CODEWITHSUNDEEP
linuxbashshellwget
nslam
nslam

Reputation: 21

remote wget command with & symbol doesn't behave as expected

Here are some test results:

I run command on my localhost, and try to execute some command on the remote host 11.160.48.88

Command 1:

ssh 11.160.48.88 "wget https://raw.githubusercontent.com/mirror/wget/master/README -O wgetReadme"

expect:

File can be downloaded and be renamed to wgetReadme

result:

work as expected

Command 2:

ssh 11.160.48.88 "wget https://raw.githubusercontent.com/mirror/wget/master/README -O wgetReadme&"

I simply add the & at the end of command, because I want this command to run in background

result:

the file wgetReadme is null on the remote server, I don't know why

Command 3:

To test if the Command 2 can be run on the remote server, I try to run the command directly on the server 11.160.48.88

wget https://raw.githubusercontent.com/mirror/wget/master/README -O wgetReadme&"

result: There are some wget transport message print to stdout, and the file is downloaded to wgetReadme. Work corretly.

Command 4:

I want to figure out if it is the SIGHUP signal kill the subprocess, and I found two evidences to prove it is not.

  1. I found this question, and I try to run this on remote server 11.160.48.88
$shopt|grep hup
huponexit       off

So the subprocess will not receive SIGHUP when ssh exits

  1. I try to run another command to prove it

ssh 11.160.48.88 "wget https://raw.githubusercontent.com/mirror/wget/master/README -O - 2>&1 > wgetReadme&"

result: The file can be downloaded to the target file correctly.

My question is why Command 2 cannot work as expected?

Upvotes: 1

Views: 417

Answers (2)

Fatih Şennik
Fatih Şennik

Reputation: 1325

Because backgrounded jobs in ssh can cause the shell to hang on logout due to a race condition that occurs when two or more threads can access shared data and they try to change it at the same time and you can also solve the problem by redirecting all three I/O streams such as > /dev/null 2>&1 & So Nohup command is useful in your case and it is a POSIX command to ignore the HUP (hangup) signal. The HUP signal is, by convention, the way a terminal warns dependent processes of logout. So I change your code as following way:

ssh -f 11.160.48.88 "sh -c 'nohup wget https://raw.githubusercontent.com/mirror/wget/master/README -O - > wgetReadme  2>&1 &'"

You can read more at https://en.wikipedia.org/wiki/Nohup

Upvotes: 3

abonne01
abonne01

Reputation: 161

& is a bash special characters which make process running in background. Then , ssh will not capture anymore output of command when you run this remotely.

You should escape it with \ to be able to run your command

in your example : wget https://raw.githubusercontent.com/mirror/wget/master/README -O wgetReadme\&"

regards

Upvotes: 0

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