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-1

I run into this weird behavior of trap.

According to trap's manpage:

When a subshell is entered, traps that are not being ignored are set to the default actions. This does not imply that the trap command cannot be used within the subshell to set new traps.

This is how I interpret it:

• If a script traps signal A, its subshell will also trap signal A but with a default action.


• The subshell can also specify another action to trap that same signal

To test my understanding, I have two scripts:

outerscript.sh

#!/bin/bash



trap "echo SIGINT in outer" SIGINT

echo PID of outer process: $$

echo -----------;

./innerscript.sh

innerscript.sh

#!/bin/bash



echo start inner script

echo PID of inner process: $$

trap "SIGINT in inner, do graceful shutdown" SIGINT

sleep 10s

echo done inner process

Then I run the outerscript with ./outerscript.sh
The outerscript will then call the innerscript and create a subshell as shown in the picture

During the sleep command, I send a SIGINT with kill -SIGINT <pid>

The result is different based on the PID receiving the signal

1. 
   

   PID to receive signal is that of outerscript

   
   
   

   Note that I still have to wait for the sleep command to complete

   
   
   

   done inner process
   
   SIGINT in outer
   
   

   
   


2. 
   

   PID to receive signal is that of innerscript

   
   
   

   Note that I still have to wait for the sleep command to complete

   
   
   

   SIGINT: command not found
   
   done inner process
   
   

   
   


3. 
   

   PID to receive signal is that of sleep

   
   
   

   Note that I DON'T have to wait for the sleep command to complete

   
   
   

   done inner process
   
   

   
   

Question

In case 2), why do I get that error? I expected innerscript to trigger its trap function.

In case 3), why does it even trigger innerscript's trap function? I expected it to just kill itself and return normally to the inner script process.

shell-script trap

share|improve this question

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  The error in case 2 is because you have trap "SIGINT ... rather than trap "echo SIGINT ...
  
  – icarus
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  In case 3 it is not triggering the trap function, it is stopping the sleep, then going onto the next line, the echo done inner process
  
  – icarus
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  oh boy, thanks for pointing out the missing echo function. I can't believe I struggled with this for hours.
  
  – Tran Triet
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you could post this as an answer, I'd gladly mark it as the solution.
  
  – Tran Triet
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  A fresh pair of eyes often helps!
  
  – icarus
  55 mins ago
  

  
  
  
  

add a comment | 

-1

I run into this weird behavior of trap.

According to trap's manpage:

When a subshell is entered, traps that are not being ignored are set to the default actions. This does not imply that the trap command cannot be used within the subshell to set new traps.

This is how I interpret it:

• If a script traps signal A, its subshell will also trap signal A but with a default action.


• The subshell can also specify another action to trap that same signal

To test my understanding, I have two scripts:

outerscript.sh

#!/bin/bash



trap "echo SIGINT in outer" SIGINT

echo PID of outer process: $$

echo -----------;

./innerscript.sh

innerscript.sh

#!/bin/bash



echo start inner script

echo PID of inner process: $$

trap "SIGINT in inner, do graceful shutdown" SIGINT

sleep 10s

echo done inner process

Then I run the outerscript with ./outerscript.sh
The outerscript will then call the innerscript and create a subshell as shown in the picture

During the sleep command, I send a SIGINT with kill -SIGINT <pid>

The result is different based on the PID receiving the signal

1. 
   

   PID to receive signal is that of outerscript

   
   
   

   Note that I still have to wait for the sleep command to complete

   
   
   

   done inner process
   
   SIGINT in outer
   
   

   
   


2. 
   

   PID to receive signal is that of innerscript

   
   
   

   Note that I still have to wait for the sleep command to complete

   
   
   

   SIGINT: command not found
   
   done inner process
   
   

   
   


3. 
   

   PID to receive signal is that of sleep

   
   
   

   Note that I DON'T have to wait for the sleep command to complete

   
   
   

   done inner process
   
   

   
   

Question

In case 2), why do I get that error? I expected innerscript to trigger its trap function.

In case 3), why does it even trigger innerscript's trap function? I expected it to just kill itself and return normally to the inner script process.

shell-script trap

share|improve this question

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  The error in case 2 is because you have trap "SIGINT ... rather than trap "echo SIGINT ...
  
  – icarus
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  In case 3 it is not triggering the trap function, it is stopping the sleep, then going onto the next line, the echo done inner process
  
  – icarus
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  oh boy, thanks for pointing out the missing echo function. I can't believe I struggled with this for hours.
  
  – Tran Triet
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you could post this as an answer, I'd gladly mark it as the solution.
  
  – Tran Triet
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  A fresh pair of eyes often helps!
  
  – icarus
  55 mins ago
  

  
  
  
  

add a comment | 

I run into this weird behavior of trap.

According to trap's manpage:

When a subshell is entered, traps that are not being ignored are set to the default actions. This does not imply that the trap command cannot be used within the subshell to set new traps.

This is how I interpret it:

• If a script traps signal A, its subshell will also trap signal A but with a default action.


• The subshell can also specify another action to trap that same signal

To test my understanding, I have two scripts:

outerscript.sh

#!/bin/bash



trap "echo SIGINT in outer" SIGINT

echo PID of outer process: $$

echo -----------;

./innerscript.sh

innerscript.sh

#!/bin/bash



echo start inner script

echo PID of inner process: $$

trap "SIGINT in inner, do graceful shutdown" SIGINT

sleep 10s

echo done inner process

Then I run the outerscript with ./outerscript.sh
The outerscript will then call the innerscript and create a subshell as shown in the picture

During the sleep command, I send a SIGINT with kill -SIGINT <pid>

The result is different based on the PID receiving the signal

1. 
   

   PID to receive signal is that of outerscript

   
   
   

   Note that I still have to wait for the sleep command to complete

   
   
   

   done inner process
   
   SIGINT in outer
   
   

   
   


2. 
   

   PID to receive signal is that of innerscript

   
   
   

   Note that I still have to wait for the sleep command to complete

   
   
   

   SIGINT: command not found
   
   done inner process
   
   

   
   


3. 
   

   PID to receive signal is that of sleep

   
   
   

   Note that I DON'T have to wait for the sleep command to complete

   
   
   

   done inner process
   
   

   
   

Question

In case 2), why do I get that error? I expected innerscript to trigger its trap function.

In case 3), why does it even trigger innerscript's trap function? I expected it to just kill itself and return normally to the inner script process.

shell-script trap

share|improve this question

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

#!/bin/bash



trap "echo SIGINT in outer" SIGINT

echo PID of outer process: $$

echo -----------;

./innerscript.sh

#!/bin/bash



echo start inner script

echo PID of inner process: $$

trap "SIGINT in inner, do graceful shutdown" SIGINT

sleep 10s

echo done inner process

share|improve this question

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

share|improve this question

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

edited 1 min ago

Kusalananda♦

159k1818 gold badges313313 silver badges500500 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

asked 1 hour ago

Tran TrietTran Triet

2501111 bronze badges

1 Answer
1

active

oldest

votes

2

There are 2 questions.
The error in case 2 is because there is no command called SIGINT on the system. The OP had

trap "SIGINT in inner, do graceful shutdown" SIGINT

but probably the desired command was

trap "echo SIGINT in inner, do graceful shutdown" SIGINT

The other question asks why the inner scripts trap function is invoked, however it is not. The sleep process is killed, the script moves onto the next line.

share|improve this answer

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges

add a comment | 

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2

There are 2 questions.
The error in case 2 is because there is no command called SIGINT on the system. The OP had

trap "SIGINT in inner, do graceful shutdown" SIGINT

but probably the desired command was

trap "echo SIGINT in inner, do graceful shutdown" SIGINT

The other question asks why the inner scripts trap function is invoked, however it is not. The sleep process is killed, the script moves onto the next line.

share|improve this answer

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges

add a comment | 

2

There are 2 questions.
The error in case 2 is because there is no command called SIGINT on the system. The OP had

trap "SIGINT in inner, do graceful shutdown" SIGINT

but probably the desired command was

trap "echo SIGINT in inner, do graceful shutdown" SIGINT

The other question asks why the inner scripts trap function is invoked, however it is not. The sleep process is killed, the script moves onto the next line.

share|improve this answer

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges

add a comment | 

There are 2 questions.
The error in case 2 is because there is no command called SIGINT on the system. The OP had

trap "SIGINT in inner, do graceful shutdown" SIGINT

but probably the desired command was

trap "echo SIGINT in inner, do graceful shutdown" SIGINT

The other question asks why the inner scripts trap function is invoked, however it is not. The sleep process is killed, the script moves onto the next line.

share|improve this answer

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges

trap "SIGINT in inner, do graceful shutdown" SIGINT

trap "echo SIGINT in inner, do graceful shutdown" SIGINT

share|improve this answer

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges

answered 56 mins ago

icarusicarus

7,31911 gold badge1717 silver badges3333 bronze badges