Temporarily declare a variable in Bash











up vote
0
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
    2 hours ago










  • If you're trying to do that for "security" purposes (ie. that var holds a password), then keep in mind that unsetting a variable does not guarantee in any way that its content will be scrubbed from memory. Simply put, keeping sensitive data in plain text in shell variables is not something that should be done for 5 secs, 5 minutes or 5 hours.
    – pizdelect
    1 hour ago

















up vote
0
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
    2 hours ago










  • If you're trying to do that for "security" purposes (ie. that var holds a password), then keep in mind that unsetting a variable does not guarantee in any way that its content will be scrubbed from memory. Simply put, keeping sensitive data in plain text in shell variables is not something that should be done for 5 secs, 5 minutes or 5 hours.
    – pizdelect
    1 hour ago















up vote
0
down vote

favorite









up vote
0
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 4 hours ago









JohnDoea

771132




771132








  • 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
    2 hours ago










  • If you're trying to do that for "security" purposes (ie. that var holds a password), then keep in mind that unsetting a variable does not guarantee in any way that its content will be scrubbed from memory. Simply put, keeping sensitive data in plain text in shell variables is not something that should be done for 5 secs, 5 minutes or 5 hours.
    – pizdelect
    1 hour 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
    2 hours ago










  • If you're trying to do that for "security" purposes (ie. that var holds a password), then keep in mind that unsetting a variable does not guarantee in any way that its content will be scrubbed from memory. Simply put, keeping sensitive data in plain text in shell variables is not something that should be done for 5 secs, 5 minutes or 5 hours.
    – pizdelect
    1 hour 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
2 hours 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
2 hours ago












If you're trying to do that for "security" purposes (ie. that var holds a password), then keep in mind that unsetting a variable does not guarantee in any way that its content will be scrubbed from memory. Simply put, keeping sensitive data in plain text in shell variables is not something that should be done for 5 secs, 5 minutes or 5 hours.
– pizdelect
1 hour ago






If you're trying to do that for "security" purposes (ie. that var holds a password), then keep in mind that unsetting a variable does not guarantee in any way that its content will be scrubbed from memory. Simply put, keeping sensitive data in plain text in shell variables is not something that should be done for 5 secs, 5 minutes or 5 hours.
– pizdelect
1 hour ago












2 Answers
2






active

oldest

votes

















up vote
3
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
    3 hours ago










  • @JeffSchaller I noticed that :-)
    – Stephen Harris
    3 hours 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
    3 hours ago




















up vote
2
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























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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    3
    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
      3 hours ago










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      3 hours 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
      3 hours ago

















    up vote
    3
    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
      3 hours ago










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      3 hours 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
      3 hours ago















    up vote
    3
    down vote










    up vote
    3
    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 3 hours ago

























    answered 3 hours ago









    Stephen Harris

    23.9k24477




    23.9k24477












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










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      3 hours 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
      3 hours ago




















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










    • @JeffSchaller I noticed that :-)
      – Stephen Harris
      3 hours 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
      3 hours ago


















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




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












    @JeffSchaller I noticed that :-)
    – Stephen Harris
    3 hours ago






    @JeffSchaller I noticed that :-)
    – Stephen Harris
    3 hours 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
    3 hours 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
    3 hours ago














    up vote
    2
    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
      2
      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
        2
        down vote










        up vote
        2
        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 3 hours ago

























        answered 3 hours ago









        Jeff Schaller

        37.6k1052121




        37.6k1052121






























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