Thanks for the tip! I don't have experience with strace, and I'm wondering if I'm misunderstanding what you're saying.

I tried running your suggested command, and I got 2,800 lines of output like this:

    execve("/nix/store/vqvj60h076bhqj6977caz0pfxs6543nb-bash-5.2-p15/bin/sh", ["sh", "-c", "echo \"60016000526001601ff3\" | xx"...], 0x7fffffffcf38 /* 106 vars */) = 0
    brk(NULL)                               = 0x4ea000
    arch_prctl(0x3001 /* ARCH_??? */, 0x7fffffffcdd0) = -1 EINVAL (Invalid argument)
    access("/etc/ld-nix.so.preload", R_OK)  = -1 ENOENT (No such file or directory)
    openat(AT_FDCWD, "/nix/store/aw2fw9ag10wr9pf0qk4nk5sxi0q0bn56-glibc-2.37-8/lib/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
    read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
    newfstatat(3, "", {st_mode=S_IFREG|0555, st_size=15688, ...}, AT_EMPTY_PATH) = 0
    mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7fc2000
    mmap(NULL, 16400, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ffff7fbd000
    mmap(0x7ffff7fbe000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1000) = 0x7ffff7fbe000

Am I doing it wrong? Or is there more training involved before one could usefully integrate this into debugging? Because to me, the output is pretty inscrutable.

There is a lot of output here, but you can grep around or filter with strace CLI. If you used -f option you should get PID numbers later on. Then you can look for all execve's to see how PIDs map to parts of the pipeline. For now maybe grep the log file with something like: "grep -e clone -e execve -e write -e read". You can do this with strace CLI, but I never remember the syntax and usually analyze the log extensively.

I think something like this could work:

  strace -f -e execve,clone,write,read -o strace.log sh -c '...'

Clone is fork, so a creation of a new process, before eventual execve (with echo there will probably be just clone).

strace tells you every syscall the process under it makes. So very helpful to understanding how a program interacts with the operating system - and I/O as all IO mechanisms are managed by the operating system.

As for how to filter this I'll leave that to the other comments, but I personally would look at the man page or Google around for tips