redirect command output to the beginning of a file in bash

There are different ways of doing it. First of all, but, it is important to remember that the file /etc/resolv.conf is sensitive and modifying it erroneously can have bad consequences. Therefore, always backs up before modifying .

As we see in How to insert a text at the beginning of a file? we can use these methods:

echo "hola" | cat - fichero > nuevo_fichero

sed '1s/^/hola\n/' fichero > nuevo_fichero

{ printf "hola\n"; cat fichero; } > nuevo_fichero

They all create a new file. If you want to edit the file itself, use sed -i.bak '...' fichero . This will modify fichero and create a backup called fichero.bak .

Tests:

$ cat fichero
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$ echo "hola" | cat - fichero
hola
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$ sed '1s/^/hola\n/' fichero
hola
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Editing the same file with i :

$ sed -i.bak '1s/^/hola\n/' fichero
$ cat fichero
hola                   # ¡el original se modificó!
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$ cat fichero.bak
1                      # ¡es una copia de seguridad!
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