I have a 2 files as below

File1: 

HEADER
XXXXX
YYYYY
ZZZZZ
EOF

[download]

File2: 

AAAAA
BBBBB
CCCCC
DDDDD
EOF

[download]

How can I get the first line from File1 and insert it into the first line of File2?File 2 ultimately should look like below:

HEADER
AAAAA
BBBBB
CCCCC
DDDDD
EOF

[download]

Is there a single liner which can do this ?

In reply to Single line insert to the start of a file by darklord_999

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