Read line by line and print matches line by line

Question

I am new to shell scripting, it would be great if I can get some help with the question below.

I want to read a text file line by line, and print all matched patterns in that line to a line in a new text file.

For example:

$ cat input.txt

SYSTEM ERROR: EU-1C0A  Report error -- SYSTEM ERROR: TM-0401 DEFAULT Test error
SYSTEM ERROR: MG-7688 DEFAULT error -- SYSTEM ERROR: DN-0A00 Error while getting object -- ERROR: DN-0A52 DEFAULT Error -- ERROR: MG-3218 error occured in HSSL
SYSTEM ERROR: DN-0A00 Error while getting object -- ERROR: DN-0A52 DEFAULT Error
SYSTEM ERROR: EU-1C0A  error Failed to fill in test report -- ERROR: MG-7688

The intended output is as follows:

$ cat output.txt

EU-1C0A TM-0401
MG-7688 DN-0A00 DN-0A52 MG-3218
DN-0A00 DN-0A52
EU-1C0A MG-7688

I tried the following code:

while read p; do
    grep -o '[A-Z]\{2\}-[A-Z0-9]\{4\}' | xargs
done < input.txt > output.txt

which produced this output:

EU-1C0A TM-0401 MG-7688 DN-0A00 DN-0A52 MG-3218 DN-0A00 DN-0A52 EU-1C0A MG-7688 .......

Then I also tried this:

while read p; do
    grep -o '[A-Z]\{2\}-[A-Z0-9]\{4\}' | xargs > output.txt
done < input.txt

But did not help :(

Maybe there is another way, I am open to awk/sed/cut or whatever... :)

Note: There can be any number of Error codes (i.e. XX:XXXX, the pattern of interest in a single line).

Answer 1

% awk 'BEGIN{RS=": "};NR>1{printf "%s%s", $1, ($0~/\n/)?"\n":" "}' input.txt 
EU-1C0A TM-0401
MG-7688 DN-0A00 DN-0A52 MG-3218
DN-0A00 DN-0A52
EU-1C0A MG-7688

Explanation in longform:

awk '
    BEGIN{ RS=": " } # Set the record separator to colon-space
    NR>1 {           # Ignore the first record
        printf("%s%s", # Print two strings:
            $1,      # 1. first field of the record (`$1`)
            ($0~/\n/) ? "\n" : " ")
                     # Ternary expression, read as `if condition (thing
                     # between brackets), then thing after `?`, otherwise
                     # thing after `:`.
                     # So: If the record ($0) matches (`~`) newline (`\n`),
                     # then put a newline. Otherwise, put a space.
    }
' input.txt 

Previous answer to the unmodified question:

% awk 'BEGIN{RS=": "};NR>1{printf "%s%s", $1, (NR%2==1)?"\n":" "}' input.txt 
EU-1C0A TM-0401
MG-7688 MG-3218
DN-0A00 DN-0A52
EU-1C0A MG-7688

edit: With safeguard against :-injection (thx @e0k). Tests that the first field after the record seperator looks like how we expect it to be.

awk 'BEGIN{RS=": "};NR>1 && $1 ~ /^[A-Z]{2}-[A-Z0-9]{4}$/ {printf "%s%s", $1, ($0~/\n/)?"\n":" "}' input.txt