How to plan a task to run after another already running task in bash? [duplicate]

Question

I'm looking for something like command1 ; command2 i.e. how to run command2 after command1 but I'd like to plan execution of command2 when command1 is already running.

It can be solved by just typing the command2 and confirming by Enter supposed that the command1 is not consuming standard input and that the command1 doesn't produce to much text on output making it impractical to type (typed characters are blended with the command1 output).

Answer 1

Generally what I do is: Ctrl+Z fg && command2

1. Ctrl+Z to pause it and let you type more in the shell.
2. Optionally bg, to resume command1 in the background while you type out command2.
3. fg && command2 to resume command1 in the foreground and queue up command2 afterwards if command1 succeeds. You can of course substitute ; or || for the && if you so desire.

Answer 2

1. start a command

   command1
   

2. pause the execution by pressing Ctrl+Z

3. find the job number of the paused command (it's usually already printed to console when to console when to command is paused) by executing

   jobs
   

4. let command1 continue in background

   bg
   

5. plan execution of command2 to await finish of command1

   wait -n <command1 job number> ; command2
   

Documentation Job Control Builtins

Answer 3

There are several alternatives.

• With one ampersand, 'push to background', the second program starts after the first one starts, but they will probably run alongside each other.

  command1 & command2
  

• With two ampersands, 'logical and', the second program will start only if the first program finished successfully.

  command1 && command2
  

• With a semicolon separating the commands in the command line, the the second program will start, when the first program finished, even if it failed or was interrupted.

  command1 ; command2
  

• You can use wait <PID> to wait for the first command to finish, if it is already running from the same shell (in the same terminal window).

• Otherwise if the first command is already running from another shell (in another window), you can use a small while loop using ps to check if the PID is still found by ps. When it is no longer found, the second command will be started.

  This demo example for bash checks if top is running via the PID, and runs the command

  echo "*** $USER, I am ready now ***"
  

  if/when top is no longer running.

  pid=$(ps -A|grep top|cut -d ' ' -f 1); \
  while [ "$pid" != "" ]; do ps -A|grep "$pid" > /dev/null; \
  if [ $? -eq 0 ]; then sleep 5;else pid=""; fi; done; \
  echo "*** $USER, I am ready now ***"
  

Answer 4

The standard way is just to let command 1 start in the background and then have command 2 start either in the background or in the foreground. The full command is this:

command1 & command2

& is not a connector between these two commands. It can be used with command1 alone to have it started in the background:

command1 &

And this is right syntax for starting both commands in the background:

command1 & command2 &

Answer 5

If you have a choice to use two terminals, here's what you can do:

• Assuming you are already running command1 in a terminal (let's call it terminal1)
• Start a new terminal (let's call it terminal2)
• Find process ID of command1 (let's call it command1_pid). In terminal2, run command ps -ef | grep <command1> (you might want to properly form the filter string in grep command so that you do not accidently find another process having as a substring).
• Run command wait <command1_pid> ; command2

Basically, wait is a bash built-in command that waits until the provided PIDs exit and then return. You are simply delegating your waiting for command1 to finish, on to bash's wait command.