exec < filename - what does this do?

Question

I have found the following code snippet (sh):

    TESTFILE=$TEST_PATH/test.out
    if [ -f $TESTFILE ]
    then
        exec < $TESTFILE
    else
        echo "$TEST_PATH: no test.out file"
    fi

the focus of my question is, what is this line doing? (the rest i understand!)

        exec < $TESTFILE

The file is not executable, nor is another process invoked, and i am having no luck finding what exec does on a plain text file.

Solution 1

From the POSIX description of exec:

exec - execute commands and open, close, or copy file descriptors

In this case, there is no command, so only file descriptors are modified. Normally, redirections you write on the command line affect the command on that line. exec is how you do redirections on the currently executing shell.

Solution 2

Intuitive understanding may be helped by observing this pattern:

cmd                  # fork; execute cmd in child
cmd < /dev/null      # fork; redirect stdin and execute cmd in child
exec cmd             # execute cmd; no fork
exec cmd < /dev/null # redirect stdin and execute cmd; no fork
exec < /dev/null     # redirect stdin; no fork and no execution

See, exec isn't really a thing. It's an anti-thing. It doesn't mean "execute" at all. It means "don't fork". It makes the command's redirection and/or command execution happen in the current process.

Comments

[NWS]

so as there is actually no need to modify the file or its descriptors, its essentially a null operation ?

[jlliagre]

It is not completely a null operation. The remaining of the scripts will pick its standard input from $TESTFILE starting from this instruction instead of whatever was stdin before, presumably the keyboard for interactive scripts.

[jw013]

@NWS Whether something is a null operation depends on what it does, not whether what it does is actually needed. A null operation does nothing by definition except possibly pass some time. Modifying file descriptors is not nothing, so that is not a null operation. Running exec by itself could be considered a null operation, except for setting $? to 0.

[jmend]

This is actually a very useful hack, I had no idea you could change stdin of the running shell.