How to escape quotes in shell within both usage of ssh and sudo?

Question

In one word:

question and example could test locally:

sh -c "echo 'how to print single quote here'"

details:

I have a config like this:

upload_server = ('192.168.1.1', 10051)

now I need a shell script to replace server config. Locally I can do it with sed:

sed -E 's/(    upload_server = ).*/\\1('\"'$a'\"', 10051)/g' config.py

but there's huge amount of machines have to change the config.

a proper plan is to use ssh with for..in:

for i in `cat hosts`; do ssh $i "cmd"; done

but to change config also need a sudo, it looks like this:

for i in `cat hosts`; do ssh $i "sudo -c \"cmd\""; done

and now within sed:

upload_server=123.123.123.123
upload_server_port=1000
cmd="sed -E 's/(    upload_server = ).*/\\1('\"\"'\"'${upload_server}'\"'\"\"', ${upload_server_port})/g' config.py"
ssh $i "sudo -c \"${cmd}\""

the result of config:

upload_server = (123.123.123.123, 1000)

which should be like this:

upload_server = ('123.123.123.123', 1000)

Now I'm confused a lot in quotes usage. :(

Answer 1

In this case, you can bypass the quoting issue entirely by passing the sed script on standard input. SSH routes the local standard input to the remote process, sudo just leaves it open, and sed can read its script on stdin. You can pass content for stdin with a here document.

for i in `cat hosts`; do
  ssh "$i" 'sudo sed -f - …' <<'EOF'
s/(    upload_server = ).*/\1("$a", 10051)/g
EOF
done

More generally, you can pass a shell snippet on stdin through SSH without worrying about quoting, as long as you don't need to also pass data on stdin.

for i in `cat hosts`; do
  ssh "$i" sudo sh <<'EOF'
sed 's/(    upload_server = ).*/\1("$a", 10051)/g' …
EOF
done

It isn't clear from your question whether you intended to insert "$a" into the configuration file, or "something" where the value of the variable a in the local shell is something. If you want to refer to the local variable, then use a here document with interpolation, and protect $\` in the here document with backslashes. Note that due to the parsing by sed, and by the remote shell in the second snippet, the value of the variable must not contain any of the characters \/' or a newline.

  ssh "$i" sudo sh <<EOF
sed 's/(    upload_server = ).*/\\1("$a", 10051)/g' …
EOF

Answer 2

$ ssh localhost sh -c \"echo \\\'how to print single quotes here\\\'\"
'how to print single quotes here'

$ ssh localhost echo \\\'how to print single quotes here\\\'
'how to print single quotes here'

To understand the rules, remember that one level of quoting is stripped by the local shell, then SSH invokes a remote shell which strips a second level of quoting.