Pfthb

I'm trying to decode a file, which is mostly encoded with base64. What I want to do is to decode the following, while still maintaining the [_*_].

example.txt

wq9cXyjjg4QpXy/Crwo=
[_NOTBASE64ED_]
aGkgdGhlcmUK
[_CONSTANT_]
SGVsbG8gV29ybGQhCg==

Sometimes it'll be in this form

aGkgdGhlcmUK[_CONSTANT_]SGVsbG8gV29ybGQhCg==

Desired output

¯_(ツ)_/¯
[_NOTBASE64ED_]
hi there
[_CONSTANT_]
Hello World!
hi there[_CONSTANT_]Hello World!

Error output

¯_(ツ)_/¯
4��!:�@�H�B�8ԓ��[��ܛBbase64: invalid input

What I've tried

base64 -di example.txt
base64 -d example.txt
base64 --wrap=0 -d -i example.txt

I tried to individually base64 the [_*_] using grep -o. Then find and

replacing them through a weird arrangement with arrays, but I couldn't

get it to work.
base64ing it all, then decoding. Results in double base64ed rows.

The file is significantly downsized!
Encoded using base64 --wrap=0, while loop, and if/else statement.
The [_*_] still need to be there after being decoded.

I am sure someone has a more clever solution than this. But try this

#! /bin/bash

MYTMP1=""
function printInlineB64()
 sed -e 's/[/n[/g' -e 's/]/]n/g'))
 OUTPUT=""
 for line in "$lines[@]"; do
 MYTMP1=$(base64 -d <<< "$line" 2>/dev/null)
 if [ "$?" != "0" ]; then
 OUTPUT="$OUTPUT$line"
 else
 OUTPUT="$OUTPUT$MYTMP1"
 fi;
 done
 echo "$OUTPUT"


MYTMP2=""
function printB64Line()

 local line=$1

 # not fully base64 line
 if [[ ! "$line" =~ ^[A-Za-z0-9+/=]+$ ]]; then 
 printInlineB64 "$line"
 return
 fi;

 # likely base64 line
 MYTMP2=$(base64 -d <<< "$line" 2>/dev/null)
 if [ "$?" != "0" ]; then
 echo $line
 else
 echo $MYTMP2
 fi;



FILE=$1
if [ -z "$FILE" ]; then
 echo "Please give a file name in argument" 
 exit 1;
fi;

while read line; do
 printB64Line "$line"
done < $FILE

and here is output

$ cat example.txt && echo "==========================" && ./base64.sh example.txt
wq9cXyjjg4QpXy/Crwo=
[_NOTBASE64ED_]
aGkgdGhlcmUK
[_CONSTANT_]
SGVsbG8gV29ybGQhCg==
==========================
¯_(ツ)_/¯
[_NOTBASE64ED_]
hi there
[_CONSTANT_]
Hello World!

$ cat example2.txt && echo "==========================" && ./base64.sh example2.txt
aGkgdGhlcmUK[_CONSTANT_]SGVsbG8gV29ybGQhCg==
==========================
hi there[_CONSTANT_]Hello World!

You need a loop that reads each line and tests whether it's base64 or non-base64, and processes it appropriately.

while read -r line
do
 case "$line" in
 [*]) echo "$line" ;;
 *) base64 -d <<< "$line" ;;
 esac
done << example.txt

I would suggest using other languages other than sh but here is a solution using cut. This would handle the case where there are more than one [_constant_] in a line.

#!/bin/bash

function decode() base64 -d)
 const=$(echo $line 

while read -r line; do
 decode $line
done < example.txt

If Perl is an option, you can say something like:

perl -MMIME::Base64 -lpe '$_ = join("", grep chomp($_ = decode_base64($_)), 1 split(/(?=[)|(?<=])/))' example.txt

The code below is equivalent to the above but is broken down into steps for the explanation purpose:

#!/bin/bash

perl -MMIME::Base64 -lpe '
 @ary = split(/(?=[)|(?<=])/, $_);
 foreach (@ary) 
 if (! /^[/) 
 chomp($_ = decode_base64($_));
 
 
 $_ = join("", @ary);
' example.txt

• 
  -MMIME::Base64 option loads the base64 codec module.


• 
  -lpe option makes Perl bahave like AWK to loop over input lines and implicitly handle newlines.


• The regular expression (?=[)|(?<=]) matches the boundary between the base64 block and the maintaining block surrounded by [...].


• The split function divides the line into blocks on the boundary and store them in an array.


• Then loop over the array and decode the base64-encoded entry if found.


• Finally merge the substring blocks into a line to print.