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@emersonf
Last active March 27, 2024 10:11
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A Bash script to compute ETag values for S3 multipart uploads on OS X.
#!/bin/bash
if [ $# -ne 2 ]; then
echo "Usage: $0 file partSizeInMb";
exit 0;
fi
file=$1
if [ ! -f "$file" ]; then
echo "Error: $file not found."
exit 1;
fi
partSizeInMb=$2
fileSizeInMb=$(du -m "$file" | cut -f 1)
parts=$((fileSizeInMb / partSizeInMb))
if [[ $((fileSizeInMb % partSizeInMb)) -gt 0 ]]; then
parts=$((parts + 1));
fi
checksumFile=$(mktemp -t s3md5)
for (( part=0; part<$parts; part++ ))
do
skip=$((partSizeInMb * part))
$(dd bs=1m count=$partSizeInMb skip=$skip if="$file" 2>/dev/null | md5 >>$checksumFile)
done
echo $(xxd -r -p $checksumFile | md5)-$parts
rm $checksumFile
@skchronicles
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Linux users

Here is an equivalent script if you are not using OSX. I hope this helps!

#!/bin/bash
set -euo pipefail
if [ $# -ne 2 ]; then
    echo "Usage: $0 file partSizeInMb";
    exit 0;
fi
file=$1
if [ ! -f "$file" ]; then
    echo "Error: $file not found." 
    exit 1;
fi
partSizeInMb=$2
fileSizeInMb=$(du -m "$file" | cut -f 1)
parts=$((fileSizeInMb / partSizeInMb))
if [[ $((fileSizeInMb % partSizeInMb)) -gt 0 ]]; then
    parts=$((parts + 1));
fi
checksumFile=$(mktemp -t s3md5.XXXXXXXXXXXXX)
for (( part=0; part<$parts; part++ ))
do
    skip=$((partSizeInMb * part))
    $(dd bs=1M count=$partSizeInMb skip=$skip if="$file" 2> /dev/null | md5sum >> $checksumFile)
done
etag=$(echo $(xxd -r -p $checksumFile | md5sum)-$parts | sed 's/ --/-/')
echo -e "${1}\t${etag}"
rm $checksumFile

@shntnu
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shntnu commented Jul 20, 2022

Thank you @skchronicles

I think there's an error in the parts calculations, now fixed below

https://gist.github.com/emersonf/7413337?permalink_comment_id=3244707#gistcomment-3244707

#!/bin/bash
set -euo pipefail
if [ $# -ne 2 ]; then
    echo "Usage: $0 file partSizeInMb";
    exit 0;
fi
file=$1
if [ ! -f "$file" ]; then
    echo "Error: $file not found." 
    exit 1;
fi
partSizeInMb=$2
partSizeInB=$((partSizeInMb * 1024 * 1024)) ### I added this
fileSizeInB=$(du -b "$file" | cut -f 1) ### I edited this
parts=$((fileSizeInB / partSizeInB)) ### I edited this and the next line
if [[ $((fileSizeInB % partSizeInB)) -gt 0 ]]; then 
    parts=$((parts + 1));
fi
checksumFile=$(mktemp -t s3md5.XXXXXXXXXXXXX)
for (( part=0; part<$parts; part++ ))
do
    skip=$((partSizeInMb * part))
    $(dd bs=1M count=$partSizeInMb skip=$skip if="$file" 2> /dev/null | md5sum >> $checksumFile)
done
etag=$(echo $(xxd -r -p $checksumFile | md5sum)-$parts | sed 's/ --/-/')
echo -e "${1}\t${etag}"
rm $checksumFile

@rajivnarayan
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rajivnarayan commented Jul 23, 2022

Thanks, this is quite useful.

I modified the script to speedup the hash computation and avoid generating temporary files. Link to script

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