"ladyrick" <ladyr...@qq.com> writes: > Description: > A heredoc starts with "cat <<'EOF'" is expected to not expand > anything just like in a single quote string. But when this > heredoc is in a $() or <(), history is expanded. > > > Repeat-By: > This works: > ``` > cat <<'EOF' > !! > EOF > ``` > This doesn't work: > ``` > cat <(cat <<'EOF' > !! > EOF > ) > ``` > This doesn't work neither: > ``` > echo "$(cat <<'EOF' > !! > EOF > )" > ```
It's a messy case; what's really happening the the last two examples is that the entire thing is read and processed as one "line", because bash reads all of the command or process substitution as a unit. Then it performs history expansion, and then it interprets the line. So the history expansion is done before bash even recognizes that there is a here-doc. In the first example, bash reads "cat <<'EOF'" and starts processing it. At that point, it recognizes that it must read a here-doc and does so. Dale
