Comment from start of file to a pattern matched line using sed

As a one-liner to demonstrate the concept :

echo -e 'a\nb\nc\nPATTERN\nd\ne\nf' | sed '0,/PATTERN/ s/^/#/'

You just have to adapt to your context :

• as for the 'PATTERN'
• I assumed '#' as the commenting character
• and regarding how you can apply this to all your files. If they all are 'fileXXX.txt', you can run : sed -i '0,/PATTERN/ s/^/#/' file*txt