TCL programming

Posted on November 12, 2015 by ronaldpringadi

A reusable expect dispatcher I kept around for running the same kind of operation across a list of servers — untar an index, restart a service, patch a config file. The trick is that the script reads the first command-line argument and dispatches to a same-named proc, so one script holds many small ops:

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./script.exp UntarLuceneindexes
./script.exp stopLuceneServices
./script.exp startLuceneServices

That’s the $functionName line at the bottom — it literally calls whatever proc name you passed in. Crude, but very handy when you’re iterating on a runbook.

The script below assumes you’ve set up SSH keys so ssh root@host connects without a password prompt. That keeps secrets out of the script and the shell history.

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#!/usr/bin/expect -f
# Expect dispatcher. Beware: whitespace matters in Tcl.

# Multi-line comments use a set / curly-brace trick:
# set comment {
#     your multi-line comment
# }

# ---------------------------------------------------
# Function definitions
# ---------------------------------------------------

if {[llength $argv] == 0} {
    send_user "Usage: script.exp FUNCTION_NAME\n"
    send_user "Example: script.exp UntarLuceneindexes\n"
    exit 1
}
set functionName [lindex $argv 0]

# Servers used by the Lucene-related procs.
set aLuceneServers {your_server_1 your_server_2 your_server_3}

proc printPass {} {
    send_user "\n\[PASS\]\n"
}

proc printFail {} {
    send_user "\n\[FAIL\]\n"
    exit 1
}

proc simpleTest {} {
    set aProductionServers {your_server_1 your_server_2 your_server_3 your_server_4 your_server_5 your_server_6}
    foreach host $aProductionServers {
        send_user "Processing ... '$host'\n"
        set timeout 60
        spawn ssh root@$host

        send "hostname\r"
        expect $host

        send "uptime\r"
        expect "load average"

        send "exit\r"
    }
}

proc UntarLuceneindexes {} {
    global aLuceneServers
    set sCurrentLuceneTar "index2_20151009.tar.bz2"
    foreach host $aLuceneServers {
        send_user "\n================================================\n"
        send_user "Processing ... '$host'\n"
        send_user "================================================\n"
        set timeout 60
        spawn ssh root@$host

        send "hostname\r"
        expect $host

        send "su - path\r"
        send "pwd\r"
        expect "/home/path"

        send "ls $sCurrentLuceneTar | wc -l\r"
        expect "1"

        send "tar -xjvf $sCurrentLuceneTar &\r"
        send "sleep 2\r"
        send "ps aux | grep index2 | grep tar | wc -l\r"
        send "disown %1\r"
        expect "1"

        send_user "Done processing ... '$host'\n"
    }
}

proc updateDBRef {} {
    set apathServers {your_server_6}
    set newDB "some_db_20150925"
    foreach host $apathServers {
        send_user "\n================================================\n"
        send_user "Processing ... '$host'\n"
        send_user "================================================\n"
        set timeout 60
        spawn ssh root@$host

        send "hostname\r"
        expect $host

        send "su - path\r"
        send "pwd\r"
        expect "/home/path"

        set propsFile "/home/path/path-current/SomeConfig.properties"
        send "grep -v 'com.somecompany.setting.server.db.pathdb2.url=' $propsFile > $propsFile.new\r"
        send "echo 'com.somecompany.setting.server.db.pathdb2.url=jdbc:postgresql://127.0.0.1:5432/$newDB' >> $propsFile.new\r"
        send "mv $propsFile.new $propsFile\r"

        send_user "Done processing ... '$host'\n"
    }
}

proc stopLuceneServices {} {
    global aLuceneServers
    foreach host $aLuceneServers {
        send_user "\n================================================\n"
        send_user "Processing ... '$host'\n"
        send_user "================================================\n"
        set timeout 60
        spawn ssh root@$host

        send "hostname\r"
        expect $host

        send "service lucene stop\r"
        expect {
            "stopped PID"           { printPass }
            "lucene is not running" { printPass }
            timeout                 { printFail }
        }

        send_user "Done processing ... '$host'\n"
    }
}

proc startLuceneServices {} {
    global aLuceneServers
    foreach host $aLuceneServers {
        send_user "\n================================================\n"
        send_user "Processing ... '$host'\n"
        send_user "================================================\n"
        set timeout 60
        spawn ssh root@$host

        send "hostname\r"
        expect $host

        send "service lucene start\r"
        expect {
            "started PID" { printPass }
            timeout       { printFail }
        }

        send_user "Done processing ... '$host'\n"
    }
}

# ---------------------------------------------------
# Run
# ---------------------------------------------------

set timeout 60
log_file -noappend expect.log  ;# default is append; -noappend overwrites

$functionName

Tcl or Expect? The post title says Tcl, but almost every interesting line above — spawn, expect, send, send_user, log_file, the #!/usr/bin/expect -f shebang — comes from Expect, an extension that sits on top of Tcl and lets you drive interactive programs (SSH, telnet, vendor CLIs) by pattern-matching on their output. Pure Tcl is the language; Expect is what makes a script like this one possible. 🤖

Use SSH keys, not expect-and-send-password. The original version of this script had blocks like expect “*assword: “ followed by send “YOUR_SERVER_PASSWORD\r”. That works, but it puts a plaintext password in your script and your shell history. The cleaner answer is SSH keypair auth — the script above assumes that’s already set up, so the password block disappears entirely. Reserve the expect-and-send-password pattern for the cases where it’s truly unavoidable: old network gear, vendor CLIs, devices that don’t accept keys.

You don’t always need Expect. If all you need is to run the same shell command across a list of servers, plain SSH in a loop is simpler and easier to debug:

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for host in your_server_1 your_server_2 your_server_3; do
  ssh "$host" "service lucene stop"
done

For larger fleets, parallel-ssh (pssh) or ansible -m shell -a “…” mygroup are usually saner. Expect earns its keep when you have to navigate an interactive prompt that doesn’t accept piped commands — confirm dialogs, paged output, vendor consoles. 💡

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