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I have Postgres running in a docker container on a remote host. I'm attempting to ssh to that remote host, and use a heredoc to pass a script to that shell. In that script, I attempt to execute a psql query against the postgres database that resides in the container on that host, and assign the output to a variable. I'm sorry, I know that's a bit convoluted, but maybe some code will be more coherent (nor not)

If I run this, I get nothing out of my "echo" of $dbVersion... in fact, due to some other experimentation, I'm fairly convinced it never gets to the "echo" line at all, but for the life of me I don't know why, because...

ssh root@$serverIP13 <<EOF
postgresId=\$(docker ps | grep 'postgres_db' | awk '{print \$1}')
dbVersion=\$(docker exec -i \$postgresId psql -X -A -d myDb  -U myUser -t -c "SELECT Max(Version) FROM DbVersion;")
echo \$dbVersion
EOF

...after turning and twisting my code this way and that... I finally found a way that works, and I get a successful "echo" of my version number. I'm quite new to bash, so I'm hoping someone out there can discern this mystery for me and help me understand a bit better how bash works in this regard, and why the below succeeds, and the above fails.

ssh root@$serverIP13 <<EOF
postgresId=\$(docker ps | grep 'postgres_db' | awk '{print \$1}')
dbVersion=\$(echo "psql -X -A -d myDb -U myUser -t -c 'SELECT Max(Version) FROM DbVersion;'" | docker exec -i \$postgresId bash)
echo \$dbVersion
EOF
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  • Is the database listening on a port accessible from the network? It'd be much easier to run psql -h "$serverIP13" ... from your local system if so, skipping both the ssh and docker exec steps. Commented Aug 11, 2020 at 10:46
  • I always appreciate folks with common sense! The only port I have open is 22, as I'm trying to constrain my exposure, but... I'm going to add this to my backlog of things to try to do in a future sprint because... it is the simpler solution, and simpler solutions are less likely to break. Thank you for the suggestion. Commented Aug 11, 2020 at 21:21
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    ssh -L 5432:localhost:5432 root@$serverIP13 to port forward over the ssh connection could help too. That's a couple of network hops, but would let you use a local psql client. Commented Aug 11, 2020 at 23:56

2 Answers 2

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The normal way to get a docker exec -i output is to call for docker to run a shell (which, in turn, runs your command.
See "Bash / Docker exec: file redirection from inside a container" (I mentioned in 2015)

dbVersion=\$(docker exec -i sh -c '\$postgresId psql -X -A -d myDb  -U myUser -t -c "SELECT Max(Version) FROM DbVersion;"')
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1 Comment

I'll try this out when I get back online tonight. This seems like the "why" I was looking for
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Following script worked without invoking bash :

ssh root@$serverIP13 << 'EOF'
postgresId=$(docker ps | grep 'postgres_db' | awk '{print $1}')
dbVersion=$(docker exec $postgresId psql -X -A -d postgres -U postgres -c "select current_database()")
echo $dbVersion
EOF

I use 'EOF' to avoid having to escape.

To pass local values to remote :

ssh root@$serverIP13 /bin/bash -s 1 2 3 << 'EOF'
echo $1 $2 $3
EOF

You should not use -i if you put docker exec in $(...)

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2 Comments

Thank you for this. I often have variables coming into my script that need to be expanded, but in this case, I do not so good call. That said... is there a way to force variable expansion if you use a quoted 'EOF'?
No, then you have to use an unescaped EOF and quote everything else like in your original attempt.

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