CODEWITHSUNDEEP
randomzsh
wafel
wafel

Reputation: 85

generating random string zsh

I am trying to generate a random string with the following code:

for pic in `ls *.jpg`; do
  rdn=`echo $RANDOM | sha256sum | cut -d" " -f1`
  mv "$pic" ${rnd}.jpg
done

This part of the script runs from within a directory containing lots of jpeg files and it should randomize their filenames. The problem is that the $RANDOM variable does not update during the iteration, and therefore gives the same hash every time. I tried to use /dev/urandom, and it works, but is a lot slower than $RANDOM. What can I do to "regenerate" $RANDOM every time it is read?

Upvotes: 6

Views: 9284

Answers (3)

Ivan Marinov
Ivan Marinov

Reputation: 2786

On my mac (using macOS High Sierra), the /dev/urandom gives me binary bytes, so the above solution results in tr: Illegal byte sequence, so I used base64 to convert bytes to characters:

cat /dev/urandom | base64 | tr -dc '0-9a-zA-Z' | head -c100

or I found a solution without base64 so you can get punctuation as well:

cat /dev/urandom | LC_ALL=C tr -dc '\''[:alnum:]\!\@\#$\-\.\,'\'' | head -c40

Upvotes: 10

Matthew Franglen
Matthew Franglen

Reputation: 4532

You can do this more simply just using cat, tr and head. For example:

cat /dev/urandom | tr -dc '0-9a-zA-Z' | head -c100

The tr command in this pipeline will delete any character from stdin which does not match the specified character set. The head will print the first 100 characters and then exit, terminating the overall command.

This will generate a 100 character string containing alphanumeric characters. To make this into a rename you just need to use command substitution:

for file in *.jpg
    mv -n ${file} $(cat /dev/urandom | tr -dc '0-9a-zA-Z' | head -c100).jpg

In zsh a for loop with a single statement does not need to be surrounded with do or done. The -n flag to mv will prevent it from overwriting an existing file - just in case you get really unlucky with the random strings.

Upvotes: 6

kojiro
kojiro

Reputation: 77167

for pic in *.jpg; do # Iterate over the jpgs in the current directory.
  dd if=/dev/urandom count=1 2>/dev/null | sha256sum | ( # Gather 512 bytes from /dev/urandom
    read rnd _ # Read the first "word" in the sha256sum output
    mv "$pic" ${rnd}.jpg # rename the jpg.
  )
done

Piping to read causes an implicit subshell, so I create an explicit subshell to guarantee I can still access the rnd parameter. Also, don't parse ls

By the way, are you sure you don't just want to base64 the output? It's cheaper than sha256sum by far and I don't see what you're getting out of sha256sum. Plus it'd make the code easier to read:

for pic in *.jpg; do
  mv "$pic" "$(base64 </dev/urandom | tr -dc 'a-zA-Z0-9' | head -c20).jpg"
done

Upvotes: 1

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