I'm trying to learn how to use the signal trap command on Bash. The trap command is quite easy to use, but the issue that hangs in my head is the importance of using single and double quotes.
What is the difference between the single and double quotes used with the trap command ?
Can you give a few simple examples that clearly show the difference between the two uses ?
The trap command just takes string to be processed as shell commands, like eval. That string will be processed for expansions, so the difference between double and single quotes is when the first expansions happen. In double-quotes, they happen when the trap is set, in single quotes when it triggers.
E.g. this will print foo=1 when the read is interrupted, but with single quotes it would print foo=2:
#!/bin/bash foo=1 trap "echo foo=$foo; exit" INT foo=2 read # hit Ctrl-C here Of course, even with double quotes, the resulting command gets expanded also when the trap triggers, so having foo=$0 on the line before the trap above, would result in the script printing the script's name on interrupt.
trapcommand wrt. single or double quotes. If in doubt, use a function:deb(){ echo "' ${BASH_COMMAND[@]}"; }; trap deb DEBUG