Shell – Why is “echo” so much faster than “touch”

commandechoshell

I'm trying to update the timestamp to the current time on all of the xml files in my directory (recursively). I'm using Mac OSX 10.8.5.

On about 300,000 files, the following echo command takes 10 seconds:

for file in `find . -name "*.xml"`; do echo >> $file; done

However, the following touch command takes 10 minutes! : 

for file in `find . -name "*.xml"`; do touch $file; done

Why is echo so much faster than touch here?

Best Answer

In bash, touch is an external binary, but echo is a shell builtin:

$ type echo
echo is a shell builtin
$ type touch
touch is /usr/bin/touch

Since touch is an external binary, and you invoke touch once per file, the shell must create 300,000 instances of touch, which takes a long time.

echo, however, is a shell builtin, and the execution of shell builtins does not require forking at all. Instead, the current shell does all of the operations and no external processes are created; this is the reason why it is so much faster.

Here are two profiles of the shell's operations. You can see that a lot of time is spent cloning new processes when using touch. Using /bin/echo instead of the shell builtin should show a much more comparable result.

Using touch

$ strace -c -- bash -c 'for file in a{1..10000}; do touch "$file"; done'
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 56.20    0.030925           2     20000     10000 wait4
 38.12    0.020972           2     10000           clone
  4.67    0.002569           0     80006           rt_sigprocmask
  0.71    0.000388           0     20008           rt_sigaction
  0.27    0.000150           0     10000           rt_sigreturn
[...]

Using echo

$ strace -c -- bash -c 'for file in b{1..10000}; do echo >> "$file"; done'
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 34.32    0.000685           0     50000           fcntl
 22.14    0.000442           0     10000           write
 19.59    0.000391           0     10011           open
 14.58    0.000291           0     20000           dup2
  8.37    0.000167           0     20013           close
[...]
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