sh: neither as fast as sloths nor as elegant as hagfish

[fivemack]

I have a directory with 244 files with names like m12331246123468911531238951802368109467.mlog, which I want to rename to names like C038.123312.mlog

time for u in m*mlog; do B=$(echo $u | cut -dm -f2 | cut -d. -f1); echo $u C${#B}.$(echo $B | cut -c1-6).mlog; done

takes 17 seconds

time for u in m*mlog; do B=$(echo $u | cut -dm -f2 | cut -d. -f1); echo $u C${#B}.${B:0:6}.mlog; done

takes eight seconds

time for u in m*mlog; do B=${u:1}; B=${B%.mlog}; echo $u C${#B}.${B:0:6}.mlog; done

takes 0.2 seconds.

Of course, when I replace 'echo' with 'mv' it still takes fourteen seconds, but I am not that shocked that mv over NFS might be slow.

Which suggests that doing $() to start a new shell is taking something like a hundredth of a second on a one-year-old PC. I didn't know that. On the other hand, if I start writing code this dense in unclear bashisms, my colleagues at work will disembowel me with spoons.

PS: if I stop running a CPU-intensive program on each of my eight cores, starting new processes gets about fifteen times faster. I can understand if it got twice as fast, but I really don't understand fifteen.

Comments

[pm215]

On the other hand, if I start writing code this dense in unclear bashisms, my colleagues at work will disembowel me with spoons.

I think that if your code is preceded by a comment which says 'for each filename of format m12331246123468911531238951802368109467.mlog, print "C038.123312" where the number before the dot is the length of the original digit string and the number after is its first six digits', then it's reasonably clear; if it doesn't then it's pretty unclear whichever variant you use. (In particular with a comment it's immediately clear to the reader whether they actually need to check the implementation against the intention in order to achieve whatever goal they had in mind when they started reading the file, and there's a record of what your intention actually was!)

[emperor]

+1