Generate a random filename in unix shell

Question

Hello,

I would like to generate a random filename in unix shell (say tcshell). The filename should consist of random 32 hex letters, e.g.:

c7fdfc8f409c548a10a0a89a791417c5

(to which I will add whatever is neccesary). The point is being able to do it only in shell without resorting to a program.

Answer 1

Tested in zsh, should work with any BASH compatible shell!

#!/bin/zsh

SUM=`md5sum <<EOF
$RANDOM
EOF`

FN=`echo $SUM | awk '// { print $1 }'`

echo "Your new filename: $FN"

Example:

$ zsh ranhash.sh
Your new filename: 2485938240bf200c26bb356bbbb0fa32
$ zsh ranhash.sh
Your new filename: ad25cb21bea35eba879bf3fc12581cc9

Answer 2

Assuming you are on a linux, the following should work:

cat /dev/urandom | tr -cd 'a-f0-9' | head -c 32

This is only pseudo-random if your system runs low on entropy, but is (on linux) guaranteed to terminate. If you require genuinely random data, cat /dev/random instead of /dev/urandom. This change will make your code block until enough entropy is available to produce truly random output, so it might slow down your code. For most uses, the output of /dev/urandom is sufficiently random.

If you on OS X or another BSD, you need to modify it to the following:

cat /dev/urandom | env LC_CTYPE=C tr -cd 'a-f0-9' | head -c 32

Answer 3

why do not use unix mktemp command:

$ TMPFILE=`mktemp tmp.XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX` &&  echo $TMPFILE
tmp.MnxEsPDsNUjrzDIiPhnWZKmlAXAO8983

Answer 4

Grab 16 bytes from /dev/random, convert them to hex, take the first line, remove the address, remove the spaces.

head /dev/random -c16 | od -tx1 -w16 | head -n1 | cut -d' ' -f2- | tr -d ' '

Assuming that "without resorting to a program" means "using only programs that are readily available", of course.

Answer 5

As you probably noticed from each of the answers, you generally have to "resort to a program".

However, without using any external executables, in Bash and ksh:

for i in {0..31}; do string+=$(printf "%x" $(($RANDOM%16)) ); done; echo $string

in zsh:

for i in {0..31}; do string+=$(printf "%x" $(($RANDOM%16)) ); dummy=$RANDOM; done; echo $string

Note that because of using the mod operator on a value that ranges from 0 to 32767 the distribution of digits using the snippets above will be skewed (not to mention the fact that the numbers are pseudo random in the first place).

In any case, the correct way to do this is using mktemp as in oraz's answer.