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echo $GOPATH will print:

/mnt/star/program/go/package:/mnt/star/git_repository/workspace/go_workplace

There are 2 directories, I want to append the first directory's sub-directory bin/ to $PATH.

If I write $PATH=$PATH:$GOPATH/bin, then actually it appends 2 directories to $PATH:

• 
  /mnt/star/program/go/package
  
  This only contains directories, it should be /mnt/star/program/go/package/bin.


• 
  /mnt/star/git_repository/workspace/go_workplace/bin
  
  This actually shouldn't be added to $PATH.

BTW, there are cases that $GOPATH only contains one directory, so that simply appending $GOPATH/bin will work.

I am looking for a solution that fits both cases. So, how do I write this in a bash config file?

You can use:

PATH="$PATH:$GOPATH%%:*/bin"

Or

PATH="$PATH:$GOPATH%:*/bin"

Both will work because there can be at most one :.

It will remove the part after :. So, in your first case, it will remove the second directory and in your second case, there will be no pattern like :*, so there will be no change in the directory name.

PATH="$PATH:$GOPATH%%:*/bin"

The asterisk is a glob, not a regular expression. Two percentage signs means to remove the maximum that it can match from the back, so even if there were three or more directories, you would only get the first one.

If there's nothing to remove, it doesn't remove anything, so you get the only path if there's only one.

Here’s an alternative answer using arrays; unfortunately this requires two steps to assign to the answer but I mention it anyway since it’s more general and potentially more versatile than the other answers:

IFS=: read -ra arr_gopath <<< "$GOPATH"
PATH="$PATH:$arr_gopath[0]/%//bin"

The advantage with this approach is that you can easily specify any component of the GOPATH, not just the first one. Or, to append all elements, followed by /bin, to PATH:

PATH="$PATH:$(IFS=:; echo "$arr_gopath[*]/%//bin")"

In both cases (splitting, joining) we set the internal field separator (IFS) to the colon character :. The substitution $var/%//bin appends /bin to the end of a chosen variable.

That said, if you wanted to append all GOPATH components, suffixed with /bin, to the PATH then the following would be simpler and doesn’t require arrays. It does, however, require that GOPATH is not empty:

PATH="$PATH:$GOPATH//://bin/bin"

($var//pattern/subst replaces all occurrences of pattern with subst, while $var/pattern/subst, used above, only replaces the first one.)