Temporarily declare a variable in Bash











up vote
1
down vote

favorite












To declare a variable in Bash (say in a Bash script-file and not in Bash function) I do for example:



x=y


But when I finish using it inside that script-file I do unset x.



Is there a way (without using a function), to unset the variable after say 5 minutes, both in one line?



A plausible approach might be x=y && echo "unset x" | at now + 5 minutes.






bash variable at







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  • 1




    You need to do unset x only if you are sourcing the script file. Otherwise the script runs in a subshell and does not affect your shell environment, so you don't need to unset it. I use parentheses to run one-liners with temporary variables on the command line all the time, like ( for i in {1..20}; do dosomething; done), and after I execute this command I don't have $i in my shell.
    – Weijun Zhou
    45 mins ago















up vote
1
down vote

favorite












To declare a variable in Bash (say in a Bash script-file and not in Bash function) I do for example:



x=y


But when I finish using it inside that script-file I do unset x.



Is there a way (without using a function), to unset the variable after say 5 minutes, both in one line?



A plausible approach might be x=y && echo "unset x" | at now + 5 minutes.






bash variable at







share|improve this question


















  • 1




    You need to do unset x only if you are sourcing the script file. Otherwise the script runs in a subshell and does not affect your shell environment, so you don't need to unset it. I use parentheses to run one-liners with temporary variables on the command line all the time, like ( for i in {1..20}; do dosomething; done), and after I execute this command I don't have $i in my shell.
    – Weijun Zhou
    45 mins ago













up vote
1
down vote

favorite









up vote
1
down vote

favorite











To declare a variable in Bash (say in a Bash script-file and not in Bash function) I do for example:



x=y


But when I finish using it inside that script-file I do unset x.



Is there a way (without using a function), to unset the variable after say 5 minutes, both in one line?



A plausible approach might be x=y && echo "unset x" | at now + 5 minutes.






bash variable at







share|improve this question













To declare a variable in Bash (say in a Bash script-file and not in Bash function) I do for example:



x=y


But when I finish using it inside that script-file I do unset x.



Is there a way (without using a function), to unset the variable after say 5 minutes, both in one line?



A plausible approach might be x=y && echo "unset x" | at now + 5 minutes.






bash variable at




bash variable at






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 1 hour ago









JohnDoea

691132




691132








  • 1




    You need to do unset x only if you are sourcing the script file. Otherwise the script runs in a subshell and does not affect your shell environment, so you don't need to unset it. I use parentheses to run one-liners with temporary variables on the command line all the time, like ( for i in {1..20}; do dosomething; done), and after I execute this command I don't have $i in my shell.
    – Weijun Zhou
    45 mins ago














  • 1




    You need to do unset x only if you are sourcing the script file. Otherwise the script runs in a subshell and does not affect your shell environment, so you don't need to unset it. I use parentheses to run one-liners with temporary variables on the command line all the time, like ( for i in {1..20}; do dosomething; done), and after I execute this command I don't have $i in my shell.
    – Weijun Zhou
    45 mins ago








1




1




You need to do unset x only if you are sourcing the script file. Otherwise the script runs in a subshell and does not affect your shell environment, so you don't need to unset it. I use parentheses to run one-liners with temporary variables on the command line all the time, like ( for i in {1..20}; do dosomething; done), and after I execute this command I don't have $i in my shell.
– Weijun Zhou
45 mins ago




You need to do unset x only if you are sourcing the script file. Otherwise the script runs in a subshell and does not affect your shell environment, so you don't need to unset it. I use parentheses to run one-liners with temporary variables on the command line all the time, like ( for i in {1..20}; do dosomething; done), and after I execute this command I don't have $i in my shell.
– Weijun Zhou
45 mins ago










2 Answers
2






active

oldest

votes

















up vote
4
down vote













You can't use at jobs because they run in a different context, and can't affect the current shell.



But we can do something similar. This code will trigger an alarm signal, which we can catch and perform action on



#!/bin/bash



x=100



trap 'unset x' SIGALRM



mypid=$$



( /bin/sleep 3 ; kill -ALRM $mypid) &



for a in 1 2 3 4 5 6

do

  echo Now x=$x

  sleep 1

done


This example is only 3 seconds long to demonstrate the solution; you can pick your delay as you need.



In action:



Now x=100

Now x=100

Now x=100

Now x=

Now x=

Now x=


You can easily make it one line with ;...






share|improve this answer























  • Wow - great minds think alike -- and within 30 seconds of each other! :)
    – Jeff Schaller
    1 hour ago










  • @JeffSchaller I noticed that :-)
    – Stephen Harris
    1 hour ago








  • 1




    @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
    – Stephen Harris
    1 hour ago




















up vote
3
down vote













Sure:



trap 'unset x; trap - USR1' USR1; { sleep 5m; kill -USR1 $$; } &


This sets a trap on the USR1 signal, then (cheating with a semicolon to put it on one line) groups together the sleep and kill commands into a background job. When that job completes, it will send the USR1 signal to the current shell, which will execute the trap. The trap unsets x and then clears the trap.



No functions!






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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    4
    down vote













    You can't use at jobs because they run in a different context, and can't affect the current shell.



    But we can do something similar. This code will trigger an alarm signal, which we can catch and perform action on



    #!/bin/bash
    

    
x=100
    

    
trap 'unset x' SIGALRM
    

    
mypid=$$
    

    
( /bin/sleep 3 ; kill -ALRM $mypid) &
    

    
for a in 1 2 3 4 5 6
    
do
    
  echo Now x=$x
    
  sleep 1
    
done


    This example is only 3 seconds long to demonstrate the solution; you can pick your delay as you need.



    In action:



    Now x=100
    
Now x=100
    
Now x=100
    
Now x=
    
Now x=
    
Now x=


    You can easily make it one line with ;...






    share|improve this answer























    • Wow - great minds think alike -- and within 30 seconds of each other! :)
      – Jeff Schaller
      1 hour ago










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      1 hour ago








    • 1




      @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
      – Stephen Harris
      1 hour ago

















    up vote
    4
    down vote













    You can't use at jobs because they run in a different context, and can't affect the current shell.



    But we can do something similar. This code will trigger an alarm signal, which we can catch and perform action on



    #!/bin/bash
    

    
x=100
    

    
trap 'unset x' SIGALRM
    

    
mypid=$$
    

    
( /bin/sleep 3 ; kill -ALRM $mypid) &
    

    
for a in 1 2 3 4 5 6
    
do
    
  echo Now x=$x
    
  sleep 1
    
done


    This example is only 3 seconds long to demonstrate the solution; you can pick your delay as you need.



    In action:



    Now x=100
    
Now x=100
    
Now x=100
    
Now x=
    
Now x=
    
Now x=


    You can easily make it one line with ;...






    share|improve this answer























    • Wow - great minds think alike -- and within 30 seconds of each other! :)
      – Jeff Schaller
      1 hour ago










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      1 hour ago








    • 1




      @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
      – Stephen Harris
      1 hour ago















    up vote
    4
    down vote










    up vote
    4
    down vote









    You can't use at jobs because they run in a different context, and can't affect the current shell.



    But we can do something similar. This code will trigger an alarm signal, which we can catch and perform action on



    #!/bin/bash
    

    
x=100
    

    
trap 'unset x' SIGALRM
    

    
mypid=$$
    

    
( /bin/sleep 3 ; kill -ALRM $mypid) &
    

    
for a in 1 2 3 4 5 6
    
do
    
  echo Now x=$x
    
  sleep 1
    
done


    This example is only 3 seconds long to demonstrate the solution; you can pick your delay as you need.



    In action:



    Now x=100
    
Now x=100
    
Now x=100
    
Now x=
    
Now x=
    
Now x=


    You can easily make it one line with ;...






    share|improve this answer














    You can't use at jobs because they run in a different context, and can't affect the current shell.



    But we can do something similar. This code will trigger an alarm signal, which we can catch and perform action on



    #!/bin/bash
    

    
x=100
    

    
trap 'unset x' SIGALRM
    

    
mypid=$$
    

    
( /bin/sleep 3 ; kill -ALRM $mypid) &
    

    
for a in 1 2 3 4 5 6
    
do
    
  echo Now x=$x
    
  sleep 1
    
done


    This example is only 3 seconds long to demonstrate the solution; you can pick your delay as you need.



    In action:



    Now x=100
    
Now x=100
    
Now x=100
    
Now x=
    
Now x=
    
Now x=


    You can easily make it one line with ;...







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited 1 hour ago

























    answered 1 hour ago









    Stephen Harris

    23.9k24477




    23.9k24477












    • Wow - great minds think alike -- and within 30 seconds of each other! :)
      – Jeff Schaller
      1 hour ago










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      1 hour ago








    • 1




      @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
      – Stephen Harris
      1 hour ago




















    • Wow - great minds think alike -- and within 30 seconds of each other! :)
      – Jeff Schaller
      1 hour ago










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      1 hour ago








    • 1




      @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
      – Stephen Harris
      1 hour ago


















    Wow - great minds think alike -- and within 30 seconds of each other! :)
    – Jeff Schaller
    1 hour ago




    Wow - great minds think alike -- and within 30 seconds of each other! :)
    – Jeff Schaller
    1 hour ago












    @JeffSchaller I noticed that :-)
    – Stephen Harris
    1 hour ago






    @JeffSchaller I noticed that :-)
    – Stephen Harris
    1 hour ago






    1




    1




    @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
    – Stephen Harris
    1 hour ago






    @JeffSchaller I rolled back your edit; I specifically put a 3 second pause in as an example; a 300 second (5minute) sleep would mean the for loop won't demonstrate the solution working. I'll edit the answer to make that clear.
    – Stephen Harris
    1 hour ago














    up vote
    3
    down vote













    Sure:



    trap 'unset x; trap - USR1' USR1; { sleep 5m; kill -USR1 $$; } &


    This sets a trap on the USR1 signal, then (cheating with a semicolon to put it on one line) groups together the sleep and kill commands into a background job. When that job completes, it will send the USR1 signal to the current shell, which will execute the trap. The trap unsets x and then clears the trap.



    No functions!






    share|improve this answer



























      up vote
      3
      down vote













      Sure:



      trap 'unset x; trap - USR1' USR1; { sleep 5m; kill -USR1 $$; } &


      This sets a trap on the USR1 signal, then (cheating with a semicolon to put it on one line) groups together the sleep and kill commands into a background job. When that job completes, it will send the USR1 signal to the current shell, which will execute the trap. The trap unsets x and then clears the trap.



      No functions!






      share|improve this answer

























        up vote
        3
        down vote










        up vote
        3
        down vote









        Sure:



        trap 'unset x; trap - USR1' USR1; { sleep 5m; kill -USR1 $$; } &


        This sets a trap on the USR1 signal, then (cheating with a semicolon to put it on one line) groups together the sleep and kill commands into a background job. When that job completes, it will send the USR1 signal to the current shell, which will execute the trap. The trap unsets x and then clears the trap.



        No functions!






        share|improve this answer














        Sure:



        trap 'unset x; trap - USR1' USR1; { sleep 5m; kill -USR1 $$; } &


        This sets a trap on the USR1 signal, then (cheating with a semicolon to put it on one line) groups together the sleep and kill commands into a background job. When that job completes, it will send the USR1 signal to the current shell, which will execute the trap. The trap unsets x and then clears the trap.



        No functions!







        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited 1 hour ago

























        answered 1 hour ago









        Jeff Schaller

        37.6k1052121




        37.6k1052121






























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