Exactly how do backslashes work within backticks?

by

The logic is quite simple as such. So we look at bash source code (4.4) itself

subst.c:9273

case '`': /* Backquoted command substitution. */
{
    t_index = sindex++;

    temp = string_extract(string, &sindex, "`", SX_REQMATCH);
    /* The test of sindex against t_index is to allow bare instances of
        ` to pass through, for backwards compatibility. */
    if (temp == &extract_string_error || temp == &extract_string_fatal)
    {
    if (sindex - 1 == t_index)
    {
        sindex = t_index;
        goto add_character;
    }
    last_command_exit_value = EXECUTION_FAILURE;
    report_error(_("bad substitution: no closing \"`\" in %s"), string + t_index);
    free(string);
    free(istring);
    return ((temp == &extract_string_error) ? &expand_word_error
                                            : &expand_word_fatal);
    }

    if (expanded_something)
    *expanded_something = 1;

    if (word->flags & W_NOCOMSUB)
    /* sindex + 1 because string[sindex] == '`' */
    temp1 = substring(string, t_index, sindex + 1);
    else
    {
    de_backslash(temp);
    tword = command_substitute(temp, quoted);
    temp1 = tword ? tword->word : (char *)NULL;
    if (tword)
        dispose_word_desc(tword);
    }
    FREE(temp);
    temp = temp1;
    goto dollar_add_string;
}

As you can see calls a function de_backslash(temp); on the string which updates the string in c. The code the same function is below

subst.c:1607

/* Remove backslashes which are quoting backquotes from STRING.  Modifies
   STRING, and returns a pointer to it. */
char *
    de_backslash(string) char *string;
{
  register size_t slen;
  register int i, j, prev_i;
  DECLARE_MBSTATE;

  slen = strlen(string);
  i = j = 0;

  /* Loop copying string[i] to string[j], i >= j. */
  while (i < slen)
  {
    if (string[i] == '\\' && (string[i + 1] == '`' || string[i + 1] == '\\' ||
                              string[i + 1] == '$'))
      i++;
    prev_i = i;
    ADVANCE_CHAR(string, slen, i);
    if (j < prev_i)
      do
        string[j++] = string[prev_i++];
      while (prev_i < i);
    else
      j = i;
  }
  string[j] = '\0';

  return (string);
}

The above just does simple thing if there is \ character and the next character is \ or backtick or $, then skip this \ character and copy the next character

So if convert it to python for simplicity

text = r"\\\\$a"

slen = len(text)
i = 0
j = 0
data = ""
while i < slen:
    if (text[i] == '\\' and (text[i + 1] == '`' or text[i + 1] == '\\' or
                             text[i + 1] == '$')):
        i += 1
    data += text[i]
    i += 1

print(data)

The output of the same is \\$a. And now lets test the same in bash

$ a=xxx

$ echo "$(echo \\$a)"
\xxx

$ echo "`echo \\\\$a`"
\xxx

More Related Contents:

  1. What is the benefit of using $() instead of backticks in shell scripts? [duplicate]
  2. Which characters need to be escaped when using Bash?
  3. How to execute a bash command stored as a string with quotes and asterisk [duplicate]
  4. Batch equivalent of Bash backticks
  5. How to avoid heredoc expanding variables? [duplicate]
  6. How do I escape the wildcard/asterisk character in bash?
  7. Variable containing multiple args with quotes in Bash
  8. How do I pass in the asterisk character ‘*’ in bash as arguments to my C program?
  9. Is there an easy way to pass a “raw” string to grep?
  10. Bash script to cd to directory with spaces in pathname
  11. Command to escape a string in bash
  12. How do I properly escape data for a Makefile?
  13. How to match a single quote in sed
  14. How to echo a variable containing an unescaped dollar sign in bash
  15. escaping newlines in sed replacement string
  16. Make Bash handle variables like Batch
  17. Some troubles with using sed and awk [closed]
  18. Add a new element to an array without specifying the index in Bash
  19. Lookup shell variables by name, indirectly [duplicate]
  20. How do I use su to execute the rest of the bash script as that user?
  21. Why start a shell command with a backslash?
  22. Find and replace filename recursively in a directory
  23. How can bash script do the equivalent of Ctrl-C to a background task?
  24. How does bash tab completion work?
  25. How to resolve symbolic links in a shell script
  26. Forward function declarations in a Bash or a Shell script?
  27. Expand variables in sed
  28. Looping through find output in Bash where file name contains white spaces
  29. How to include nohup inside a bash script?
  30. What is the proper way to test a Bash function’s return value?

Leave a Comment