How capture all bash output (from inside a script) into variable and then turn output back to stdout?

Question

I have a bash script that runs a bunch of other commands (e.g. docker). I want the script to be able to capture all the output into a variable and then echo out a custom return at the end.

Example:

#!/usr/bin/env bash
set -euo pipefail

# Capture into this (PSEUDO CODE)
declare CapturedOutput
$(Capture Output > CapturedOutput)

# Run some commands like...
docker-compose ... up -d

# Stop capturing (PSEUDO CODE)
$(Stop Capture Output > CapturedOutput)

echo "something"

So if someone called my script like ./runit.sh and the docker command had output, they wouldn't see it but would only see:

> ./runit.sh

something

Possible duplicate of Capture output of a bash command, parse it and store into different bash variables — Léa Gris
– Léa Gris, Commented Aug 14, 2019 at 19:21
@LéaGris It's not a duplicate. That's capturing the output from outside the script. I want to capture it inside — Don Rhummy
– Don Rhummy, Commented Aug 14, 2019 at 19:22
Is it just the output of docker you want to capture in a variable? — David C. Rankin
– David C. Rankin, Commented Aug 14, 2019 at 19:29
Are you actually going to use CapturedOutput? Redirecting a command group to /dev/null might be sufficient. — chepner
– chepner, Commented Aug 14, 2019 at 19:33
@DavidC.Rankin No, I want to redirect the output of all the commands I run in the script — Don Rhummy
– Don Rhummy, Commented Aug 15, 2019 at 14:02

John Bollinger · Accepted Answer · 2019-08-14 20:09:00Z

1

The most straightforward way to capture output into a variable is to use command substitution. You can easily wrap that around a large chunk of script:

#!/usr/bin/env bash
set -euo pipefail

# To capture standard error, too:
# exec 3>&2 2>&1

CapturedOutput=$(

# Run some commands like...
docker-compose ... up -d

)

# To restore standard error:
# exec 2>&3- 

echo "something"

The caveat is that the commands from which output is being captured run in a subshell, but I'm having trouble coming up with an alternative for capturing into a variable in the same shell in which the commands themselves run.

answered Aug 14, 2019 at 20:09

John Bollinger

191k11 gold badges103 silver badges207 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Don Rhummy Over a year ago

Would this work differently than wrapping it all in a function?

John Bollinger Over a year ago

I'm uncertain what specific alternative you have in mind, @DonRhummy. You could put docker-compose and all the other commands whose output is to be captured in a function, and that might indeed be a sensible thing to do, but you would still need to use command substitution -- either inside the function or of the function call -- to capture the output to a variable.

Don Rhummy Over a year ago

Unfortunately, this sin't working. I think because it's in a subshell now, docker is unable to mount the volumes. I get the error:

ERROR: for redis  Cannot start service redis: error while creating mount source path '/host_mnt/c/Users/myname/redis-test/customstartup.sh': mkdir /host_mnt/c: file exists

John Bollinger Over a year ago

@DonRhummy, I believe you that your particular script, written with use of a command substitution as I have described, does not successfully do the job you want it to do. It's not at all clear, however, why running the commands in a subshell would be directly responsible for the particular error described in that message, and in fact I find the prospect dubious. The issues with use of subshells largely revolve around (not) communicating information back to the parent shell, and especially around redirections and shell variable modifications performed in the subshell not affecting the parent.

Don Rhummy Over a year ago

I know, but when I run it not in a subshell it works, but the moment I put it in a subshell it throws the error

|

Jeff Y · Accepted Answer · 2019-08-14 20:08:51Z

0

I think you want to look at the "Command Substitution" section on bash's man page.

To translate the pseudocode in your question to bash's format, something like:

CapturedOutput=$(docker-compose ... up -d)
CapturedOutput=$CapturedOutput $(docker-compose ... up -d)
...

answered Aug 14, 2019 at 20:08

Jeff Y

2,4661 gold badge14 silver badges18 bronze badges

Collectives™ on Stack Overflow

How capture all bash output (from inside a script) into variable and then turn output back to stdout?

2 Answers 2

6 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related