Shell – bash adds extra single quotes

quotingshellssh

I have a problem executing my script.
When executing it in debug mode (bash -x), I can see that bash is adding extra quotes.
Therefor my script is failing.

Here this is within my script:

testvar="\"sudo /home/pi/shared/blink.sh 27 off\""
ssh -n -q -q -o BatchMode=yes -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o ConnectTimeout=5 $1 ${testvar}

This is the output:

ssh -n -q -q -o BatchMode=yes -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o ConnectTimeout=5 192.168.42.105 '"sudo' /home/pi/shared/blink.sh 27 'off"'

Best Answer

Bash is displaying single quotes so as to show a command that is valid input syntax. It is not running a command which contains these single quotes in a parameter to the ssh command.

ssh … '"sudo' /home/pi/shared/blink.sh 27 'off"'

tells you that the last 4 parameters of the ssh command are "sudo, /home/pi/shared/blink.sh, 27 and off".

On the remote host, the ssh daemon joins the words of the commands with spaces as separators, so the remote command that you are executing is

"sudo /home/pi/shared/blink.sh 27 off"

This attempts to execute a command whose name is sudo /home/pi/shared/blink.sh 27 off, which of course doesn't exist.

Remove the double quotes from your definition of testvar.

It doesn't matter here, but it probably matters in your real case: instead of ${testvar}, write "$testvar" (or "${testvar}" if you want but the braces are optional). Always put double quotes around variable substitutions unless you know why you need to leave them out. "$testvar" expands to the value of the variable testvar, whereas $testvar when not in double quotes treats the value of testvar as a whitespace-separated list of glob patterns.

Related Solutions

SSH Sudo Expect – Run Command with Sudo on Remote Machine

I think you missed a level of quotes escaping. At this high level of escaping, it is best to simply make a little script for every stage where otherwise quotes would be required.

Otherwise, you could try this modified version (but mind you, I don't encourage this coding style!)

function _remoteInstallation(){
    local retval=1
    local debToInstall=$(basename "$1")
    local remoteMachine="$2"
    spawned=$(expect -d -c "
          set timeout 1800
          spawn \"/usr/bin/ssh -t borrajax@$remoteMachine /usr/bin/sudo /usr/bin/dpkg -i /home/borrajax/Documents/$debToInstall\"
          expect {
                \"Are you sure you want to continue connecting\" { send \"yes\r\"; exp_continue }
                \"password\" { send \"myPassword\r\";  exp_continue }
                \"[sudo] password\" { send \"myPassword\r\";  exp_continue }
                default { exit 1 }
          }
    " )
    retval=$?
    return $retval
}

Bash – In the code “{ exec >/dev/null; } >/dev/null” what is happening under the hood

Let's follow

{ exec >/dev/null; } >/dev/null; echo "Hi"

step by step.

There are two commands:

a. { exec >/dev/null; } >/dev/null, followed by

b. echo "Hi"

The shell executes first the command (a) and then the command (b).
The execution of { exec >/dev/null; } >/dev/null proceeds as follows:

a. First, the shell perform the redirection >/dev/null and remembers to undo it when the command ends.

b. Then, the shell executes { exec >/dev/null; }.

c. Finally, the shell switches standard output back to where is was. (This is the same mechanism as in ls -lR /usr/share/fonts >~/FontList.txt -- redirections are made only for the duration of the command to which they belong.)
Once the first command is done the shell executes echo "Hi". Standard output is wherever it was before the first command.

Best Answer

Related Solutions

SSH Sudo Expect – Run Command with Sudo on Remote Machine

Bash – In the code “{ exec >/dev/null; } >/dev/null” what is happening under the hood

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