
@NaftuliTzviKay Right. My $CHECKSUM explanation is correct but as $CHECKSUM does not appear in my code it is not helpful. But as the "$(<"$file.sha")" removes the newline this does not matter (unless you use the files in other ways, too). This problem can probably not be avoided wit cut. I will add an awk alternative to my answer. –  Hauke Laging Apr 28 at 19:50

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