CODEWITHSUNDEEP
bashheredoc
Polaris96
Polaris96

Reputation: 1

why does bash read need a redirect

Forgive me if this is obvious but I can't reason it out. The intent was to assign a heredoc to a variable in bash. The correct code on this site is: 

read -r -d '' VAR <<'EOF'
abc'asdf"
$(dont-execute-this)
foo"bar"''
EOF

This works very well. But I don't see why read wouldn't load the data into VAR without a redirect.

-d sets the delimiter as single quote. Shouldn't bash absorb all the following data until it encounters a ' and put the lot into VAR?

This doesn't happen, of course. I'm just wondernig why not.

Upvotes: 0

Views: 57

Answers (1)

Charles Duffy
Charles Duffy

Reputation: 295315

read reads from stdin -- not from your script's source file. It would be pretty useless (for prompting the user, reading from a separate file, etc) otherwise. See BashFAQ #1 for more on its use.

If you run a script at a terminal, then, read by default reads from that terminal; if you pipe content into your script, read by default reads from that content; etc.

read -r -d '' var, specifically, reads from stdin until a NUL character is seen, or an end-of-file condition takes place (in the latter case, it exits with a nonzero status, but still populates the destination variable).

Using <<'EOF' redirects your stdin from a temporary file containing the heredoc's contents. Thus, even though the file contains no NULs, it will eventually hit EOF, and thus control will return.

Upvotes: 3

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