How to escape single quotes within single quoted strings

Question

Let's say, you have a Bash alias like:

alias rxvt='urxvt' 

which works fine.

However:

alias rxvt='urxvt -fg '#111111' -bg '#111111'' 

won't work, and neither will:

alias rxvt='urxvt -fg \'#111111\' -bg \'#111111\'' 

So how do you end up matching up opening and closing quotes inside a string once you have escaped quotes?

alias rxvt='urxvt -fg'\''#111111'\'' -bg '\''#111111'\'' 

seems ungainly although it would represent the same string if you're allowed to concatenate them like that.

Answer 1

If you really want to use single quotes in the outermost layer, remember that you can glue both kinds of quotation. Example:

 alias rxvt='urxvt -fg '"'"'#111111'"'"' -bg '"'"'#111111'"'"  #                     ^^^^^       ^^^^^     ^^^^^       ^^^^  #                     12345       12345     12345       1234 

Explanation of how '"'"' is interpreted as just ':

1. ' End first quotation which uses single quotes.
2. " Start second quotation, using double-quotes.
3. ' Quoted character.
4. " End second quotation, using double-quotes.
5. ' Start third quotation, using single quotes.

If you do not place any whitespaces between (1) and (2), or between (4) and (5), the shell will interpret that string as a one long word.

Answer 2

I always just replace each embedded single quote with the sequence: '\'' (that is: quote backslash quote quote) which closes the string, appends an escaped single quote and reopens the string.

---

I often whip up a "quotify" function in my Perl scripts to do this for me. The steps would be:

s/'/'\\''/g    # Handle each embedded quote $_ = qq['$_']; # Surround result with single quotes. 

This pretty much takes care of all cases.

Life gets more fun when you introduce eval into your shell-scripts. You essentially have to re-quotify everything again!

For example, create a Perl script called quotify containing the above statements:

#!/usr/bin/perl -pl s/'/'\\''/g; $_ = qq['$_']; 

then use it to generate a correctly-quoted string:

$ quotify urxvt -fg '#111111' -bg '#111111' 

result:

'urxvt -fg '\''#111111'\'' -bg '\''#111111'\''' 

which can then be copy/pasted into the alias command:

alias rxvt='urxvt -fg '\''#111111'\'' -bg '\''#111111'\''' 

(If you need to insert the command into an eval, run the quotify again:

 $ quotify  alias rxvt='urxvt -fg '\''#111111'\'' -bg '\''#111111'\''' 

result:

'alias rxvt='\''urxvt -fg '\''\'\'''\''#111111'\''\'\'''\'' -bg '\''\'\'''\''#111111'\''\'\'''\'''\''' 

which can be copy/pasted into an eval:

eval 'alias rxvt='\''urxvt -fg '\''\'\'''\''#111111'\''\'\'''\'' -bg '\''\'\'''\''#111111'\''\'\'''\'''\'''