Linux shell script programming fundamentals: making a command-line tool that executes a command to execute several

Question

Example with the command-line tool flock.
The documentation just defines the syntax:

flock [options] <file> -c <command>

But -c seems not to be mandatory. This line executes a program if locking of a file is possible (the '-n' switch disable wait for lockfile release):

flock -n lockfile.lck echo "File is Locked"

I would like, instead, to make a script like:

flock -n $TMP/lockfile.lck (
        echo "Congratulations:"
        echo "you have acquired the lock of the file"
        )

As you can see, it is nearly the same, but executing more than one line.
Is it possible?

And, for the case when locking is not possible:

flock -n lockfile.lck echo "Lock acquired" || echo "Could not acquire lock"

I would like to know if it can be implemented something like :

flock -n $TMP/lockfile.lck (
        echo "Congratulations:"
        echo "you have acquired the lock of the file"
        ) || (
        echo "How sad:"
        echo "the file was locked."
        )

Thanks.

twalberg · Accepted Answer · 2014-04-03 19:57:55Z

1

Since the parentheses there are part of the shell syntax, and not commands in their own right, you would have to do something like this:

flock -n lockfile.lock bash -c '( command list )'

Depending on what your exact set of commands are, the necessary quoting/escaping might be a slight additional challenge, but if that gets too hard, then just put your commands in a proper shell script, and call that instead of the bash -c ... syntax.

EDIT : In fact, using a shell this way, the parentheses are no longer necessary for command grouping (and in fact would be less efficient due to spawning an additional subshell), so this would suffice:

flock -n lockfile.lock bash -c 'command list'

edited Apr 3, 2014 at 19:57

answered Apr 3, 2014 at 19:47

twalberg

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3 Comments

Sopalajo de Arrierez Over a year ago

I don't know if the parentheses are needed. I was just creating an example of some character that will allow the script to separate the commands in different lines, instead of putting al the lines together between ;. I can prescind of parentheses or use any other character.

twalberg Over a year ago

Well, the above would work even without parentheses (which aren't necessary), and would still give you the ability to run multiple commands under the lock - just flock -n lockfile.lock bash -c 'cmd1; cmd2; cmd3 ...'.

glenn jackman Over a year ago

For readability, within a single quoted string, you can put as many newlines as required.

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