Shell Programming and Scripting

Hello,
I had to make an ssh connection to remote server from a bash shell script. I've summarized my research and I want to share it with you. I hope it will be useful to somebody or it will get improved.
Bash has an option, which is called "input redirection", I think. It is something similar to this:

Code:

#!/bin/sh

mysql -u tsurko -p mydb <<EOF
SELECT * FROM test_table;
SELECT * FROM other_table;
quit
EOF
exit

This script will connect to MySQL, will run the two queries and will quit. In similar way we can make an ssh connection in bash script. Here is some example code:

Code:

#!/bin/sh

ssh example.com <<EOF
touch test_file
exit
EOF
exit

This way you can execute anything you want on the remote server. I have the following errors, while running the script:

Code:

Warning: no access to tty (Bad file descriptor).
Thus no job control in this shell.

As far as I know this is known issue and it has been discussed in the forum. I haven't tried to fix it - it works for me.
Here is an example, using shell variables in the script:

Code:

#!/bin/sh

TEST_VAR=sample_file

ssh example.com <<EOF
touch $TEST_VAR
exit
EOF

echo "I'm back"
exit

In this case the value of variable TEST_VAR is acquired from the local environment. Ny local I mean the environment, where the script is executed. So on the remote machine we have executed the command "touch sample_file".
Here is another example showing how to use environment variables, declared on the remote server:

Code:

#!/bin/sh

TEST_VAR=sample_file

ssh example.com <<EOF
touch \$TEST_VAR
exit
EOF

echo "I'm back"
exit

Let's assume that we have TEST_VAR set on the remote server. We have escaped the $ character in our script, so $TEST_VAR is treated as string, not as a variable. On the remote server we have executed "touch $TEST_VAR". and TEST_VAR's value is acquired from there.
And finally let's assume that we have to run some commands on two or more machines. It's very inconvenient to have them two or more times (for each session). In this case the following workaround can be used. A text file is created and the desired commands are saved there. Then we can use "`cat <filename>`" in the script and they will be executed on the server. :

Code:

#!/bin/sh

ssh example.com <<EOF
`cat test.cmd`
exit
EOF

ssh example2.com <<EOF
`cat test.cmd`
exit
EOF

exit

and more convenient solution (thanks to ddreggors):

Code:

#!/bin/sh

ssh example1.com `cat test.cmd`
ssh example2.com `cat test.cmd`

Don't forget that the file must contain only commands, written in the way we want them executed on the server. The file can't be a bash script - any variables and so on won't be parsed.
That's it. I'll be glad to hear any suggestions, corrections, critics or just opinions. Corrections about my English are also welcome - it's not my mother tongue.

Just a thought here...

I notice you use:

Code:

ssh example2.com <<EOF
`cat test.cmd`
exit
EOF

when really (for a single command like that) you can use:

Code:

ssh example2.com `cat test.cmd`

or do you actually have multiple commands in that ssh command and only show one here?

If you do then you can use this:

Code:

ssh example2.com `cat test.cmd`; `cat test2.cmd`; `test3.cmd`

Yes, you are right. Your variant is better! I'll edit my post.

No reason you have to use a "here script" with <<EOF, you can pipe commands directly to the standard input of ssh, for example using echo:

Code:

$ D=/tmp; F1=x1; echo "F2=$D/$F1
echo on \$(hostname): escaped: D=\$D F1=\$F1 F2=\$F2
echo on \$(hostname): not escaped: D=$D F1=$F1 F2=$F2
df $D >\$F2; ls -l \$F2; cat \$F2; rm -rf \$F2; ls -l \$F2
" | ssh qneill@qneill-linux

this shows a couple of local variables D and F1 (expanded locally when the local echo is executed), and a remote variable F2 (expands to nothing locally, but remotely expands to the assigned value). The commands produce this output:

Code:

on qneill-linux: escaped: D= F1= F2=/tmp/x1
on qneill-linux: not escaped: D=/tmp F1=x1 F2=
-rw-r--r-- 1 qneill 30101 125 2009-04-14 15:25 /tmp/x1
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda1            147581948  78980980  61104240  57% /
ls: /tmp/x1: No such file or directory

The commands shown use the local variable D and remote variable F2, creating, using, and removing a file (/tmp/x1 on the remote machine).

If all of your variable expansions are constant, or only depend on remote things, you can use single quotes and avoid having to escape all the $ signs:

Code:

echo 'D=/tmp; F1=x1; F2=$D/$F1
echo on $(hostname): D=$D F1=$F1 F2=$F2
df $D >$F2; ls -l $F2; cat $F2; rm -rf $F2; ls -l $F2
' | ssh qneill@qneill-linux

which yields

Code:

on qneill-linux: D=/tmp F1=x1 F2=/tmp/x1
-rw-r--r-- 1 qneill 30101 125 2009-04-14 15:34 /tmp/x1
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda1            147581948  78980960  61104260  57% /
ls: /tmp/x1: No such file or directory

Once you start using this mechanism for anything complicated, you will quickly run into many quoting issues. Consider writing a script that generates code on the remote system, or a templated script that can be parameterized locally or remotely as needed, then executed remotely with a single call to ssh.
--
qneill