understand: bash -c 'set w x y z; IFS=":-;"; echo "$*"'

vincix · 01-01-2018, 11:47 AM

I'd like to understand the following command:
bash -c 'set w x y z; IFS=":-;"; echo "$*"'
The result is:
w:x:y:z

I'm supposing that a new bash instance is being run, then the variables w, x, y, z are set.

First question: given that we have a $* at the end, that means that it lists all positional parameters. But how do w, x, y and z become parameters if they're variables?

Secondly: Does the :-; assigning for IFS mean that any of the three characters work as delimiters? And the first is chosen by default in this case?

Thirdly: I don't understand exactly how the -c option works in this case. What's its role exactly? I did read the man pages. It says that commands are read from string. Does that mean it simply makes the following commands (set, IFS, echo) possible for that bash instance?

rknichols · 01-01-2018, 12:08 PM

Quote:

Originally Posted by vincix

I'd like to understand the following command:
bash -c 'set w x y z; IFS=":-;"; echo "$*"'
The result is:
w:x:y:z

I'm supposing that a new bash instance is being run, then the variables w, x, y, z are set.

First question: given that we have a $* at the end, that means that it lists all positional parameters. But how do w, x, y and z become parameters if they're variables?

They are not variables. The command "set w x y z" sets the positional parameters $1 through $3 to the values "w", "x", "y", and "z".

Quote:

Secondly: Does the :-; assigning for IFS mean that any of the three characters work as delimiters? And the first is chosen by default in this case?

Yes.

Quote:

Thirdly: I don't understand exactly how the -c option works in this case. What's its role exactly? I did read the man pages. It says that commands are read from string. Does that mean it simply makes the following commands (set, IFS, echo) possible for that bash instance?

When invoked with "-c", bash simply takes the string that follows as a single command line to be executed. It is very much like running

Code:

echo 'set w x y z; IFS=":-;"; echo "$*"' | bash

the principal difference being what the commands within that string will see as their standard input (irrelevant in this case since none of these specific commands make use of stdin).