statement blocks Mechanism in Shell scripting?

Question

In Beginning Linux Programming book. There is a secition about Statement Block. In that Portion the explanation looks like the following.

Statement Blocks

If you want to use multiple statements in a place where only one is allowed, such as in an AND or OR list, you can do so by enclosing them in braces {} to make a statement block. For example, see the following code:

get_confirm && {
    grep −v "$cdcatnum" $tracks_file > $temp_file 
    cat $temp_file > $tracks_file
    echo
    add_record_tracks
}

Please explain how the code is executing in the statement block...

Solution

The code within { ... } execute exactly the way it would have executed without the curly braces, except now it's dependent on the exit status of get_confirm.

If get_confirm exits with a zero exit status ("success"), the block executes, otherwise not.

It's equivalent to

if get_confirm; then
    # the contents of the block goes here
fi

(which I think looks nicer)

The book's example is IMHO not a good example of a common use of { ... }. A better example would have been

{ echo 'hello world'; date; } >file

which uses a single redirection to redirect the standard output of both commands in the { ... } block to the same file.

This is similar to (and has the same effect, in this instance, as)

( echo 'hello world'; date ) >file

but the statements within { ... } executes in the same environment as the shell, whereas the statements in ( ... ) executes in a subshell (a separate environment).

You can see the difference with

{ a=42; }; echo $a

and

( a=1973 ); echo $a

The first will output 42 whereas the second will not output 1973 (the assignment happens in a subshell and it can't affect the environment outside).

Notice about grammar: The closing } of a { ... } block must follow a newline or a ;. { echo 'hello' } is not valid, while both { echo 'hello'; } and

{ 
    echo 'hello'
}

are.

Comments

[Hauke Laging]

The code within the braces is executed the same way as if there were no braces.

[Dan M.]

@Kusalananda, sorry if it's offtopic but is this {} syntax - bash specific, or is it POSIX-compliant?

[Dan M.]

@Kusalananda thanks. I'll look into other causes that may explain why my script has an empty output under dash but not zsh and bash.

[Kusalananda]

@DanM. You could ask a question about it.