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Abhishek Kamal
Asked: 2019-01-29 08:34:48 +0800 CST

Run another command when previous command completes [duplicate]

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Today I opened gnome-terminal and I wrote 

ls && sleep 4 && gnome-terminal

to open another terminal after the completion of ls command and waiting for 4 seconds.
So it successfully opened a new terminal after previous commands completely ran (including sleep 4).

After that, next time I typed a new command 

ls -lR && sleep 4 && gnome-terminal

The command ls -lR completed after 3 seconds, but after that none of the commands sleep 4 and gnome-terminal ran successfully.

What is the problem?

command-line bash syntax

2 Answers

  1. Best Answer

    && means to run it if the previous command was successful. In Unix that generally means exit status 0.

    $ ls barfoofba && echo "This will not be echo'd"
ls: cannot access 'barfoofba': No such file or directory
$ ls bar && echo "This will be echo'd"  
This will be echo'd

    In the first instance, ls did not succeed, so it exited with a non-zero exit status, and bash did not run the second command.

    In the second example, I ls'd a existing file, so ls exited with 0 as exit status, and the command was executed.

    If you want to run commands unconditionally, e.g. not dependent on the result of the first, you may separate them with ; like this

     command1; command2; command3

    and so forth.

    Thus you may do 

    ls -lR ; sleep 4 ; gnome-terminal

    In addition to && and ; you have || which is the opposite of &&: Only run the command if the previous command failed with a non-zero exit status. If the first command succeeds, the next will not be executed. If it fails, the next will be executed.

    So in short:

    • &&: Run if preceding command exited with 0
    • ;: Run unconditionally
    • ||: Run if preceding command exited with a non-zero exit status.
    • &: Run both commands in paralell, the first in background and second in foreground.
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  2. The command ls -lR exited with an exit-status different than zero, so the following commands have not been executed. Most probably ls was unable to open a subdirectory. It didn't happen in your first command because you didn't use the -R-option.

    From man bash:

            command1 && command2
       command2  is  executed if, and only if, command1 returns an exit status
       of zero.

    From man ls:

    Exit status:
       0      if OK,
       1      if minor problems (e.g., cannot access subdirectory)
       2      if serious trouble (e.g., cannot access command-line argument).
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