Linux while read loop

Question

I have the following script:

#!/bin/bash

function doPing () {
    pings=0

    #cat /home/scripts/test.txt | while read server ; do
    #while [ $pings -le 3 ] ; do
        echo success1 $pings
        if [ 1 -eq 1 ]; then {
            pings=$(expr $pings + 1)
            echo success- $pings
        } else if [ 1 -eq 2 ]; then {
            pings=$(expr $pings + 1)
            echo not
        } else {
            pings=$(expr $pings + 1)
            echo known
        } fi
        fi

        echo success3 $pings
    done

    echo -e "\nSuccessfully pinged $pings.\n"
}

doPing

test.txt contains a few lines of server names, it does not matter actually.

My problem is that when I uncomment the line #while ..., I get:

success1 0
success- 1
success3 1
success1 1
success- 2
success3 2
success1 2
success- 3
success3 3
success1 3
success- 4
success3 4
success1 4
success- 5
success3 5

Successfully pinged 0.

but when I uncomment the line #cat ..., I get:

success1 0
success- 1
success3 1
success1 1
success- 2
success3 2
success1 2
success- 3
success3 3
success1 3
success- 4
success3 4

Successfully pinged 4.

How can I make it so that the #while output will be some number pinged, like #cat ..., not zero? Please help. Thanks.

This is knit-picky (and not your problem), but you may want to adjust #!/bin/bash to be #!/usr/bin/env bash. That's more portable to other systems as it uses bash that's specified by the environmental vars. As far as the issue, I bet someone will beat me to an answer - but I am looking at it locally now. — nerdwaller
– nerdwaller, Commented Oct 4, 2013 at 14:10

Lesmana · Accepted Answer · 2013-10-04 15:20:20Z

2

The problem is that the pipe creates a subshell and changes to a variable in a subshell does not propagate to the parent.

For more information read this explanation about variables in pipelines.

answered Oct 4, 2013 at 15:20

Lesmana

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1 Comment

Gordon Davisson Over a year ago

...and the solution is simple: remove the useless use of cat! Don't use a pipeline, use while ... done </home/scripts/test.txt.

andrej · Accepted Answer · 2013-10-05 05:19:01Z

0

This is what I did:

...
while read value ; do
...
done < /home/scripts/test.txt
...

… although I do not know what -r means after read.

answered Oct 5, 2013 at 5:19

andrej

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1 Comment

andrej Over a year ago

Yes, but what is the practical implication of this, that is what I do not get yet. Can it be that file then looks as one big line, even if in reality has more then one line? Thank you

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