Get 20% of lines in File randomly - shell

Question

This is my code:
nb_lignes=`wc -l $1 | cut -d " " -f1`
for i in $(seq $nb_lignes)
do
m=`head $1 -n $i | tail -1`
//command
done
Please how can i change it to get Get 20% of lines in File randomly to apply "command" on each line ?
20% or 40% or 60 % (it's a parameter)
Thank you.

Answer 1

This will randomly get 20% of the lines in the file:
awk -v p=20 'BEGIN {srand()} rand() <= p/100' filename
So something like this for the whole solution (assuming bash):
#!/bin/bash
filename="$1"
pct="${2:-20}" # specify percentage
while read line; do
: # some command with "$line"
done < <(awk -v p="$pct" 'BEGIN {srand()} rand() <= p/100' "$filename")
If you're using a shell without command substitution (the <(...) bit), you can do this - but the body of the loop won't be able to have any side effects in the outer script (e.g. any variables it sets won't be set anymore once the loop completes):
#!/bin/sh
filename="$1"
pct="${2:-20}" # specify percentage
awk -v p="$pct" 'BEGIN {srand()} rand() <= p/100' "$filename" |
while read line; do
: # some command with "$line"
done

Answer 2

Try this:
file=$1
nb_lignes=$(wc -l $file | cut -d " " -f1)
num_lines_to_get=$((20*${nb_lignes}/100))
for (( i=0; i < $num_lines_to_get; i++))
do
line=$(head -$((${RANDOM} % $nb_lignes)) $file | tail -1)
echo "$line"
done
Note that ${RANDOM} only generates numbers less than 32768 so this approach won't work for large files.
If you have shuf installed, you can use the following to get a random line instead of using $RANDOM.
line=$(shuf -n 1 $file)

Answer 3

you can do it with awk.see below:
awk -v b=20 '{a[NR]=$0}END{val=((b/100)*NR)+1;for(i=1;i<val;i++)print a[i]}' all.log
the above command prints 20% of all the lines starting from begining of the file.
you just have to change the value of b on command line to get the required % of lines.
tested below:
> cat temp
1
2
3
4
5
6
7
8
9
10
> awk -v b=10 '{a[NR]=$0}END{val=((b/100)*NR)+1;for(i=1;i<val;i++)print a[i]}' temp
1
> awk -v b=20 '{a[NR]=$0}END{val=((b/100)*NR)+1;for(i=1;i<val;i++)print a[i]}' temp
1
2
>

Answer 4

shuf will produce the file in a randomized order; if you know how many lines you want, you can give that to the -n parameter. No need to get them one at a time. So:
shuf -n $(( $(wc -l < $FILE) * $PCT / 100 )) "$file" |
while read line; do
# do something with $line
done
shuf comes standard with GNU/Linux distros afaik.