How can I add a backslash before all spaces?

Solution 1:

tr can't do multiple characters. Use one of these instead:

1. sed

   echo "$line" | sed 's/ /\\ /g' 
   

   or

   sed 's/ /\\ /g' <<< "$line"
   

2. Perl

   echo "$line" | perl -pe 's/ /\\ /g'  
   

   or

   perl -pe 's/ /\\ /g'<<< "$line"
   

   Perl also has a nifty function called quotemeta which can escape all odd things in a string:

   line='@!#$%^&*() _+"'
   perl -ne 'print quotemeta($_)' <<< $line
   

   The above will print

   \@\!\#\$\%\^\&\*\(\)\ _\+\"\
   

3. You can also use printf and %q:

   %q  quote the argument in a way that can be reused as shell input
   

   So, you could do

   echo "$line" | printf "%q\n" 
   

   Note that this, like Perl's quotemeta will escape all special characters, not just spaces.

   printf "%q\n" <<<$line
   

4. If you have the line in a variable, you could just do it directly in bash:

   echo ${line// /\\ }
   

Solution 2:

There is AWK missing in the list of all the possible solutions :)

$ echo "Hello World" | awk '{gsub(/ /,"\\ ");print}'                                                                             
Hello\ World