How to escape $ in sed over ssh command

dollar-signescapingsedssh

I am trying to create a patch that users can use to remotely edit a file in a pre-defined way using sed, and I could do this manually on each computer, but it would take a long time.

The line I am struggling with is as follows:

host=[hostname]
port=[portnum]
ssh -t $host -p $port "cp ~/file1 ~/file1.bak ; sed -i \"s/fcn1('param1', $2)\n/fcn2('param2'):$zoom\n/g\" ~/file1"

This makes a backup of file1 and then edits a line in the file. I actually want to edit more than one line, but this line demonstrates the problems:
The command works, provided no $ signs are used within the sed command.

I have tried a number of ways of escaping these $ signs but cannot seem to find one that works.
I can use a . wildcard in the find, but obviously not in the replace string.

I would use single quotes for the sed command, in order to avoid expanding the $2, but single quotes are already used inside the command.

Does anyone have any ideas of how to overcome this problem? Thanks in advance for any suggestions!

Best Answer

This should work as well:

ssh -t $host -p $port "cp ~/file1 ~/file1.bak && sed -i \"s/fcn1('param1', \\\$2)/fcn2('param2'):\\\$zoom/g\" file1"

You need 3 back slashes as you have to escape the $ sign in the string passed in the remote bash to sed. And you have to escape that back slash and the $ sign when sending it over via ssh.

Use `\r` instead of `\n`.

Substituting by \n inserts a null character into the text. To get a newline, use \r. When searching for a newline, you’d still use \n, however. This asymmetry is due to the fact that \n and \r do slightly different things:

\n matches an end of line (newline), whereas \r matches a carriage return. On the other hand, in substitutions \n inserts a null character whereas \r inserts a newline (more precisely, it’s treated as the input CR). Here’s a small, non-interactive example to illustrate this, using the Vim command line feature (in other words, you can copy and paste the following into a terminal to run it). xxd shows a hexdump of the resulting file.

echo bar > test
(echo 'Before:'; xxd test) > output.txt
vim test '+s/b/\n/' '+s/a/\r/' +wq
(echo 'After:'; xxd test) >> output.txt
more output.txt

Before:
0000000: 6261 720a                                bar.
After:
0000000: 000a 720a                                ..r.

In other words, \n has inserted the byte 0x00 into the text; \r has inserted the byte 0x0a.

Unix – How to remove the passphrase for the SSH key without having to create a new key

Short answer:

$ ssh-keygen -p

This will then prompt you to enter the keyfile location, the old passphrase, and the new passphrase (which can be left blank to have no passphrase).

If you would like to do it all on one line without prompts do:

$ ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]

Important: Beware that when executing commands they will typically be logged in your ~/.bash_history file (or similar) in plain text including all arguments provided (i.e. the passphrases in this case). It is, therefore, is recommended that you use the first option unless you have a specific reason to do otherwise.

Notice though that you can still use -f keyfile without having to specify -P nor -N, and that the keyfile defaults to ~/.ssh/id_rsa, so in many cases, it's not even needed.

You might want to consider using ssh-agent, which can cache the passphrase for a time. The latest versions of gpg-agent also support the protocol that is used by ssh-agent.

Best Answer

Related Solutions

How to replace a character by a newline in Vim

Use \r instead of \n.

Unix – How to remove the passphrase for the SSH key without having to create a new key

Related Topic

Use `\r` instead of `\n`.