remove all text after a certain character that appears on odd lines

Question

remove all text after a certain character that appears on all odd lines

this works for every line in the file with sed

sed 's/;.*//' 

how to edit this sed line so it just does all odd lines

Answer 1

With GNU sed:

$ sed '1~2s/;.*//' file

The address range 1~2 will match line 1, and then every second line after that. The general case is first~step and the example in the manual is sed -n 1~2p which will print all odd lines.

This is a GNU extension.

The equivalent thing in awk:

$ awk 'NR % 2 { sub(";.*", "") } { print }' file

Or, as suggested by Arrow in comments:

$ awk -F';' 'NR % 2 { $0 = $1 } { print }' file

which has the same effect, but instead of explicitly substituting out everything after the first ; it uses ; as the field separator (with -F';') and then outputs only the first field (by replacing the whole input line with only the first field for odd rows).

Answer 2

This probably might work with all sed version, tested on GNU sed though

$ cat ip.txt 
foo;bar-baz;xyz
a;b;c
good bad. hi there
d;e
cool;hot;normal

$ sed 's/;.*//;n' ip.txt 
foo
a;b;c
good bad. hi there
d;e
cool

The n command will fetch next line but won't change anything, effectively allowing to change only odd lines

With perl

perl -pe 's/;.*// if $.%2' ip.txt

where if $.%2 adds condition to perform substitution only if line number is odd