Small script to verify hash of a downloaded file

Question

You know how whenever you download a file you should really compare the hash of the download to the one provided on the website? This makes absolute sense, but it's a pain to do it letter for letter, digit for digit. So, I wrote this little script to take care of the job. Any comments are welcome.

#!/bin/bash
# hash_checker - program to verify a downloaded file
error_exit()
{
 echo "1ドル" 1>&2
 exit 1
}
usage="usage: hash_checker downloaded_file hash_provided -a algorithm"
downloaded_file=
hash_given=
hash_calc=
algo="sha256"
# check if file and hash were provided
if [ $# -lt 2 ]; then
 error_exit "$usage"
fi
# parsing the provided hash and file
downloaded_file="1ドル"
hash_given="2ドル"
# parsing the algorithm, if provided
if [ "3ドル" != "" ]; then
 if [ "3ドル" = "-a" ]; then
 algo="4ドル"
 else
 error_exit "$usage"
 fi
fi
# check if input is a valid file
if [ ! -f "$downloaded_file" ]; then
 error_exit "Invalid file! Aborting."
fi
# calculate the hash for the file
hash_calc="$($algo'sum' $downloaded_file)"
hash_array=($hash_calc)
hash_calc=${hash_array[0]}
# compare the calculated hash to the provided one
if [ "$hash_calc" = "$hash_given" ]; then
 echo "The hashes match. File seems to be valid."
else
 echo "The hashes do not match. File does not seem to be valid."
fi

• Toby Speight: Why not just use the -c option to sha1sum or md5sum to do the comparison?
• iuvbio: Didn't know that was an option. Kinda expected something like this must exist, thanks.

Answer 1

Notes:

• I'd use getopts for arg parsing -- lots of examples on stackoverflow about how to use it.
• always quote your variables

• you should validate the algorithm:

  sum_exe="${algo}sum"
  if ! type -P "$sum_exe" >/dev/null; then
   error_exit "'$algo' is an unknown checksum algorithm"
  fi
  

• have the checksum program read from stdin, then you don't have to do your (削除) incorrect (削除ここまで) unsafe word parsing since the program will not print a filename

  hash_calc=$( "$sum_exe" < "$downloaded_file" )
  

  ---

  As the above doesn't work, let's use read from a process substitution

  read -r hash_calc _ < <("$sum_exe" < "$downloaded_file")
  

• iuvbio: Thanks for your notes. Reading from stdin the filename is not printed, but ` -` is appended to the hash, and thus the test fails. Is there a way around that other than more word parsing?
• glenn jackman: I've updated my answer