Understanding SSH and exit codes in Bash scripts.

ssh-exit-codes.sh

#!/bin/bash
#
# SSH Exit Codes
#
# Using SSH in scripting is pretty standard, but sometimes you want to stop execution of a script
# if a command inside an SSH session fails to exit cleanly (return 0).  The key to remember is that
# the ssh command's exit code will be that of the *last executed* command inside the ssh session, just
# like a bash script ends with the exit code of the last command executed unless you specifically
# call exit.
#
# See the examples below for various ways of handling ssh connections running multiple commands
# and how that interacts with it's exit value and the usage of "set -e" in the shell.

REMOTE_SERVER="insert-test-server-here.example.org"

echo "Testing exit codes from HEREDOC ssh connections with a set -e"

ssh $REMOTE_SERVER << EOF
  set -e
  someNonExistantCommand
  hostname
EOF

echo "Return of SSH Connection: $?"
# This version returns non-zero because of failed command, and never runs hostname
# The set -e allows this to happen.


###############################################################################
echo
echo "Testing exit codes from HEREDOC ssh connections without a set -e"

ssh $REMOTE_SERVER << EOF
  someNonExistantCommand
  hostname
EOF

echo "Return of SSH Connection: $?"
# This returns zero and runs hostname
# Without the set -e designated in the ssh shell's environment, all commands
# will happily run.  Since hostname shouldn't fail, it will return a 0



###############################################################################
echo
echo "Testing exit codes from ssh connections with a set -e using quotations and semicolons"

ssh $REMOTE_SERVER "
  set -e ;
  someNonExistantCommand ;
  hostname
"

echo "Return of SSH Connection: $?"
# This version returns non-zero because of failed command, and never runs hostname
# The set -e stops execution at the bad command.



###############################################################################
echo
echo "Testing exit codes from ssh connections without a set -e using quotations and semicolons"

ssh $REMOTE_SERVER "
  someNonExistantCommand ;
  hostname
"

echo "Return of SSH Connection: $?"
# This returns zero and runs hostname
# Similar to the HEREDOC version, the shell continues to run commands after a failed one.
# The semicolon doesn't stop additional execution from happening.



###############################################################################
echo
echo "Testing exit codes from ssh connections with a set -e using quotations and ampersands"

ssh $REMOTE_SERVER "
  set -e &&
  someNonExistantCommand &&
  hostname
"

echo "Return of SSH Connection: $?"
# This version returns non-zero because of failed command, and never runs hostname
# Again, the set -e here stops our execution before ever running hostname.



###############################################################################
echo
echo "Testing exit codes from ssh connections without a set -e using quotations and ampersands"

ssh $REMOTE_SERVER "
  someNonExistantCommand &&
  hostname
"

echo "Return of SSH Connection: $?"
# This version returns non-zero because of failed command, and never runs hostname
# Here, it's actually the && that stops our execution.  This boolean operator will
# only continue if the previous command executes cleanly, so when the bad command
# exits with a non-zero return code, it stops execution and hostname never runs.