How to append a conditional command to a command that is already running?

Question

Let's say I have three commands that I run like this:

a && b && c

This will run b after a completes, and c after b completes.

What if I did this:

a && b

While this is running, I want to append c as a conditional command. Can it be done?

Answer 1

I would do this:

• a && b
• press ctrl-Z to suspend the current pipeline
• bg 1 to run it in background
• wait %1 && c

wait waits for the specified job to complete, and returns its result status, so that the above behaves like a && b && c would have.

You can also define an alias or function, eg alias bgwait='bg 1 && wait %1'. Then, after pressing ctrl-Z, all you have to type is bgwait && c. Supporting multiple jobs is left as an exercise for the reader.

Answer 2

The result of the last operation is stored in $?. 0 means no errors occurred, anything else is a problem.

You can e.g. use it like this

if [ $? -eq 0 ]; then c; fi

Make it a function or similar in your .bash_profile and you can run it a bit more comfortably.

Answer 3

For NT, you can type your new command and hit ENTER while a current command is running.

So for example,

c:\blah>dir /s

where blah is big enough that you'll have time to type while stuff scrolls onto the screen.

now all this stuff scrolls on the screen

and while it is appearing, type mem

then after dir /s is complete, mem will run.