How to use AWK to continuously output lines from a file - linux

Question

I have a file with multiple lines, and I want to continuously output some lines of the file, such as first time, print from line 1 to line 5, next time, print line 2 to line 6, and so on.
I find AWK as a very useful function and I tried to write a code on my own, but it just outputs nothing.
Following is my code
#!/bin/bash
for n in `seq 1 3`
do
N1=$n
N2=$((n+4))
awk -v n1="$N1" -v n2="$N2" 'NR == n1, NR == n2 {print $0}' my_file >> new_file
done
For example, I have an input file called my_file
1 99 tut
2 24 bcc
3 32 los
4 33 rts
5 642 pac
6 23 caas
7 231 cdos
8 1 caee
9 78 cdsa
Then I expect an output file as
1 99 tut
2 24 bcc
3 32 los
4 33 rts
5 642 pac
2 24 bcc
3 32 los
4 33 rts
5 642 pac
6 23 caas
3 32 los
4 33 rts
5 642 pac
6 23 caas
7 231 cdos

Answer 1

Could you please try following, written and tested with shown samples in GNU awk. Where one needs to mention all lines which needs to be printed in lines_from variable, then there is a variable named till_lines which tells us how many lines we need to print from a specific line(eg--> from 1st line print next 4 lines too). On another note, I have tested OP's code and it worked fine for me, its generating the output file with new_file since calling awk in bash loop is NOT good practice hence adding this as an improvement too here.
awk -v lines_from="1,2,3" -v till_lines="4" '
BEGIN{
num=split(lines_from,arr,",")
for(i=1;i<=num;i++){ line[arr[i]] }
}
FNR==NR{
value[FNR]=$0
next
}
(FNR in line){
print value[FNR] > "output_file"
j=""
while(++j<=till_lines){ print value[FNR+j] > "output_file" }
}
' Input_file Input_file
When I see contents of output_file I could see following:
cat output_file
1 99 tut
2 24 bcc
3 32 los
4 33 rts
5 642 pac
2 24 bcc
3 32 los
4 33 rts
5 642 pac
6 23 caas
3 32 los
4 33 rts
5 642 pac
6 23 caas
7 231 cdos
Explanation: Adding detailed explanation for above.
awk -v lines_from="1,2,3" -v till_lines="4" ' ##Starting awk program from here and creating 2 variables lines_from and till_lines here, where lines_from will have all line numbers which one wants to print from. till_lines is the value till lines one has to print.
BEGIN{ ##Starting BEGIN section of this program from here.
num=split(lines_from,arr,",") ##Splitting lines_from into arr with delimiter of , here.
for(i=1;i<=num;i++){ ##Running a for loop from i=1 to till value of num here.
line[arr[i]] ##Creating array line with index of value of array arr with index of i here.
}
}
FNR==NR{ ##Checking condition FNR==NR which will be TRUE when 1st time Input_file is being read.
value[FNR]=$0 ##Creating value with index as FNR and its value is current line.
next ##next will skip all further statements from here.
}
(FNR in line){ ##Checking condition if current line number is coming in array then do following.
print value[FNR] > "output_file" ##Printing value with index of FNR into output_file
j="" ##Nullifying value of j here.
while(++j<=till_lines){ ##Running while loop from j=1 to till value of till_lines here.
print value[FNR+j] > "output_file" ##Printing value of array value with index of FNR+j and print output into output_file
}
}
' Input_file Input_file ##Mentioning Input_file names here.

Answer 2

Another awk variant
awk '
BEGIN {N1=1; N2=5}
arr[NR]=$0 {}
END {
while (arr[N2]) {
for (i=N1; i<=N2; i++)
print arr[i]
N1++
N2++
}
}
' file