Reverse even lines

Shell Programming and Scripting
1

I'm trying to reverse every even line in my file using the awk command below but it prints only the odd lines but nothing else:

$ awk '(NR % 2) {print}; !(NR % 2) {print | "rev";}' myfile

Any idea what I might have done wrong?

Thank you.

2

Show input and expected output

3

Input:

@MVM-RI-I124161:36:000000000-A326L:1:1101:14597:1343 1:N:0:NTTACTCG
TCAGCGAAAACGCTTCGGAATGGAGGATATCTATTTTCCAAAATCCTCTGTGGTCTTTTTGCAGCATTCTGGAAAAAGACCACAGAGGATTTTGGAAAATAGATATATTTAATGGAGGCAAGTGTGAAAAATCACATTGAAATGCA
+
>>111>1111B1AEFE?000ACB10?0FBFGHCHHHHGFFD1CFECGGFFFFBFFGGHHH?A21B0EGGHEE1B100//E>0?/010/0FGGGD/1@10B1@1FDGBDGH2>F2110AEEAFGFHBFHBDAFGFGFGHBGFBGBFF

Output:

@MVM-RI-I124161:36:000000000-A326L:1:1101:14597:1343 1:N:0:NTTACTCG
ACGTAAAGTTACACTAAAAAGTGTGAACGGAGGTAATTTATATAGATAAAAGGTTTTAGGAGACACCAGAAAAAGGTCTTACGACGTTTTTCTGGTGTCTCCTAAAACCTTTTATCTATAGGAGGTAAGGCTTCGCAAAAGCGACT
+
FFBGBFGBHGFGFGFADBHFBHFGFAEEA0112F>2HGDBGDF1@1B01@1/DGGGF0/010/?0>E//001B1EEHGGE0B12A?HHHGGFFBFFFFGGCEFC1DFFGHHHHCHGFBF0?01BCA000?EFEA1B1111>111>>

Thanks.

4 
 awk '!(FNR%2) {t="";l=length;for(i=1;i<=l;i++) t=substr($0,i,1) t;$0=t}1' myFile
5

This is doing the trick but any idea why my command doesn't work? And can you please explain your command? Thank you.

6 
$ cat input
1
1 2
1 2 3
1 2 3 4

$ awk '(NR % 2 == 0)  { print }' input
1 2
1 2 3 4

$ awk '(NR % 2 == 1)  { print }' input
1
1 2 3
7

Try

$ awk  '{if(NR%2==0)print $0 | "rev" ;else print }' file
8 
pesky awk-s - don't flush output  unless you explicitly close the pipped cmd
awk 'NR % 2 {print;next}; {print | "rev";close("rev")}' myfile

# for every EVEN line
!(FNR%2) {
   # initialize a temp var t to an empty string
   t=""
   # temp var 'l' contains the length of the current record/line
   l=length
   # iterate through the current line assigning the reversed chars to 't'
   for(i=1;i<=l;i++) 
      t=substr($0,i,1) t
   # assign the final reversed record/line back to $0
   $0=t
}
# 1 in awk is default/shortcut to 'print' current/$0 record/line
1
1 Like
9

try also:

awk '{if (NR % 2) {print} else {r="rev" ; print | r; close(r);}; }' input
10

gawk / mawk / bwk variation to vgersh99's suggestion:

awk '!(NR%2){for(i=2;i<=NF;i++) $1=$i $1; $0=$1}1' FS= file
11

Thank you all for your help.