Shell.escape arg quotes a string (if necessary) to avoid interpretation of characters which are special to the shell.
Shell.escape is used internally by jenga when displaying the "+" output lines, which show what commands are run; seen by the user when running jenga -verbose or if the command results in a non-zero exit code. It is exposed in the API since it is useful when constructing actions such as: Action.progress ~dir ~prog:"bash" ~args, which should ensure args are suitably quoted.
Examples: (1) escape "hello" -> "hello" (2) escape "foo bar" -> "'foo bar'" (3) escape "foo'bar" -> "'foo'\\''bar'"
Note the arg and result strings in the above examples are expressed using ocaml string literal syntax; in particular, there is only single backslash in the result of example 3.
Example (1): No quoting is necessary. Example (2): simple single quoting is used, since arg contains a space, which is special to the shell. Example (3): the result is single quoted; the embedded single quote is handled by: un-quoting, quoting using a bashslash, then re-quoting.