The trouble you're having is with the backslashes inside the string (as @Matt correctly pointed out in the comments.)
Vim strings (using double-quotes as delimiters) interpret the backslash as a special character, so you need to escape them with an additional backslash if you want to use them in a double-quoted string.
let s:os = system("sed -n 's/^NAME=\\(.*\\)/\\1/p' /etc/os-release")
Alternatively, you might want to use single-quoted strings in Vim, since those do not treat the backslash as special and don't require escaping it. This is pretty fine in this particular case, since using double-quotes for the sed expression in the shell command is OK, since they do not contain any shell metacharacters such as
let s:os = system('sed -n "s/^NAME=\(.*\)/\1/p" /etc/os-release')
On the other hand, you could actually do that same operation using Vimscript primitives only, which should be more efficient (since you're not spawning a couple of external processes anymore) and more portable (since it doesn't depend on which particular shell you're using or whether a
sed can even be found in
Consider using the following snippet instead:
let s:os = matchlist(readfile('/etc/os-release'), '^NAME=\(.*\)')
:help readfile() and
You might want to add some error handling, in case the file doesn't exist or the variable match can't be found there.