Clash Royale CLAN TAG

Why is the DEBUG trap executed more times than expected?

The following code triggers the debug trap 3 times. I am however expecting only 2 executions - one for foo, the second for echo hello:

foo

echo hello

foo() {
echo hello
}

set -T
trap 'echo oops' DEBUG
foo


Output:

oops
oops
oops
hello


Expected output:

oops
oops
hello


Bash versions tested:

Bash

GNU bash, version 4.3.30(1)-release (x86_64-unknown-linux-gnu)
GNU bash, version 4.4.19(1)-release (x86_64-pc-linux-gnu)
GNU bash, version 5.0.0(1)-alpha (x86_64-pc-linux-gnu)


I am pretty sure I just misunderstood the manual and that I am missing something really simple/obvious here.

1 Answer
1

I assume you understand that DEBUG trap enables running the command before every other command and set -T enables the trap to all sub-shells and functions also. So once you set the trap you have three executions happening.

DEBUG

set -T

trap

foo

trap

echo oops

trap

echo

trap

echo

trap

hello

trap

See the sequence when ran from the debug mode

$ bash -x script.sh
+ set -T
+ trap 'echo oops' DEBUG
++ echo oops # <--- triggered by call to 'foo'
oops
+ foo
++ echo oops # <--- triggered by call to 'echo oops' inside trap definition
oops
++ echo oops # <--- result of the actual command to be run
oops
+ echo hello # <--- result of the function call 'foo'
hello


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