how to use a literal # in a vim string?

Question

I am trying to define a string in vimscript containing a literal # which then gets passed to a shell command using execute.

function! g:Call_imagemagick_with_hex(hex_color)
    let a:display_command = "! display -size 300x300 xc: '\#" . a:hex_color . "'"
    execute a:display_command
    "echo(a:display_command)
endfunction

and I would call this function like

:call Call_imagemagick_with_hex('34495e')

what I want the shell to see via execute' is:

display -size 300x300 xc: '#34495e'

The problem is that if # appears unescaped in the string passed to execute, vim tries to expand it - it seems # is like % but represents the previous buffer. e.g. If I use

let a:display_command = "! display -size 300x300 xc: '#" . a:hex_color . "'"

vim tries to expand # and throws an error. If I use

let a:display_command = "! display -size 300x300 xc: '\#" . a:hex_color . "'"

the shell sees:

display -size 300x300 xc: '\#34495e'

and the shell command doesn't work. From learning vimscript the hardway

Using single quotes tells Vim that you want the string exactly as-is, with no escape sequences. The one exception is that two single quotes in a row will produce one single quote.

It seems if ' does protects against meta-character expansion, it's possibly only in certain contexts.
Otherwise, how can I pass a # character to an underlying shell command?

Answer 1

You should use system instead of execute ! :

call system("display -size 300x300 xc: '#" . a:hex_color . "'")

You can use the return value in a variable if you want, e.g.:

let l:files = system('ls')

In this case you can prefer using the systemlist() that returns a list:

let l:files = systemlist('ls')

If you need to use special vim char in this context you can always use expand.

Reference: :h system()