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Reference

Magic Aliases: A Layering Loophole in the Bourne Shell (2003, by Simon Tatham)

(Simon Tatham is the author of PuTTY!)

mquote (magic quote) - No more quoting headaches in Bash

Usage

mquote [-dnpQv] ...

  -d    Use "declare -p" for quoting.
  -n    No run. Only print the command after quoting without executing it.
        This is implicit if there is no ``..`` in the argument.
  -p    Use "printf %q" for quoting.
  -Q    Use "${var@Q}" for quoting. This requires Bash 4.4+.
  -v    Verbose. Print the command before execution.

Note

  • Bash 3.2+ is required.
  • The mquote alias only works in interactive Bash shell and command history must be enabled (set -o history).
  • In the following examples which use ssh, we assume the remote shell is also Bash.

Examples

Example 1: Quote a whole command

Say we have this demo command:

$ echo '111"222' | awk -F\" '{ print $1 + $2 }'
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How should the command be quoted if we want to define it as an alias? Just pass the whole command literally to mquote:

$ mquote echo '111"222' | awk -F\" '{ print $1 + $2 }'
>>> echo" '111\"222' | awk -F\\\" '{ print \$1 + \$2 }'"

Then, copy-n-paste the output to the alias command:

$ alias foo=echo" '111\"222' | awk -F\\\" '{ print \$1 + \$2 }'"
$ foo
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Or you can copy-n-paste the mquote output to a var assignment:

$ cmd=echo" '111\"222' | awk -F\\\" '{ print \$1 + \$2 }'"
$ eval "$cmd"
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$ ssh 127.0.0.1 "$cmd"
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Example 2: Pass a verbatim command through ssh

Still using this example:

$ echo '111"222' | awk -F\" '{ print $1 + $2 }'
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To run it over ssh, we can ask mquote to automatically quote the command by using ``..``:

$ mquote ssh 127.0.0.1 ``echo '111"222' | awk -F\" '{ print $1 + $2 }'``
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That's to say, mquote will automtically quote the part between ``..`` on the fly.

With -v (verbose) we can see the real command after quoting:

$ mquote -v ssh 127.0.0.1 ``echo '111"222' | awk -F\" '{ print $1 + $2 }'``
>>> ssh 127.0.0.1 echo\ \'111\"222\'' | awk -F\" '\''{ print $1 + $2 }'\'
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With -n (no run) it'll only print the command without executing it:

$ mquote -n ssh 127.0.0.1 ``echo '111"222' | awk -F\" '{ print $1 + $2 }'``
>>> ssh 127.0.0.1 echo" '111\"222' | awk -F\\\" '{ print \$1 + \$2 }'"
$ mquote -n -d ssh 127.0.0.1 ``echo '111"222' | awk -F\" '{ print $1 + $2 }'``
>>> ssh 127.0.0.1 "echo '111\"222' | awk -F\\\" '{ print \$1 + \$2 }'"
$ mquote -n -p ssh 127.0.0.1 ``echo '111"222' | awk -F\" '{ print $1 + $2 }'``
>>> ssh 127.0.0.1 echo\ \'111\"222\'\ \|\ awk\ -F\\\"\ \'\{\ print\ \$1\ +\ \$2\ \}\'
$ mquote -n -Q ssh 127.0.0.1 ``echo '111"222' | awk -F\" '{ print $1 + $2 }'``
>>> ssh 127.0.0.1 'echo '\''111"222'\'' | awk -F\" '\''{ print $1 + $2 }'\'''

Example 3: Pass a verbatim command through nested ssh

$ echo '111"222' | awk -F\" '{ print $1 + $2 }'
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To run the above command with 2 level nested ssh (ssh host1 ssh host2 ...), we can use the ``{2}..`` syntax:

$ mquote -v ssh 127.0.0.1 ssh 127.0.0.1 ``{2}echo '111"222' | awk -F\" '{ print $1 + $2 }'``
>>> ssh 127.0.0.1 ssh 127.0.0.1 echo'" '\'111'\"222'\'' | awk -F\\\" '\''{ print \$1 + \$2 }'\'\"
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For more levels quoting, use ``{3}..``, ``{4}..``, ...

$ mquote ssh 127.0.0.1 ssh 127.0.0.1 ssh 127.0.0.1 ``{3}echo '111"222' | awk -F\" '{ print $1 + $2 }'``
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And ``{1}..`` is the same as ``..``.

Example 4: Quote parameters for commands

Say we want to write an echo command which outputs " ' # $( < > \ | ). Just enclose it with ``..``:

$ mquote -v echo ``" ' # $( < > \ | )``
>>> echo '" '\'' # $( < > \ | )'
" ' # $( < > \ | )

Over ssh:

$ mquote ssh 127.0.0.1 echo ``{2}" ' # $( < > \ | )``
" ' # $( < > \ | )
$ mquote ssh 127.0.0.1 ssh 127.0.0.1 echo ``{3}" ' # $( < > \ | )``
" ' # $( < > \ | )

Example 5: Multiple ``..`` in one command

$ mquote echo foo ``$( < > )`` bar ``" ' \ |``
foo $( < > ) bar " ' \ |
$ mquote ssh 127.0.0.1 echo foo ``{2}$( < > )`` bar ``{2}" ' \ |``
foo $( < > ) bar " ' \ |

Example 6: It's never been so easy to write one-liners!

People often write sed/awk/perl/... one-liners and it's error-prone when quoting the script part (usually -e ... or -c ...) in shell. With mquote, you can focus on the util/language's script itself without worrying about shell quoting.

$ mquote echo | sed -e ``s/.*/"'/``
"'
$ mquote awk ``BEGIN { print "\"" "'" }`` /dev/null
"'
$ mquote sh -c ``echo "\"'"``
"'
$ mquote python3 -c ``print('"\'')``
"'
$ mquote perl -e ``print '"', "'", "\n"``
"'

set-x

Quite often I need to turn on set -x to debug something:

$ set -x
$ do something
$ set +x

# or

$ set -x; do something; set +x

With set-x you can just set-x do something. For example:

$ sum=0; for ((i=1; i<=5; ++i)); do ((sum += i)); done; echo $sum
15
$ set-x sum=0; for ((i=1; i<=5; ++i)); do ((sum += i)); done; echo $sum
+ eval 'sum=0; for ((i=1; i<=5; ++i)); do ((sum += i)); done; echo $sum'
++ sum=0
++ (( i=1 ))
++ (( i<=5 ))
++ (( sum += i ))
++ (( ++i ))
++ (( i<=5 ))
++ (( sum += i ))
++ (( ++i ))
++ (( i<=5 ))
++ (( sum += i ))
++ (( ++i ))
++ (( i<=5 ))
++ (( sum += i ))
++ (( ++i ))
++ (( i<=5 ))
++ (( sum += i ))
++ (( ++i ))
++ (( i<=5 ))
++ echo 15
15
+ set 0
+ set +x