bash- trimming the even lines in txt file

Question

I'd like to remove the first X characters and the last Y characters from all even lines of a file with bash.

Input:

1
AABBBBBCCC
2
GKDDABC

let X=2 and Y=3:

1
BBBBB
2
DD

Answer 1

Using awk:

$ awk -v x=2 -v y=3 '0==NR%2 {$0=substr($0,x+1,length($0)-y-x)} 1' file
1
BBBBB
2
DD

How it works:

• -v x=2 -v y=3

  The -v options define our two variables, x and y.

• 0==NR%2 {$0=substr($0,x+1,length($0)-y-x)}

  NR is the line counter. When 0 == NR%2, we are on an even line and we remove x characters from the start and y from the end. In awk, $0 is the whole line. We replace with a substring which starts at position x+1 and has length of length($0)-y-x.

• 1

  This is cryptic shorthand for print the line.