I want to search the file and replace specific pattern with its hash (SHA1) values.

For example, let file.txt has the following content:

one S56G one two three
four five V67X six

and I want to replace the pattern [A-Z][0-9]\{2\}[A-Z] with SHA1 value of the match. In the example above, the matches are S56G and V67X.

Using sed, I tried:

sed "s/[A-Z][0-9]\{2\}[A-Z]/$(echo \& | sha1sum)/g"

without success, as the result is always the hash value of '&'.

I also tried ge flag, with the command:

sed 's/[A-Z][0-9]\{2\}[A-Z]/echo & | sha1sum/ge'

which throws errors:

sh: 1: one: not found
sha1sum: one: No such file or directory
sha1sum: two: No such file or directory
sha1sum: three: No such file or directory


3 Answers 3


In your attempt the command substitution ($(…)) is performed before sed being executed and the string passed to it as parameter.

Use a scripting language which regular expression substitution supports code execution:

perl -MDigest::SHA=sha1_hex -pe 's/[A-Z][0-9]{2}[A-Z]/sha1_hex$&/ge' inputfile

php -R 'echo preg_replace("/[A-Z][0-9]{2}[A-Z]/e","sha1(\$0)",$argn),"\n";' inputfile

ruby -rdigest/sha1 -pe '$_.gsub!(/[A-Z][0-9]{2}[A-Z]/){Digest::SHA1.hexdigest$&}' inputfile

python -c 'import sys,fileinput,re,hashlib;[sys.stdout.write(re.sub("[A-Z][0-9]{2}[A-Z]",lambda s:hashlib.sha1(s.group(0)).hexdigest(),l))for l in fileinput.input()]' inputfile
  • 1
    Note that the length of python code is not necessarily due to the language's verbosity. Just my knowledge is weaker.
    – manatwork
    Oct 25, 2013 at 10:10
  • I still don't understand why my first try (using the 'g' tag) always returns the hash value of '&'. Without the pipe to 'sha1sum', the 'echo' command returns the correct match. Does 'sed' not support redirection? Oct 25, 2013 at 11:50
  • @user1734905, see if this is more clear: pastebin.com/fvcHTyKV
    – manatwork
    Oct 25, 2013 at 12:18

@manatowork has surely provided with answer. Only adding this as a curiosity ...

A bash+sha1sum variant.

function fail()
    printf "Failed on line \`%s'\n" "$line" >&2
    exit 2

declare -A sha_map;

while read -r line; do
    while [[ $line =~ $re ]]; do
        if ! [[ ${sha_map[$m]} ]]; then
            sha="$(printf "%s" "$m" | sha1sum)" || fail;
            sha_map["$m"]=${sha%% *};
    printf "%s\n" "$line";
done <"$fn"
  • I'm quite reluctant writing a script that reads the file line by line. It seems that 'sed' and 'perl', being well designed stream editor, would be able to read the file much more efficiently than a user-written bash script does. Oct 25, 2013 at 11:54
  • @user1734905: Indeed. As said, only as a curiosity. And, that said, the approach could be used in a script where the volume is small, and the primary goal (of the script) is another.
    – Runium
    Oct 25, 2013 at 12:00

Here is my solution:

cp file.txt result.txt
cat file.txt | grep -o '[A-Z][0-9]\{2\}[A-Z]' | while read i ; do
    sed -i "s/$i/$(echo -n $i | sha1sum | cut -f 1 -d ' ')/g" result.txt

Just copy all the lines and paste it into the terminal. With file.txt

one S56G one two three
four five V67X six

I get

one 03763566330069a397584344c0a640a3cba05a4c one two three
four five 7802350a2592cdc6dfdee408336919ee9e3cc5f2 six

This solution has complexity of O(n^2) and not intended to be used on large files with many pattern matches, since it processes the target file ones per pattern match.

  • I see a fast handed member downvoted you without explanation. It could be because 1) possible openssl version difference – my openssl 1.0.1e provides messy output; 2) your checksums are calculated including trailing newline, like “S56G\n” – use echo -n $i|sha1sum instead.
    – manatwork
    Oct 25, 2013 at 10:21
  • @manatwork Thank you! Nice catch :-) I have improved my answer getting ride of openssl and using echo -n. Oct 25, 2013 at 10:53
  • Your approach has the advantage to easily reduce the amount of hash calculations in case the replaceable fragments are repeated. (So in “a S56G b S56G c S56G d” calculate SHA1 once and replace all 3 occurrences.) Just pipe grep's output through sort -u to avoid performing the same calculation again and again. (sed will replace them all on the first call anyway.)
    – manatwork
    Oct 25, 2013 at 11:16
  • I thought about adding sort -u, but then I decided against it. Don't wanted to make it to complicate. The advantage of my approach is, that it's an example of bash scripting: pipe everything together, if performance doesn't matter. Oct 25, 2013 at 12:49

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