sed copy substring in following line

Question

I've a .po file I need to copy msgid value into msgstr value if msgstr is empty.

For example

msgid "Hello"
msgstr ""

msgid "Dog"
msgstr "Cane"

Should become

msgid "Hello"
msgstr "Hello"

msgid "Dog"
msgstr "Cane"

Currently, for testing purpose, I'm working with another file, but final script will works inline.

#!/bin/bash
rm it2.po
sed $'s/^msgid.*/&\\\n---&/' it.po > it2.po
sed -i '/^msgstr/d' it2.po
sed -i 's/^---msgid/msgstr/' it2.po

This script has 2 problems (at least):

1. copies msgid into msgstr also when msgstr is not empty;
2. I'm pretty sure that exist a single line or a more elegant solution.

Any help would be appreciated. Thanks in advance.

Answer 1

You may consider better tool gnu awk instead of sed:

awk -i inplace -v FPAT='"[^"]*"|\\S+' '$id != "" && $1 == "msgstr" && (NF==1 || $2 == "\"\"") {$2=id} $1 == "msgid" {id=$2} 1' file

msgid "Hello"
msgstr "Hello"

msgid "Dog"
msgstr "Cane"

-v FPAT='"[^"]*"|\\S+' makes a quoted string or any non-whitespace field an individual field.

A more readable form:

awk -i inplace -v FPAT='"[^"]*"|\\S+' '
$id != "" && $1 == "msgstr" && (NF==1 || $2 == "\"\"") {$2=id}
$1 == "msgid" {id=$2}
1' file

Answer 2

This might work for you (GNU sed):

sed -E 'N;s/(msgid "(.*)".*msgstr )""/\1"\2"/;P;D' file

Open a two line window and if the first line contains msgid and the second msgstr "", replace the msgstr value by the msgid value. Print/delete the first line and repeat.