Capturing SSH output as variable in bash script

Question

I've been struggling with this problem when writing a bash script. Basically, I want to measure the time of a program on a remote server, so I use the command: /usr/bin/time -f %e sh -c "my command > /dev/null 2>&1" to execute the program. However, it appears that I cannot capture the output of my command (SSH) to a variable at all. In fact, the result (time) keeps getting printed out to stdout.

The full code is:

respond=$(ssh ${fromNode} /usr/bin/time "-f" "%e" "'sh' '-c' 'virsh migrate --live ${VM} qemu+ssh://${toNode}/system --verbose > /dev/null 2>&1'")

The value of respond is just empty, though the time is printed out to the standard output.

If you want to capture the result of a MySql query into an array, you can do this: stackoverflow.com/a/57061108/470749 — Ryan
– Ryan, Commented Jul 16, 2019 at 15:49

Rogach · Accepted Answer · 2012-08-21 06:35:43Z

51

"time" command prints result to stderr, not to stdout. Thus it is not piped into your variable.

You should reroute stderr to stdout to achieve what you want:

 result=$(ssh host time "command" 2>&1)

And your full code can look something like this:

 respond=$(ssh ${fromNode} /usr/bin/time "-f" "%e" "'sh' '-c' 'virsh migrate --live ${VM} qemu+ssh://${toNode}/system > /dev/null 2>&1'" 2>&1)

edited Aug 21, 2012 at 6:35

answered Aug 21, 2012 at 5:39

Rogach

27.5k21 gold badges100 silver badges174 bronze badges

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

1 Comment

Andy Dang Over a year ago

Great. Works exactly as I wanted it to. Thank you very much for your great help! And by the way, I just simply use &> /dev/null for the virsh migrate part since I intended to discard the output anyway

nneonneo · Accepted Answer · 2012-08-21 05:37:45Z

3

Try swapping the order of your redirections around (to 2>&1 >/dev/null). Your current code is sending both stdout and stderr to /dev/null (so I'm kind of curious as to why anything is printed at all).

Why is this necessary? The syntax 2>&1 means 'duplicate stdout (descriptor 1) as stderr (descriptor 2)'; in effect, stderr is made into a copy of the current stdout. If you put >/dev/null first, then stdout is first redirected to /dev/null, and then stderr is pointed at the current stdout, i.e. /dev/null.

But if you put >/dev/null second, stderr will first become a copy of the current stdout (the normal output stream), before stdout is redirected. So the command's stderr prints to the tty (or the interpreter) as if it came from stdout, while stdout is silenced. This is the behaviour you want.

From man bash:

Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.

answered Aug 21, 2012 at 5:37

nneonneo

181k37 gold badges331 silver badges412 bronze badges

6 Comments

Rogach Over a year ago

Those redirections affect only 'virsh' command (silencing it), not the output from 'time' command.

Rogach Over a year ago

Though I'm curious, as well, why output of command with 'verbose' flag should be silenced :)

nneonneo Over a year ago

Ah, you are right. I misread the command. Seems my quote-matching skills need improvement :)

Rogach Over a year ago

By the way, great explanation - I always wondered about this behavior, your answer explained things.

Andy Dang Over a year ago

The verbose option is to force the virsh migrate command to keep the process foreground. It's not really needed, but just in case some virsh version may send the process into background.

|

Collectives™ on Stack Overflow

Capturing SSH output as variable in bash script

2 Answers 2

1 Comment

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related