Bash – Why can’t I capture AWS EC2 CLI Output in Bash

aws-clibashshell

I am trying to capture the output of an aws ec2 delete-snapshot in a Bash script command but I cannot get anything to capture the output. I have tried result=$(command), result=`command` etc. but when I try to echo $result there is nothing there.

Here is some example output. 

root@host:~# aws ec2 delete-snapshot --snapshot-id vid --output json>test

A client error (InvalidParameterValue) occurred when calling the DeleteSnapshot operation: Value (vid) for parameter snapshotId is invalid. Expected: 'snap-...'.
root@host:~# aws ec2 delete-snapshot --snapshot-id vid>test

A client error (InvalidParameterValue) occurred when calling the DeleteSnapshot operation: Value (vid) for parameter snapshotId is invalid. Expected: 'snap-...'.
root@host:~# cat test
root@host:~# testing=$(aws ec2 delete-snapshot --snapshot-id vid)

A client error (InvalidParameterValue) occurred when calling the DeleteSnapshot operation: Value (vid) for parameter snapshotId is invalid. Expected: 'snap-...'.
root@host:~# echo $testing

root@host:~#

I need to automate creation and deletion of snapshots, but I can't capture the output.

Has anyone else ran into this issue?

Best Answer

The > operator only redirects stdout ("standard output"), or "file descriptor 1". Error messages are usually printed on a different file descriptor, 2, stderr, ("standard error"). On your terminal screen, you are seeing both stdout and stderr.

The > operator is really more just a shortcut for 1>, and again, only redirects stdout. The 2> operator is similar to 1> but it instead of redirecting stdout, it redirects stderr.

user@host$ echo hello world >/dev/null
user@host$ echo hello world 1>/dev/null
user@host$ echo hello world 2>/dev/null
hello world
user@host$

So, to redirect both stdout and stderr to the same file, use >file 2>&1. 

user@host$ echo hi 2>/dev/null 1>&2
user@host$

This says, "redirect the echo's stderr to /dev/null, and redirect stdout to stderr. 

user@host$ curl --invalid-option-show-me-errors >/dev/null
curl: option --invalid-option-show-me-errors: is unknown
try 'curl --help' or 'curl --manual' for more information

user@host$ curl --invalid-option-show-me-errors 2>/dev/null
user@host$ 
user@host$ curl --invalid-option-show-me-errors >/dev/null 2>&1
user@host$

In modern Bash, you can also use &> to redirect both streams to the same file:

user@host$ curl --invalid-option-show-me-errors &>/dev/null
user@host$

So for you, specifically, use:

aws ec2 delete-snapshot --snapshot-id vid --output json >test 2>&1

Or

aws ec2 delete-snapshot --snapshot-id vid --output json &>test
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