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### 36.2 Shell Arguments

Lisp programs sometimes need to run a shell and give it a command that contains file names that were specified by the user. These programs ought to be able to support any valid file name. But the shell gives special treatment to certain characters, and if these characters occur in the file name, they will confuse the shell. To handle these characters, use the function `shell-quote-argument`:

Function: shell-quote-argument argument

  This function returns a string that represents, in shell syntax, an argument whose actual contents are argument. It should work reliably to concatenate the return value into a shell command and then pass it to a shell for execution.

  Precisely what this function does depends on your operating system. The function is designed to work with the syntax of your system’s standard shell; if you use an unusual shell, you will need to redefine this function. See Security Considerations.

      ;; This example shows the behavior on GNU and Unix systems.
      (shell-quote-argument "foo > bar")
           ⇒ "foo\\ \\>\\ bar"

      ;; This example shows the behavior on MS-DOS and MS-Windows.
      (shell-quote-argument "foo > bar")
           ⇒ "\"foo > bar\""

  Here’s an example of using `shell-quote-argument` to construct a shell command:

      (concat "diff -u "
              (shell-quote-argument oldfile)
              " "
              (shell-quote-argument newfile))

The following two functions are useful for combining a list of individual command-line argument strings into a single string, and taking a string apart into a list of individual command-line arguments. These functions are mainly intended for converting user input in the minibuffer, a Lisp string, into a list of string arguments to be passed to `make-process`, `call-process` or `start-process`, or for converting such lists of arguments into a single Lisp string to be presented in the minibuffer or echo area. Note that if a shell is involved (e.g., if using `call-process-shell-command`), arguments should still be protected by `shell-quote-argument`; `combine-and-quote-strings` is not intended to protect special characters from shell evaluation.

Function: split-string-and-unquote string &optional separators

  This function splits string into substrings at matches for the regular expression separators, like `split-string` does (see Creating Strings); in addition, it removes quoting from the substrings. It then makes a list of the substrings and returns it.

  If separators is omitted or `nil`, it defaults to `"\\s-+"`, which is a regular expression that matches one or more characters with whitespace syntax (see Syntax Class Table).

  This function supports two types of quoting: enclosing a whole string in double quotes `"…"`, and quoting individual characters with a backslash escape ‘`\`’. The latter is also used in Lisp strings, so this function can handle those as well.

Function: combine-and-quote-strings list-of-strings &optional separator

  This function concatenates list-of-strings into a single string, quoting each string as necessary. It also sticks the separator string between each pair of strings; if separator is omitted or `nil`, it defaults to `" "`. The return value is the resulting string.

  The strings in list-of-strings that need quoting are those that include separator as their substring. Quoting a string encloses it in double quotes `"…"`. In the simplest case, if you are consing a command from the individual command-line arguments, every argument that includes embedded blanks will be quoted.

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