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

[ckd]

I'd probably do that with perl -ne and a quick bit of string or regex manipulation followed by a rename() call; above a certain level of complexity, I stop trying to do everything in sh/bash.

(If it's something I'll use six months from now, I'll pay the setup time price and do it in Python so I can read it six months from now. For a one-off one-liner, Perl is fine.)

[pjc50.livejournal.com]

Agreed. Perl is also much less likely than bash to misbehave if you give it a file with a space or newline in the name.