Getting multiple variables from the output of docker exec command in a bash script?

Question

I would like to execute a script on a host machine (script_on_host.sh), which then reaches inside a docker container in order to grab some data using a second script (script_in_container.sh). The data is only available inside the container, and not from the host directly, hence the reason for doing it this way.

I am having trouble passing data between the two scripts. I tried the following:

script_in_container.sh

#!/bin/bash

x=1
y=2
z=6.3
echo $x
echo $y
echo $z

script_on_host.sh

#!/bin/bash

results=$(docker exec mycontainer './script_in_container.sh')
X=$results(1)
Y=$results(2)
Z=$results(3)

But it throws an error. Can someone help me with this? Is there a nice or more standard way to go about passing variables in this way? I feel like I'm maybe doing something a little clumsily here, by using echo to print multiple lines from the container script.

Answer 1

You could use readarray to build an array of results delimited by newline.

readarray -t results < <(docker exec mycontainer './script_in_container.sh')
x=${results[0]}
y=${results[1]}
z=${results[2]}

Answer 2

The simplest solution would be to use set -- to assign variables from the docker container to the positional parameters like this:

#!/bin/bash

results=$(docker exec mycontainer './script_in_container.sh')

saveIFS=$IFS
IFS=$'\n'
set -- $results
IFS=$saveIFS

echo result 1: "$1"
echo result 2: "$2"
echo result 3: "$3"

Answer 3

You can use sed, like this:

#!/bin/bash

results=$(docker exec mycontainer './script_in_container.sh')
X=$(echo "$results" |sed '1q;d')
Y=$(echo "$results" |sed '2q;d')
Z=$(echo "$results" |sed '3q;d')

The sed expression is actually two commands:

• 1q is a command to match the first line and quit
• ; is a command separator
• d deletes the current line.

In the case of reading the first line the d is kind of redundant since sed will quit after it prints line 1 anyway but for other lines the d is necessary to delete any lines that do not match your first command.

Answer 4

There are two fully-standards-compatible solutions, and they'll also be the fastest because they don't fork/exec more than necessary:

#!/bin/sh
docker exec mycontainer './script_in_container.sh' | {
  IFS= read -r X
  IFS= read -r Y
  IFS= read -r Z
  echo X=$X Y=$Y Z=$Z
}

(you can omit the IFS= prefix if you don't care about stripping initial whitespace) and

#!/bin/sh
Z="$(docker exec mycontainer './script_in_container.sh')"
X="${Z%%$'\n'*}"
Z="${Z#*$'\n'}"
Y="${Z%%$'\n'*}"
Z="${Z#*$'\n'}"
echo X=$X Y=$Y Z=$Z

%% is "remove matches recursively from the end", # is "remove one match from the start`, so removing all matches of [newline, anything] removes every line but the first, and removing one match of [anything, newline] removes the first line.

The $'\n' spelling is new in POSIX.1-2024, and not universally available; you can spell it as

'
'

(so,

X="${Z%%'
'*}"

) to increase your compatibility range.