« Bash Leanings

I recently refactored a pretty large and complex set of bash scripts.

I think Ruby influenced my Bash coding style quite a lot.

Here’s a list of some of the patterns I’ve followed.

Have a basedir variable available

script\_base="\`dirname "$0"\`"

Separation of concerns

Write loosely couple bash code in separate files, import them by doing:

. $script\_base/utils/colors.sh

Avoid global state

Limit as much as possible global state (variables), they are evil in any language

export PATH=/usr/pkg/sbin:/usr/pkg/bin:$PATH

Is my one and only export and ‘public’ variable in the script

Separation of concerns

Write independent, small functions

\# Utility function to retrive configuration properties, uses jq   
function get\_config {   
 local environment=$1 local config=$2  
 jq -r ".$environment.$config" config.json  
}

Documentation

Document your functions, sadly Bash doesn’t allow named params, so it’s pretty hard to figure out what a function takes.

Scope

Leverage local scope: Use local variables

Principle Of Least Astonishment

Check error codes often and offer meaningful log messages

\# Checks the return code of the last command run and outputs error / exit if not nil  
function check\_error {   
 if [ $? -ne 0 ];   
 then   
 error "[ERR] $1"   
 exit 1   
 fi   
}

User interaction

Use colours, Bash is fun with colours

function error {   
 echo "$(tty -s && tput setaf 1)$1$(tty -s && tput sgr0)"   
}
function ok {   
 echo "$(tty -s && tput setaf 2)$1$(tty -s && tput sgr0)"   
}
function warn {   
 echo "$(tty -s && tput setaf 3)$1$(tty -s && tput sgr0)"   
}

Defensive coding

Check error codes properly also when you run remote scripts

\# Runs a command on remote ssh  
function ssh\_exec {   
 local remote\_user=$1   
 local remote\_command=$2   
 local results
results=$(ssh -T -q "$remote\_user" "$remote\_command")  
 if [ $? -ne 0 ];   
 then   
 log "remote code execution return: $?"   
 log "remote code execution output: $results"  
 error "[ERR] Failed running remote command: $remote\_command"  
 exit 1  
 fi   
}

In Conclusion

Don’t underestimate Bash, it’s an awesome, Turing Complete language and requires no installation.

When the code starts to get too nasty, use your favourite scripting language!