Brace expansion

Installing and Using OpenWrt
1

Asked in r/openwrt but was left without an answer.

Can someone explain why brace expansion is so slooow in openwrt 24.10 compared to 23.05.05

To reproduce generate a colon delimited 3000 line file.

    for i in {1..3000}; do
    echo “$i:$i:$i:$i:$i:$i” >> myfile.txt
    done

Btw u have to explicitly call bash for aforementioned script to work

Now make another script

    #!/bin/bash
    my_file=$(< “myfile.txt”)
    my_file=${my_file//:/ }
    echo “$my_file”

And try same with

    my_file=$(tr ‘:’ ‘ ‘ <<<$my_file)

On ubuntu laptop all the test show that expansion is faster than with tr. in openwrt its exact opposite. In theory expansion should work faster too because it doesnt spawn a new process to deal with it but does everything in the same process so no subshell nothing.

I can confirm that expansion was working faster in 23.05.05

2 
for q in `seq 1 1000` ...
1 Like
3

Im sorry but what this has to do with my question?

1 Like
4

Bash is not primary shell in OpenWrt, there are dozen ways to squeeze regex from bash.
Do some ltrace/strace mayb€ you find your answer. sed s/:/\ /g is even faster, i cannot get amy of samples to scratch measurable time.

5

Unfortunately there are no library or system calls in brace expansion it relies solely on bash to do that. So strace or ltrace are useless in this case.

6

Absolutely not specific to OpenWrt.

7

I would agree but why its working with 23.05.05? Exact same bash version.