output while loop in bash script

Question

I have ips file with content:

192.168.10.10 3306
192.168.10.20 3306

and my script is:

1 #!/bin/bash
  2
  3 p=0
  4 cat /root/ips | while read host port
  5  do
  6    check_up=$(bash -c 'exec 3<> /dev/tcp/'$host'/'$port';echo $?' 2>/dev/null)
  7     if [ $check_up != 0 ]
  8         then
  9           p=$[$p+1]
 10           echo "(1):p in loop = $p"
 11     fi
 12         echo "(2):p in loop = $p"
 13  done
 14      echo "(3):p out loop = $p"
 15
 16     if [ $p % 2 != 0 ]
 17        then
 18             exit 1
 19      fi
~

and out put is:

[root@db1 ~]# ./new-script.sh
(1):p in loop = 1
(2):p in loop = 1
(1):p in loop = 2
(2):p in loop = 2
(3):p out loop = 0
./new-script.sh: line 16: [: too many arguments

why echo "(3):p out loop = $p" return 0 (first value $p)!? when last value $p is 2 ? also, how to fix the error in line 16?

@Ignacio Vazquez-Abrams I did not understand you.what do you mean? — pyramid13, Mar 06 '18 at 15:52
You're asking two questions - you should only ask one at a time, for the benefit of other people who search for answers before asking themselves. It becomes difficult for searches to find the correct url results when you combine questions. — user1404316, Mar 06 '18 at 15:58

ilkkachu · Accepted Answer · 2018-03-06T16:16:44.433

2

The standard way to do arithmetic in the shell is $((..)). Standard test/[ supports only comparisons and other tests. So, the standard-conforming version would be:

if [ "$(( p % 2 ))" -ne 0 ]; then...

^{(That only needs the quotes if your IFS contains digits, so usually they're not needed.)}

In Bash/ksh/zsh, you could use the (( .. )) construct, which works like a command and allows the test too:

if (( p % 2 != 0 )); then ...

The [[ test seems to allow some arithmetic, but it's a bit picky about the syntax/whitespace, so you probably shouldn't do that.

As for why the assignment to p doesn't persist outside the loop, see

edited Mar 06 '18 at 16:16

answered Mar 06 '18 at 15:54

ilkkachu

138,973

Note that [ p%2 -ne 0 ] works in ksh. – Stéphane Chazelas Mar 06 '18 at 16:08
And in bash. test (and [) absolutely does do integer math if you actually use correct syntax, e. g. if [ $((2+2)) -eq 4 ]; then echo yep; fi. – DopeGhoti Mar 06 '18 at 16:14
@DopeGhoti, that's not test doing the addition, it's the arithmetic expansion that does it. – ilkkachu Mar 06 '18 at 16:18
I should have said, rahter, that [ and test can to integer arithmetic comparison (with the -eq, -ne, -ge, le, -lt, and -gt operators). – DopeGhoti Mar 06 '18 at 16:47

output while loop in bash script

1 Answers1