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I have a text file and I want to delete random lines from a range. Here is an example:

Line 1: abcd

Line 2: efgh

Line 3: ijkl

Line 4: mnop

Line 5: qrst

Line 6: uvwxyz

Out of these six, I want to randomly delete, say, 3.

How to get that done? It would be great if there is a solution in vim, so one can apply it on different ranges.

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3 Answers

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Here's a solution using sed:

sed -i $((start + RANDOM % range))d filename.txt

where:

  • start is the beginning line number of your range
  • range (or end-start is the number of lines to include from start)
  • sed -i -Nd tells sed to delete line N in the input file
  • RANDOM is bash's random number generator; a special shell variable that holds a random integer between 0 and 32767 when you use it.

So, for example, to delete a random line between lines 90 and 120 in file test.txt, you'd use:

sed -i $((90 + RANDOM % 30))d test.txt
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Great! Almost there. Just three more points: What if I want to delete multiple random lines at one go? Can I tell sed to delete say, m lines? Can I tell sed to keep x number of line? Lastly, if I want to execute the same command on a range in vim, :[range] !sed .... what should follow? – deshmukh Jul 13 '12 at 5:46
To delete a range N starting or ending at a random line, simply prefix N, or suffix ,N to the sed expression. To delete multiple random lines, you can repeat the sed expression with -e i.e. sed -e $... -e $... -e $.... Keeping a certain number of lines seems difficult to do with a one-liner, but certainly with a bash script... – izx Jul 13 '12 at 6:18
Please avoid using the $[ ... ] syntax. It has been deprecated for a decade and it was removed from the bash documentation ages ago. The only reason it still works is because of bash's policy on backward compatibility. $(( )) is the documented syntax for arithmetic expansion. printf '%s\n' "$((90 + RANDOM % 30))d" w | ed -s file.txt – geirha Jul 13 '12 at 10:31
OK. This was perfect. I have added an answer by putting everything together. – deshmukh Jul 14 '12 at 4:21
@geirha: many thanks for the edit. I missed your initial comment notification. – izx Jul 15 '12 at 8:14
up vote 0 down vote

Use the following command 

:nd

where n is the line number.

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That would just delete a specific line. I want to delete a random line. – deshmukh Jul 13 '12 at 5:47
up vote 0 down vote

In order to delete multiple random lines form a specific range in the text file, here is what I did:

  • Open the file in vim
  • Go to the top of the range from where you want to remove multiple random lines. The range should have an empty line at the bottom

  • Enter the following command:

    .,/^\s*$/-1 !sed -e $((9 * $RANDOM / 32267))d

. - From here

,/^\s*$/-1 - Till the last non-blank line

!sed -e $((9 * $RANDOM / 32267))d - the sed command to delete a random line

That will delete one line at random.

Now, if you want to delete 5 more random lines, just do 5@: and vim does the rest.

This can be further improved if I can replace that 9 (number of lines from which to delete) with some expression that will be universal

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1  
bash only does integer math, so $((9 * RANDOM / 32267)) will only result in 0 or 9. You really do need modulo. $((RANDOM%9)) – geirha Jul 14 '12 at 10:02
1  
Here's an awk that reads the input into an array, then prints all but one random line from the array. !awk '{a[NR]=$0}END{srand();r=int(NR*rand())+1;for(i=1;i<=NR;++i)if(i\!=r)print a[i]}' – geirha Jul 14 '12 at 11:15
@geirha Thanks! I will still use the same 4@: technique to delete multiple lines in vim. But I wonder how can I print all but 'n' random lines using awk – deshmukh Jul 15 '12 at 6:02
1  
That's a bit more complex since you now have to keep track of what lines you've omitted already. Instead of making r an int we'll make it an array, and fill it with random numbers until it has n unique ones. awk '{a[NR]=$0}END{srand();r[int(NR*rand())+1];while(length(r)<(n<NR?n:NR))r[int(NR*‌​rand())+1];for(i=1;i<=NR;++i)if(\!(i in r))print a[i]}' n=2 n=2 at the end says to omit two lines. – geirha Jul 15 '12 at 8:08
1  
Ah, sorry, I forgot mawk's length() function only works on strings, not arrays. I only tested it with gawk. Is gawk (GNU awk) an option? – geirha Jul 23 '12 at 15:26
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