bash_ssh_break_out_of_while_loop.md

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SSh Break Out While-Loop	bash	bash, ssh	May 28, 2020	Bash While-loop will be breaked by SSH	mydoc_sidebar	bash_ssh_break_out_of_while_loop.html	bash

SSh Break Out While-Loop

=====

Issue

$ cat sn_verify1.sh
#!/bin/bash

line=$1

server=$(echo ${line} | awk '{print $1}')
local_sn=$(echo ${line} | awk '{print $2}' )

remote_sn_temp=$(ssh ${server} -q "dmidecode -t system | sed -ne '/Serial Number/s/.*: //p'")
remote_sn=${remote_sn_temp%% *}

echo ${server}
if [[ ${local_sn} == ${remote_sn} ]]; then echo same; else echo mismatch; fi

$ cat /tmp/sorted_test
test001.exmple.net CZJ14113TT
test002.exmple.net CZJ14113PM

$ while read -r line ; do ./sn_verify1.sh "${line}"; done < /tmp/sorted_test
test001.exmple.net
same

Root cause

ssh reads from standard input, therefore it eats all remaining lines. We can just connect its standard input to nowhere

#!/bin/bash

line=$1

server=$(echo ${line} | awk '{print $1}')
local_sn=$(echo ${line} | awk '{print $2}' )

remote_sn_temp=$(cat /dev/null | ssh ${server} -q "dmidecode -t system | sed -ne '/Serial Number/s/.*: //p'" )
remote_sn=${remote_sn_temp%% *}

echo ${server}
if [[ ${local_sn} == ${remote_sn} ]]; then echo same; else echo mismatch; fi

$ while read -r line ; do ./sn_verify1.sh "${line}"; done < /tmp/sorted_test
test001.exmple.net
same
test002.exmple.net
same

or apply -n option with ssh

-n      Redirects stdin from /dev/null (actually, prevents reading from stdin).  This must be used when ssh is run in the background. A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background.  (This does not work if ssh needs to ask for a password or passphrase; see also the -f option.)

$ cat sn_verify1.sh
#!/bin/bash

line=$1

server=$(echo ${line} | awk '{print $1}')
local_sn=$(echo ${line} | awk '{print $2}' )

remote_sn_temp=$(ssh ${server} -qn "dmidecode -t system | sed -ne '/Serial Number/s/.*: //p'" )
remote_sn=${remote_sn_temp%% *}

echo ${server}
if [[ ${local_sn} == ${remote_sn} ]]; then echo same; else echo mismatch; fi

$ while read -r line ; do  ./sn_verify1.sh "${line}"; done < /tmp/sorted_test
test001.exmple.net
same
test002.exmple.net
same

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