bash while loop breaks after the first line

Question

I have a simple script with while loop, but cannot understand, why it breaks after first line, from $vault_list variable:

#!/bin/bash

tech_login="$1"
vault_list=$(docker exec -i tmgnt_vault_1 vault list secret/${tech_login}-terminals | sed 1,2d)

while IFS= read -r terminal
do
    echo "line is $terminal"
    key_values=$(docker exec -i tmgnt_vault_1 vault read secret/${tech_login}-terminals/$terminal )


done <<< "$vault_list"

If I remove $key_values from while loop, it returns all values in echo "line is $terminal". Can anyone point me, what is the problem with while loop? I assume, that this can be a problem with output, but not sure.

Does docker exec -i tmgnt_vault_1 vault read secret/${tech_login}-terminals/$terminal work if you run it separately? — Geoff
– Geoff, Commented Oct 11, 2018 at 11:03
Docker or the command you run inside it eats the rest of stdin, so there's nothing left to read in the next iteration. — choroba
– choroba, Commented Oct 11, 2018 at 11:13

gaoithe · Accepted Answer · 2019-11-13 17:03:20Z

4

Hopefully this will help others.
ssh might be the command that is eating stdin.
It was for me.

e.g. ssh inside a while loop was causing the loop to exit after first iteration.

LIST="cid1 10.10.0.1 host1
cid2 10.10.0.2 host1
cid3 10.10.0.3 host2"

# this while loop exits after first iteration
# ssh has eaten rest of stdin
echo "$LIST" |while read -r cid cip chost; do
    echo $cid;
    PSINFO=$(ssh $chost docker exec -i $cid "ps -e -orss=,pid=,args=,cmd=" |grep java );
    echo PSINFO=$PSINFO; 
done;

SOLVED by directing ssh to take stdin from /dev/null using </dev/null:

# this while loop keeps on running
# ssh directed to take stdin from /dev/null
echo "$LIST" |while read -r cid cip chost; do
    echo $cid;
    PSINFO=$(ssh $chost docker exec -i $cid "ps -e -orss=,pid=,args=,cmd=" </dev/null |grep java );
    echo PSINFO=$PSINFO; 
done;

answered Nov 13, 2019 at 17:03

gaoithe

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2 Comments

Ouss Over a year ago

Yes. for sure, ssh consumes the stdin. You can use the -n flag to as in "ssh -n" instead of < /dev/null. The code will look nicer and more readable. Check man.openbsd.org/ssh.1 and look for -n.

gaoithe Over a year ago

InteReStinG. Thank you. That looks like a proper solution rather than a hack redirecting /dev/null. Good to read up on ssh -n and -f flags.

Vasyl Stepulo · Accepted Answer · 2018-10-11 11:35:17Z

2

With hint from @choroba I found right syntax for $key_values:

key_values=$(docker exec -i tmgnt_vault_1 vault read secret/${tech_login}-terminals/$terminal <<<$terminal)

I was need to pass the $terminal variable explicitly to the docker command, which can be done with a here-string, "<<

edited Oct 11, 2018 at 11:35

answered Oct 11, 2018 at 11:27

Vasyl Stepulo

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Comments

artickl · Accepted Answer · 2024-04-22 23:01:45Z

Had the same issue with aws ecs execute-command. While the loop was interrupted right after the first execution. Unfortunately, it does not have -n flag, but a trick with < /dev/null helped.

So, the correct way to run a command for each task in the cluster is:

$ aws ecs list-tasks --cluster prod-cluster --service prod-service --region prod-region \
    | grep arn:aws:ecs:prod-region \
    | sed 's_.*/__g;s/[",]//g' \
    | while read task; do 
        echo $task; 
        aws ecs execute-command --cluster prod-cluster --task ${task} --container prod-image --command "uptime" --interactive < /dev/null \
            | grep 'load average'; 
    done

It will show uptime for all tasks running currently in the cluster

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