How can i generate a random file filled with random number or character in shell script? I also want to specify size of the file.
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Which characters are allowed in the output file? any random byte or just ascii alphanumeric bytes?Amitay Dobo– Amitay Dobo2010-04-06 18:23:08 +00:00Commented Apr 6, 2010 at 18:23
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6 Answers
Use dd command to read data from /dev/random.
dd if=/dev/random of=random.dat bs=1000000 count=5000
That would read 5000 1MB blocks of random data, that is a whole 5 gigabytes of random data!
Experiment with blocksize argument to get the optimal performance.
answered Apr 6, 2010 at 17:55
Tadeusz A. Kadłubowski
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4 Comments
Progress Programmer
dd -bs=1024 count=1 if/dev/random of=test it doesn generate the right file size i want...
wRAR
@Tadeusz: One should usually use bs=1M if he wants MiB, not MB.
Amitay Dobo
After a second read of the question, i think he also wanted to save only characters (guessing alphabetic ones) and numbers to the file.
camh
That dd command is unlikely to complete as there will not be 5 gigabytes of entropy available. Use /dev/urandom if you need this much "randomness".
head -c 10 /dev/random > rand.txt
change 10 to whatever. Read "man random" for differences between /dev/random and /dev/urandom.
Or, for only base64 characters
head -c 10 /dev/random | base64 | head -c 10 > rand.txt
The base64 might include some characters you're not interested in, but didn't have time to come up with a better single-liner character converter... (also we're taking too many bytes from /dev/random. sorry, entropy pool!)
2 Comments
Amitay Dobo
oops, missed the characters and numbers part, i'm guessing you mean alphanumeric characters... need to revise.
Tadeusz A. Kadłubowski
Saving entropy: the latest trend in egologists' green living. :)
A good start would be:
http://linuxgazette.net/153/pfeiffer.html
#!/bin/bash
# Created by Ben Okopnik on Wed Jul 16 18:04:33 EDT 2008
######## User settings ############
MAXDIRS=5
MAXDEPTH=2
MAXFILES=10
MAXSIZE=1000
######## End of user settings ############
# How deep in the file system are we now?
TOP=`pwd|tr -cd '/'|wc -c`
populate() {
cd $1
curdir=$PWD
files=$(($RANDOM*$MAXFILES/32767))
for n in `seq $files`
do
f=`mktemp XXXXXX`
size=$(($RANDOM*$MAXSIZE/32767))
head -c $size /dev/urandom > $f
done
depth=`pwd|tr -cd '/'|wc -c`
if [ $(($depth-$TOP)) -ge $MAXDEPTH ]
then
return
fi
unset dirlist
dirs=$(($RANDOM*$MAXDIRS/32767))
for n in `seq $dirs`
do
d=`mktemp -d XXXXXX`
dirlist="$dirlist${dirlist:+ }$PWD/$d"
done
for dir in $dirlist
do
populate "$dir"
done
}
populate $PWD
Comments
Create 100 randomly named files of 50MB in size each:
for i in `seq 1 100`; do echo $i; dd if=/dev/urandom bs=1024 count=50000 > `echo $RANDOM`; done
1 Comment
Steve K
It's better to use mktemp to create random files. for i in seq 1 100; do myfile=
mktemp --tmpdir=. dd if=/dev/urandom bs=1024 count=50000 > $myfile doneThe RANDOM variable will give you a different number each time:
echo $RANDOM
Comments
Save as "script.sh", run as ./script.sh SIZE. The printf code was lifted from http://mywiki.wooledge.org/BashFAQ/071. Of course, you could initialize the mychars array with brute force, mychars=("0" "1" ... "A" ... "Z" "a" ... "z"), but that wouldn't be any fun, would it?
#!/bin/bash
declare -a mychars
for (( I=0; I<62; I++ )); do
if [ $I -lt 10 ]; then
mychars[I]=$I
elif [ $I -lt 36 ]; then
D=$((I+55))
mychars[I]=$(printf \\$(($D/64*100+$D%64/8*10+$D%8)))
else
D=$((I+61))
mychars[I]=$(printf \\$(($D/64*100+$D%64/8*10+$D%8)))
fi
done
for (( I=$1; I>0; I-- )); do
echo -n ${mychars[$((RANDOM%62))]}
done
echo
1 Comment
nortally
The /dev/random & base64 approach is also good, instead of piping through base64, pipe through "tr -d -c [:alnum:]", then you just need to count the good chars that come out until you're done.