Why is ls much slower than echo *?

Question

(Inspired by the comments for this question)

Why is ls this much slower than echo *?

$ time bash -c 'for i in {1..10000}; do ls -f > /dev/null; done'
bash -c 'for i in {1..10000}; do ls -f > /dev/null; done' 7.49s user 5.39s system 108% cpu 11.883 total

$ time bash -c 'for i in {1..10000}; do echo * > /dev/null; done'
bash -c 'for i in {1..10000}; do echo * > /dev/null; done' 0.16s user 0.25s system 98% cpu 0.415 total

I mean, sure, ls being slightly slower than echo * makes sense, but this seems ridiculous.

Why would ls be nearly be the case?

If it has to do with ls not needing the speed, then there's no reason for yes to be so fast.

What is ls doing under the hood that makes it so slow?

(I'm using zsh on Arch Linux, on a laptop. Times are similar on bash. I've also tested on a (Linux Mint) desktop computer with similar results. (There, ls -f is faster at around 7 seconds, echo * is still about 0.4 seconds, which is still a ridiculous difference.) On Arch Linux, ls was also unaliased, but I didn't bother for Mint, and it didn't make a difference anyway.)

If you replace ls with /bin/ls and echo with /bin/echo in your tests, is there still a large difference? — Mark Plotnick, May 13 '20 at 03:42
A follow up to @MarkPlotnick point, your echo is probably using the shell's built-in while ls is forking a new process each time. Changing echo to /bin/echo will cause the script to fork and exec that program. That'll give you more of an apples-to-apples comparison. — Andy Dalton, May 13 '20 at 03:47
@MarkPlotnick Not too sure what the convention is, do you want to add that as an answer or should I answer it myself and accept it? — FaeFeyFa, May 13 '20 at 03:54
Also for comparison, try using something like busybox, where both echo and ls are essentially builtins — muru, May 13 '20 at 03:55
@FaeFeyFa you didn't use any quotes in the post. Andy added those. — muru, May 13 '20 at 03:56

score 0 · Answer 1 · answered May 13 '20 at 03:57

Using strace, we can see echo makes far less syscalls to the kernel compared to ls:

root@4a21b0630cba:/# strace -c ls
bin  boot  dev  etc  home  lib  lib32  lib64  libx32  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0         7           read
  0.00    0.000000           0         1           write
  0.00    0.000000           0        10           close
  0.00    0.000000           0         9           fstat
  0.00    0.000000           0        26           mmap
  0.00    0.000000           0         9           mprotect
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         2           rt_sigaction
  0.00    0.000000           0         1           rt_sigprocmask
  0.00    0.000000           0         2           ioctl
  0.00    0.000000           0         8           pread64
  0.00    0.000000           0         2         2 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2         2 statfs
  0.00    0.000000           0         2         1 arch_prctl
  0.00    0.000000           0         2           getdents64
  0.00    0.000000           0         1           set_tid_address
  0.00    0.000000           0         8           openat
  0.00    0.000000           0         1           set_robust_list
  0.00    0.000000           0         1           prlimit64
------ ----------- ----------- --------- --------- ----------------
100.00    0.000000                    99         5 total
root@4a21b0630cba:/# 
root@4a21b0630cba:/# 
root@4a21b0630cba:/# 
root@4a21b0630cba:/# strace -c echo *
bin boot dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0         1           read
  0.00    0.000000           0         1           write
  0.00    0.000000           0         4           close
  0.00    0.000000           0         3           fstat
  0.00    0.000000           0         7           mmap
  0.00    0.000000           0         4           mprotect
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         6           pread64
  0.00    0.000000           0         1         1 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2         1 arch_prctl
  0.00    0.000000           0         2           openat
------ ----------- ----------- --------- --------- ----------------
100.00    0.000000                    36         2 total
root@4a21b0630cba:/#

Why is ls much slower than echo *?

1 Answers1