Bash script number generator - bash

Question

I need to generate random numbers in an specific format as test data. For example, given a number "n" I need to produce "n" random numbers and write them in a file. The file must contain at most 3 numbers per line. Here is what I have:
#!/bin/bash
m=$1
output=$2
for ((i=1; i<= m; i++)) do
echo $((RANDOM % 29+2)) >> $output
done
This outputs the numbers as:
1
2
24
21
10
14
and what I want is:
1 2 24
21 10 14
Thank you for your help!

Answer 1

Pure bash (written as a function rather than a script file)
randx3() {
local d=$' \n'
local i
for ((i=0;i<$(($1 - 1));++i)); do
printf "%d%c" $((RANDOM%29 + 2)) "${d:$((i%3)):1}"
done
printf "%d\n" $((RANDOM%29 + 2))
}
Note that it doesn't take a file argument; rather it outputs to stdout, so you would use it like this:
randx3 11 > /path/to/output
That style is often more flexible.
Here's a less hacky one which allows you to select how often you want a newline:
randx() {
local i
local m=$1
local c=${2:-3}
for ((i=1;i<=m;++i)); do
if ((i%c && i<m)); then
printf "%d " $((RANDOM%29 + 2))
else
printf "%d\n" $((RANDOM%29 + 2))
fi
done
}
Call that one as randx 11 or randx 11 7 (second argument defaults to 3).

Answer 2

Pipe the output to a command that will read 3 lines at a time:
for ((i=1; i<= m; i++)) do
echo $((RANDOM % 29+2))
done | sed -e '$!N;$!N;s/\n/ /g' >> $output

Answer 3

This is what paste was designed for:
$ for i in {0..10}; do echo $RANDOM; done | paste -d' ' - - -
14567 3240 16354
17457 25616 12772
3912 7490 12206
7342 10554
Another approach would be to build up the values in an array, then use printf.
m=$1
output=$2
vals=()
while (( m-- )); do
vals+=( $((RANDOM % 29+2)) )
done
printf '%d %d %d\n' "${vals[#]}" > "$output"

Answer 4

Shortest!!!
I need to produce "n" random numbers and write them in a file. The file must contain at most 3 numbers per line.
pr -t -3 -s\ <(for ((n=6;n--;)){ echo $((RANDOM % 29+2));}) >file
Then
cat file
11 29 27
14 21 22
YAS: Yet another bash solution
As a script:
#!/bin/bash
n=$1
file=$2
out=()
>$file
for ((i=1;i<=n;i++));do
out+=($((RANDOM%29+2)))
[ $((i%3)) -eq 0 ] && echo ${out[*]} >>$file && out=()
done
[ "$out" ] && echo ${out[*]} >>$file
Usage:
script <quantity of random> <filename>
Important remark about RANDOM%29
This way of rendering random between 2 to 30 is not equitable!
As $RANDOM give a number between 0 and 32767, there is:
for ((i=0;i<32768;i++)) ;do
((RL[$((i%29+2))]++))
done
for ((i=0;i<32;i++));do
printf "%3d %5d\n" $i ${RL[i]}
done | column
0 0 7 1130 14 1130 21 1130 28 1130
1 0 8 1130 15 1130 22 1130 29 1129
2 1130 9 1130 16 1130 23 1130 30 1129
3 1130 10 1130 17 1130 24 1130 31 0
4 1130 11 1130 18 1130 25 1130
5 1130 12 1130 19 1130 26 1130
6 1130 13 1130 20 1130 27 1130
... there is 1130 chances to obtain a number between 2 to 28, but only 1129 chances to obtain a 29 or a 30.
To prevent this, you have to drop unwanted results:
random2to30() {
local _random=32769
while (( $_random>=32741 )) ;do
_random=$RANDOM;
done;
printf -v $1 "%d" $((2+_random%29))
}
The proof:
tstr2to30() {
unset $1
local _random=32769
while (( $_random>=32741 )); do
read _random || break
done
[ "$_random" ] && printf -v $1 "%d" $((2 +_random % 29 ))
}
unset RL
while tstr2to30 MyRandom && [ "$MyRandom" ] ;do
((RL[MyRandom]++))
done < <(seq 0 32767)
for ((i=0;i<32;i++));do
printf "%3d %5d\n" $i ${RL[i]}
done | column
Give:
0 0 7 1129 14 1129 21 1129 28 1129
1 0 8 1129 15 1129 22 1129 29 1129
2 1129 9 1129 16 1129 23 1129 30 1129
3 1129 10 1129 17 1129 24 1129 31 0
4 1129 11 1129 18 1129 25 1129
5 1129 12 1129 19 1129 26 1129
6 1129 13 1129 20 1129 27 1129
Where all value do obtain exactly same (1129) chances!
Final useable script
So the script could become (Don't forget bash's shebang!):
#!/bin/bash
n=${1:-11} # default to 11 values
c=${2:-3} # default to 3 values by lines
minval=${3:-2} # default to 2 random min
maxval=${4:-30} # defailt to 30 random max
file=${5:-/dev/stdout} # default to STDOUT
rnum=$(( maxval - minval + 1 ))
rmax=$(( ( 32768 / rnum ) * rnum ))
randomGen() {
local _random=33000
while [ $_random -ge $rmax ] ;do
_random=$RANDOM
done
printf -v $1 "%d" $(( minval +_random % rnum ))
}
out=()
for ((i=1;i<=n;i++));do
randomGen MyRandom
out+=($MyRandom)
[ $((i%c)) -eq 0 ] && echo ${out[*]} >>"$file" && out=()
done
[ "$out" ] && echo ${out[*]} >>"$file"

Answer 5

This awk will insert a newline after every 3rd line or a space:
for ((i=1; i<= m; i++)); do
echo $((RANDOM % 29+2))
done | awk '{printf "%s%c", $1, (NR % 3) ? " " : "\n"}' >> $output

Answer 6

yet another way of doing it :
eval echo {1..$m} | xargs -n3 echo $((RANDOM % 29+2)) > $output