What does the "=~" operator do in shell scripts?

Question

It seems that it is sort of comparison operator, but what exactly it does in e.g. the following code (taken from https://github.com/lvv/git-prompt/blob/master/git-prompt.sh#L154)?

    if [[ $LC_CTYPE =~ "UTF" && $TERM != "linux" ]];  then
            elipses_marker="…"
    else
            elipses_marker="..."
    fi

I'm currently trying to make git-prompt to work under MinGW, and the shell supplied with MinGW doesn't seem to support this operator:

conditional binary operator expected
syntax error near `=~'
`        if [[ $LC_CTYPE =~ "UTF" && $TERM != "linux" ]];  then'

In this specific case I can just replace the entire block with elipses_marker="…" (as I know my terminal supports unicode), but what exactly this =~ does?

Answer 1

It's a bash-only addition to the built-in [[ command, performing regexp matching. Since it doesn't have to be an exact match of the full string, the symbol is waved, to indicate an "inexact" match.

In this case, if $LC_CTYPE CONTAINS the string "UTF".

More portable version:

if test `echo $LC_CTYPE | grep -c UTF` -ne 0 -a "$TERM" != "linux"
then
  ...
else
  ...
fi

Answer 2

It's a regular expression matching. I guess your bash version doesn't support that yet.

In this particular case, I'd suggest replacing it with simpler (and faster) pattern matching:

[[ $LC_CTYPE == *UTF* && $TERM != "linux" ]]

(note that * must not be quoted here)