linux - How to execute ssh-keygen without prompt

Question

I want to automate generate a pair of ssh key using shell script on Centos7, and I have tried

yes "y" | ssh-keygen -t rsa
echo "


" | ssh-keygen...
echo | ssh-keygen..

all of these command doesn't work, just input one 'enter' and the shell script stopped on "Enter passphrase (empty for no passphrase)", I just want to know how to simulate mutiple 'enter' in shell continuously.

Many thanks if anyone can help !

Answer 1

We need to accomplish two steps automatically:

1. Enter a passphrase. Use the -N flag (void string for this example):

   ssh-keygen -t rsa -N ''

2. Overwrite the key file:

Use -f to enter the path (in this example id_rsa) plus a here-string to answer yes to the following question:

ssh-keygen -q -t rsa -N '' -f ~/.ssh/id_rsa <<<y >/dev/null 2>&1

Or, under a bash like shell, If you certainly want to overwrite the previous one, use just a here-string to feed the command with all the need input:

ssh-keygen -q -t rsa -N '' <<< $'
y' >/dev/null 2>&1

From ssh-keygen man page:

  -N new_passphrase provides the new passphrase.
  -q                silence ssh-keygen.
  -f filename       specifies the filename of the key file.

---

Step by step explanation

$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/klashxx/.ssh/id_rsa):

1) To avoid entering the key use -f:

$ ssh-keygen -t rsa -f ~/.ssh/id_rsa
Generating public/private rsa key pair.
/home/klashxx/.ssh/id_rsa already exists.
Overwrite (y/n)?

ATTENTION: If you don't care about the RSA file name and certainly want to overwrite the previous one, check the instructions below point four.

2) Now we need to answer "y" automatically to the overwrite question (let's use a here-string for that job):

$ ssh-keygen -t rsa -f ~/.ssh/id_rsa <<< y
Generating public/private rsa key pair.
/home/klashxx/.ssh/id_rsa already exists.
Overwrite (y/n)? Enter passphrase (empty for no passphrase):

3) Finally we're going to use the -N flag to enter a void pass:

$ ssh-keygen -t rsa -N '' -f ~/.ssh/id_rsa <<< y
Generating public/private rsa key pair.
/home/klashxx/.ssh/id_rsa already exists.
Overwrite (y/n)? Your identification has been saved in /home/klashxx/.ssh/id_rsa.
Your public key has been saved in /home/klashxx/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:Xo0t6caMB/8TSsigxfY28JIfqYjyqxRZrFrPncx5yiU klashxx@server
The key's randomart image is:
+---[RSA 2048]----+
|                 |
|  .              |
|   o .           |
|  +   *    =     |
| +.  + BSo= o    |
|...o.+o+XO...    |
|.. .o.E==+B. .   |
|o . ...=.o...    |
|.+o.  o     ..   |
+----[SHA256]-----+

4) Extra ball, cleanup the output, just check the return code:

$ ssh-keygen -q -t rsa -N '' -f ~/.ssh/id_rsa <<<y >/dev/null 2>&1
$ echo $?
0

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An alternative path to overwrite the previous RSA file (no -f flag needed)

NOTE: Only bash like shells.

If you don't care about the RSA name and just want to overwrite it, we need to answer these two questions automatically:

1. Enter file in which to save the key: /example/path/.ssh/id_rsa already exists.

2. Overwrite (y/n)?

If we do this by hand, for the first question we just need to hit enter, and for the second, type y and press enter.

We can simulate these actions by using the following here-string:

$' y'

From the bash man page:

Words of the form $'string' are treated specially. The word expands to "string", with backslash-escaped characters replaced as specified by the ANSI C standard.

new line

So, if we use od to analyze our string:

cat - <<< $'
y' | od -c
0000000  
   y  

We see that we're getting just what we need to answer the questions.

Points 1 and 2 can be summarized into:

ssh-keygen -q -t rsa  <<< $'
y'

And the final command will be:

$ ssh-keygen -q -t rsa -N '' <<< $'
y' >/dev/null 2>&1
$ echo $?
0

---

Kudos

@lukasz-dynowski, @redochka, @mellow-yellow, @yeti and the rest of the folks in this thread.