bashsnippets.md

Bash Snippets

• Best Bash Guide Ever (apparently) - http://tldp.org/LDP/abs/html/

• See - http://bash.cumulonim.biz/BashPitfalls.html

• Bash Special Parameters - http://www.thegeekstuff.com/2010/05/bash-shell-special-parameters/#more-4383

$_

Basics and Rules of Thumb

• Always use round brackets for numeric comparisons. With square brackets you have to use -eq instead of == ascii comparison

Variable Handing

Die unless arguments are correct

die () {
    echo >&2 "$@"
    exit 1
}

[ "$#" -eq 3 ] || die "Usage: $0 arguments "

####Check Argument Set Or Default Value

[ -z "$RPATH" ] && RPATH="/usr/bin/R"

Shifting variables

# Grab the servername then shift
SERVER=$1
shift

Increment variable

counter=$((counter+1))

Looping and Conditional Constructs

Simple if

if (( $counter == 3 )); then    		
	kill -9 $PID
else
	counter=$((counter+1))
fi


[[ -f file ]] 
[[ -d dir ]]
[[ -r entry ]]
[[ -w entry ]]
[[ -x entry ]]

[[ -n var ]]  // non zero
[[ -z var ]] // zero length

Maths

let
(( j > 0 ))
typeset -i k=$1

Exit Status

Set exit status with exit n

Previous exit status with $? Previous pid with $!

Simple for loop

for JMETERTEST in "$@"
do
    <stuff>
done

Executing Things

Backtick

Take the output from a variable and store this:

numscripts=`ls -l metrics*.rb 2> /dev/null | wc -l`

] Alternatively, use a subshell using $()

Subshell

for cmd in $(ls metrics*.rb)

Parsing output of a command to check whether something is present

In this case an R package

#!/usr/bin/env bash

function checkpackage() {
	output=""
	output=`Rscript -e 'installed.packages(lib.loc = "/opt/mango/navigator/rlibs")' | grep "$1.*\"$1\""`
	
	if [ -n "$output" ]; then
		echo "$1: PASS"
	else
		echo "$1: FAIL"		
	fi
}

checkpackage "RNMImport"

Find files and execute them

#!/bin/bash

for i in $(find . -name *.ctl); do	
	directory=`dirname $i`
    filename=`basename $i`
	`cd $directory; execute $filename >> output.txt`
done

Finding a process by name, shutting it down and forcefully terminating just in case

#!/bin/bash

function restartservice() {

        service $1 stop;

        RETRY_LIMIT=3
        PID="$(ps -ef | grep $1 | grep java | awk ' { print $2 } ')"

        counter=1
        while kill -0 $PID >/dev/null 2>&1
        do
                echo "Waiting for tomcat to die, $counter"
                sleep 3

                if (( $counter == $RETRY_LIMIT )); then
                        kill -9 $PID > /dev/null 2>&1
                else
                        counter=$((counter+1))
                fi

        done

        service $1 start; 
}

restartservice tomcat7

Working With Arrays

Adding an item to an array

remotepids=("${remotepids[@]}" $pid)

Checking an array length

if (( ${#remotepids[@]} > 0 )); then    
echo -ne "\nStarted processes [ ${remotepids[@]} ] \n"
echo ${remotepids[@]} > $pidsfile

Dates

`date +"%m-%d-%y_%T"-$serverhost`

Functions

See: http://www.linuxjournal.com/content/return-values-bash-functions

local sets a local variable

Basic

function clean {
    echo "Parameter is $1"
} 

Pass variables with

clean tomcatlogdir

Redirect output/error to dev null

command > /dev/null 2>&1 &

Processing items in an array

declare -a arr=("a", "b", "c")

## now loop through the above array
for srv in "${arr[@]}"
do
    echo "Handling: $srv"
done