I’ve been coding a service which communicates over SSH today. I ran into a issue when trying to run long commands and listening to their output. It was kind of tiresome to find out all tricks to do, so here’s my code:
import sys
import time
import select
import paramiko
host = 'test.example.com'
i = 1
#
# Try to connect to the host.
# Retry a few times if it fails.
#
while True:
print "Trying to connect to %s (%i/30)" % (host, i)
try:
ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect(host)
print "Connected to %s" % host
break
except paramiko.AuthenticationException:
print "Authentication failed when connecting to %s" % host
sys.exit(1)
except:
print "Could not SSH to %s, waiting for it to start" % host
i += 1
time.sleep(2)
# If we could not connect within time limit
if i == 30:
print "Could not connect to %s. Giving up" % host
sys.exit(1)
# Send the command (non-blocking)
stdin, stdout, stderr = ssh.exec_command("my_long_command --arg 1 --arg 2")
# Wait for the command to terminate
while not stdout.channel.exit_status_ready():
# Only print data if there is data to read in the channel
if stdout.channel.recv_ready():
rl, wl, xl = select.select([stdout.channel], [], [], 0.0)
if len(rl) > 0:
# Print data from stdout
print stdout.channel.recv(1024),
#
# Disconnect from the host
#
print "Command done, closing SSH connection"
ssh.close()The code should be quite self explainatory, but here’s what it does in short:
- Connect to the SSH server
- Send the command (non-blocking)
- Create a loop, waiting for the channel to get an exist code
- The loop is looking for data to pring (stdout.channel.recv_ready()) and prints any data it receives