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Old 06-19-2008, 04:13 PM   #1
pdklinux79
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how to pick random file name from a list of filenames in a text file.


My problem is :

ls -l /data1/cur/*.cdt >>fi.txt

this file has list of column names

i want some way to pick a random name from this fi.txt.

how to do it in bash shell scripting?

thanks
 
Old 06-19-2008, 05:34 PM   #2
unSpawn
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Could try
Code:
a=($(< fi.txt)); rand () { i=`od -cN10 /dev/random | sha1sum`; i=${i// */}; i="${i//[^0-9]/}"; n=${#a[@]}; n=${#n}; i="${i:0:$n}"; [ $i -le ${#a[@]} ] && echo "${i}" || rand; }; for (( n = 0 ; n < 10; n++)); do echo ${a[${RANDOM:2:3}]}; done
. Looks a bit wack but that's because $RANDOM isn't as random as it should be. Hmm. Spose an easier method will come your way RSN ;-p
 
Old 06-19-2008, 05:35 PM   #3
Mr. C.
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Code:
awk "NR==$(($RANDOM % $(wc -l fi.txt| awk '{print $1}')))" fi.txt
or to break it down into multiple stages:

Code:
nlines=$(wc -l fi.txt| awk '{print $1}')
rand=$((RANDOM % $nlines))
awk "NR==$rand" fi.txt

Last edited by Mr. C.; 06-19-2008 at 06:17 PM.
 
Old 06-19-2008, 05:38 PM   #4
unSpawn
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Yeah, *way* easier.
 
Old 06-19-2008, 06:15 PM   #5
rjlee
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Quote:
Originally Posted by pdklinux79 View Post
My problem is :

ls -l /data1/cur/*.cdt >>fi.txt

this file has list of column names

i want some way to pick a random name from this fi.txt.

how to do it in bash shell scripting?

thanks
Just take the first line when sorted randomly:

Code:
sort -R fi.txt | head -n 1
You can pipe that through cut if you want just the filename.

Of course, this isn't truly random since it hashes the keys, so you will get the same chance of each file regardless of the number of times it occurs. Also, it's only pseudorandom, as are all answers not seeding from /dev/urandom.
 
Old 06-19-2008, 06:18 PM   #6
Mr. C.
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Quote:
Originally Posted by rjlee View Post
Just take the first line when sorted randomly:

Code:
sort -R fi.txt | head -n 1

Ouch! Sorting is one of the most expensive operations possible.
 
Old 06-19-2008, 06:54 PM   #7
kenoshi
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Quote:
Originally Posted by Mr. C. View Post
Code:
awk "NR==$(($RANDOM % $(wc -l fi.txt| awk '{print $1}')))" fi.txt
or to break it down into multiple stages:

Code:
nlines=$(wc -l fi.txt| awk '{print $1}')
rand=$((RANDOM % $nlines))
awk "NR==$rand" fi.txt
This will work only if $rand doesn't return a 0, which it does eventually. Also, you don't need the awk for nlines.

E.g.
Code:
rand=$((RANDOM % $(wc -l < fi.txt)))
if [ "$rand" = "0" ]; then ((rand++)) ; fi
awk "NR==$rand" fi.txt
 
Old 06-19-2008, 07:13 PM   #8
Mr. C.
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The redirection is a clever way to avoid the filename as part of the output. Good call.

I ignored the 0 case, but shouldn't have. Instead, because the line numbers are 1 based vs. 0 based, we can bias by one:

rand=$((1 + (RANDOM % $nlines)))

With 10 lines, mod will give 0 to 9. Biasing by 1, yields 1 to 10, exactly what we want.
 
Old 06-20-2008, 11:38 AM   #9
pdklinux79
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Thanks a ton guys... Its working and my friday at work is a cool breeze. Have agood weekend.
 
Old 06-20-2008, 02:46 PM   #10
rjlee
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Quote:
Originally Posted by Mr. C. View Post
Ouch! Sorting is one of the most expensive operations possible.
Ah, but it's easier to read.

And depending on the size of the list of files, speed may not be an issue. Slow but readable and reliable is sometimes better than highly optimised, and if speed were essential then I would be using C or assemblerů
 
  


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