将转义符添加到字符串中的特殊字符（作为函数）

Question

So I what I am looking to find working is something like this:

$ echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\?
This is a special character string. \~`!@#$%^&*()-=+[]{}|;:'",<>/?

I want to be able to add backslashes to only the special characters. I have seen examples around here that do indeed work, however I can't seem to get them working as a pipable function.

Here's what I would like to have it look like:

$ echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\? | addESC
This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\?

$ echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\? | addESC | addESC
This\\\ is\\\ a\\\ special\\\ character\\\ string.\\\ \\\\\\~\\\`\\\!\\\@\\\#\\\$\\\%\\\^\\\&\\\*\\\(\\\)\\\-\\\=\\\+\\\[\\\]\\\{\\\}\\\|\\\;\\\:\\\'\\\"\\\,\\\<\\\>\\\/\\\?

Using bash on High Sierra.

Edit as per Andreas' request

Example of escaping characters I've used that I confirm to work, just not in a function: https://unix.stackexchange.com/questions/141312/how-to-escape-special-characters-in-a-string

I have tried:

$ echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\? | printf '%q\n'

$ printf '%q\n' <<< printf '%q\n' <<< echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\?

$ echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\? | xargs printf '%q\n'

Answer 1

Before I attempt to answer, I have a some warnings. I'm not sure what the actual goal is here, so depending on what that is, there are several potential problems.

First, it's impossible in general to reconstruct how a string was quoted/escaped on the command line, because there are many different ways to express the same string in shell syntax. For example, all of the following commands pass exactly the same argument to echo , and therefore print exactly the same thing:

echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\?
echo This\ is\ a\ special\ character\ string.\ \\~\`\!@#\$%^\&*\(\)-=+[]{}\|\;:\'\",\<\>/?
echo 'This is a special character string. \~`!@#$%^&*()-=+[]{}|;:'"'"'",<>/?'
echo $'This is a special character string. \~`!@#$%^&*()-=+[]{}|;:\'",<>/?'
...and many more

(Note: technically, the second of those might pass something different, if the current directory happens to contain one or more files with specific really weird names.)

Second, some versions of echo do some additional escape processing. Some do this only when passed the -e option. Some print "-e" if you try to pass the -e option. It's a mess.

Third, what needs to be escaped and how really depends on what you're going to use it for (and specifically, how it's going to be parsed). Different situations involve different parsing rules, and you have to add escapes appropriate for the specific processing that the output is going to be subject to. In my answer, I concentrated on reversing the specific escaping in your example.

My solution: you can use sed in a pipeline to add escapes before any of a list of characters, specified as a bracket expression. It's slightly tricky because "]" and "-" are delimiters in a bracket expression; the trick there is to specify "]" as the first character and "-" as the last, so they're not mistaken for their other meanings. Also, I'm going to write this as a single-quoted string, so the single-quote requires special handing. Like this:

sed 's/[][ \~`!@#$%^&*()=+{}|;:'"'"'",<>/?-]/\\&/g'

Or as a function:

addESC() { sed 's/[][ \~`!@#$%^&*()=+{}|;:'"'"'",<>/?-]/\\&/g'; }

Example:

$ echo This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\? | addESC 
This\ is\ a\ special\ character\ string.\ \\\~\`\!\@\#\$\%\^\&\*\(\)\-\=\+\[\]\{\}\|\;\:\'\"\,\<\>\/\?

As for why your attempts didn't work: In the first, printf doesn't read from stdin, it expects arguments. The third tries to fix this with xargs , but xargs does its own quote/escape parsing and removal, which messes it up. In the second, <<< takes a string , not a command ; to apply it to the output of a command, you'd use something like <<< "$(command)" . Also, in all versions, bash printf's %q quotes and/or escapes specifically as needed for consumption by bash itself, which doesn't match the escaping in your example.