what does the number 1 in the shellescape function mean in vim?

Question

A vim command confuses me. I have read and re-read :help shellescape() several times. I still don't understand the meaning of the number 1 in shellescape(expand('%:p'), 1).

Here's the full command I'm trying to understand:

:nnoremap <F4> :exe ':silent  !"c:\Program Files\Mozilla Firefox\firefox.exe"'shellescape(expand('%:p'), 1)<CR>   

Let's break down this long command piece by piece.

• the command is to map an exe command into F4 in the whole.
• :exe is to execute some command.
• :silent ! is to execute a shell command silently
• "c:\Program Files\Mozilla Firefox\firefox.exe" can call my firefox program.
• shellescape(), there are blanks to be escaped.
• expand('%:p') can get current file name in full expression that is path + filename.

What does this excerpt from the help page mean?

With a |non-zero-arg| {special} and 'shell' containing "csh" in the tail it's
escaped a second time.

Is there some meaning such the same as 1 or 2 in s/(ha.*)(world)/\2\1/g?
please take a simple example in detail.

And i have two questions related to the topic.
1.In which way i can get how many type of shells in my gvim?
2.In which situation can i change 1 into 2 in shellescape()?

:nnoremap <F4> :exe ':silent  !"c:\Program Files\Mozilla Firefox\firefox.exe"'shellescape(expand('%:p'), 2)<CR> 

Answer 1

There are basically two different uses for shellescape(); one is the Vimscript system() function, the other the :! Ex command. While most escaping needs are identical in both uses (e.g. to deal with whitespace, arguments must be enclosed in quotes), the :! command requires some additional escaping, because on the command-line, Vim assigns special meaning to symbols like % (replacing it with the current file name). This does not happen for system().

Therefore, to tell shellescape() which escaping mode to use, it has the additional {special} argument; if you pass 1 (or any other value that evaluates to "true"), the additional characters are escaped, too.

TL;DR: Pass 1 for :! commands, 0 or omit the argument for use with system().