Script queries

LYC · 03-28-2020, 08:59 PM

Hi,

When I execute the following in command line, it works

$ for i in {1..930}; do date >> output.log; prstat -a 1 1 >> output.log; sleep 60; done

When I put them in a file as follows and run the file, it only run once:

#!/usr/bin/sh

for i in {1..930}
do
date >> output.log
prstat -a 1 1 >> output.log
sleep 60
done

Seeking for advice?

MadeInGermany · 03-28-2020, 10:26 PM

The { } expansion is bash - not standard shell.

Code:

#!/bin/bash

for i in {1..930}
do 
  date
  prstat -a 1 1
  sleep 60
done >> output.log

Also it is more efficient to redirect the whole loop, so the output file is opened once when the loop starts (and closed when the loop ends).

Turbocapitalist · 03-28-2020, 10:58 PM

POSIX shell is guaranteed portable and faster. But Bash has more features and is the default on most GNU/Linux systems and older OS X.

So if you need portability to run this on a BSD, avoid functions unqiue to bash:

Code:

#!/bin/sh

for i in $(seq 1 930)
do 
  date
  prstat -a 1 1
  sleep 60
done >> output.log

Edit: Oops. That is POSIX but not working on OpenBSD because of the lack of seq which is part of the coreutils package. Here's an actually portable version.

Code:

#!/bin/sh

i=0
while test $i -lt 930
do 
  date
  prstat -a 1 1
  i=$(($i+1))
  sleep 60
done >> output.log

pan64 · 03-29-2020, 03:30 AM

offtopic, but I think $(seq 1 930) is definitely slower than {1..930}.
I would prefer your second solution.

Turbocapitalist · 03-29-2020, 03:33 AM

Quote:

Originally Posted by pan64

offtopic, but I think $(seq 1 930) is definitely slower than {1..930}.

Quite right. The part about POSIX being faster should be constrained to mean when external programs are avoided.

MadeInGermany · 03-29-2020, 04:39 AM

The POSIX compliant

Code:

i=0
while test $i -lt 930
do
  i=$((i+1))
  echo $i
done
echo $i

behaves like

Code:

for i in {1..930}

or

Code:

for i in $(seq 1 930)

To complete this discourse, I want to also mention the posixly correct

Code:

i=0
while test $((i+=1)) -le 930
do
  echo $i
done
echo $i

that behaves like

Code:

for ((i=1; i<=930; i++))