Shell – echo result from subshell in zsh

echosubshellzsh

In zsh, the following 2 commands results differently:

a=$(</etc/hosts) && echo $a
echo $(</etc/hosts)

The 1st prints contents line by line, while the 2nd prints contents as a whole in one single line.

I guess it is because the subshell returns result line by line to the main echo process, but I can not confirm that.

Someone can help me to clear that?

Best Answer

In zsh, unquoted variables are not automatically split and glob, while unquoted command substitution will be split into words using values in IFS.

In your case, the first command saved content of file /etc/hosts to variable a, echo $a prints value of a variable without split and glob, you get the contents of /etc/hosts (split and glob are not performed in RHS of variable assignment).

echo $(</etc/hosts) used unquoted command substitution, the result will be split into words, so you get content of /etc/hosts as all words delimited by space.

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Ok, I think I have a clue what is going on.

The behaviour you are observing in zsh and bash when executing echo $(mc) is caused by mc.

When you run mc normally, it doesn't react when Ctrl+c is pressed, since it ignores SIGINT. The way to end mc is by pressing F10 and Enter.

When you run echo $(mc) the input goes to the mc process, so it is no wonder that when you press Ctrl+c nothing will happen, because mc ignores SIGINT. But when you run echo $(mc) and press F10 and Enter it will react. (When I then press F10 and Enter again it actually quits; it should quit at the first try but I have no idea why it does not.)

From this fact I deduce that Midnight Commander is running normally, but the output from it is put into the shell buffer, so it can later be used by echo. Something we should consider normal.

Also you think that bash and zsh are hanging, but I would say that they are not hanging but waiting for the echo command to return. But echo can only return when mc returns, which it only does when F10 and Enter are pressed. With this we can also explain what happens when you press Ctrl+z. By pressing the combination mc goes to sleep like it would if you had run mc normally and echo is still waiting for the sleeping mc.

So everything is normal behavior. Except for what fish does. fish seems to start commands run with echo ($command) in the background. It kind of makes sense, since such a command normally does not need any input. For the why and how I have no answer. But you can see that it runs in the background when you enter

echo (mc)
jobs

Best Answer

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