CODEWITHSUNDEEP
bash
Al3n
Al3n

Reputation: 120

What's the meaning of "expect <<- DONE"?

What is this usage mean: 

expect <<- DONE
...
DONE

as in eof not recognized in Expect Script

especially the <<- part.

Upvotes: 3

Views: 439

Answers (1)

janos
janos

Reputation: 124646

In man bash, if you search for <<- (by typing: /<<- and Enter), you will find:

   If the redirection operator is <<-, then all leading tab characters are
   stripped from input lines and  the  line  containing  delimiter.   This
   allows  here-documents within shell scripts to be indented in a natural
   fashion.

For example:

$ cat << EOF
>       hello
> there
> EOF
    hello
there

The same thing but using <<- instead of <<

$ cat <<- EOF
>       hello
> there
> EOF
hello
there

The leading TAB character on the "hello" line is stripped.

As the quotation from the man page said, this is useful in shell scripts, for example:

if cond; then
    cat <<- EOF
    hello
    there
    EOF
fi

It is customary to indent the lines within code blocks as in this if statement, for better readability. Without the <<- operator syntax, we would be forced to write the above code like this:

if cond; then
    cat << EOF
hello
there
EOF
fi

That's very unpleasant to read, and it gets much worse in a more complex realistic script.

Keep in mind though, as @glenn-jackman pointed out:

Note that only tab characters are removed, not arbitrary whitespace. Be careful that your text editor does not convert tabs to spaces.

Upvotes: 7

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