Bash cheat sheet

Getoptions with flags

while getopts "f" option; do   
 case ${option} in
 f)
     flag="1"
 *)
     echo "unknown parameter"
     exit;;
  esac
done

Getoptions with args

while getopts "a:" option; do   
 case ${option} in
 a)
     arg="${OPTARG}"
 *)
     echo "unknown parameter"
     exit;;
  esac
done

Condition -OR-

if "$isA" || "$isB"; then
 do-something
fi 

• Condition if var is defined

if [ ${var+x} ]; then 
 do-something
fi

Print Text

cat << EOF
Any ...
text ... 
EOF
}

Read tokenized configuration file

. /etc/os-release
echo $VERSION_ID

Search in file, return bool

if grep -q search-arg "/etc/file.conf"; then
 do-something
fi

Search and replace

sed -i -r  's/(.*)umask 022(.*)$/\t#Changed\n\tumask 027/gi' /etc/profile

• CRLF within string

scriptmsg+="Hello\n"
scriptmsg+="World\n"
echo -e $scriptmsg

NL=$'\n'
echo -e " Hello${NL}Wolrd"

echo "
Hello
World
"

Code sample of a function call using args and the use of the return code

#!/bin/bash
main() {
 declare test_path='/etc'
 declare test_file='motd'
 chkFile "${test_path}/${test_file}"	
 myreturnvalue=$?
 #echo "myreturnvalue is "$myreturnvalue
 if [ $myreturnvalue == "1" ] ; then  
  echo "File exist: ${test_path}/${test_file}"
 else
  echo "File does NOT exist: ${test_path}/${test_file}"
 fi
}
function chkFile(){
 if [ -f $1  ]; then   #Check for file
  return 1
 else 
  return 0
 fi
}
main "$@"; exit 0

Code sample of a array

• Declare

declare -a array=()

• Add data

array+=("Red")  
array+=("Blue")
array+=("Yellow")

• Enum

# get length of an array
arraylength=${#array[@]}

# use for loop to read all values and indexes
for (( i=0; i<${arraylength}; i++ ));
do
  echo "index: $i, value: ${array[$i]}"
done

• Return

index: 0, value: Red
index: 1, value: Blue
index: 2, value: Yellow

Search in file for string

if ! grep -oP '^\s*net.ipv4.ip_forward=1' "/etc/sysctl.conf"; then 
 echo "Add by OpenVPN-Installer Script $DateTime" >> "/etc/sysctl.conf"
 echo "net.ipv4.ip_forward=1" >> "/etc/sysctl.conf"
fi

Check if a user exist

if getent passwd ${ovpnuser} > /dev/null 2>&1; then
  echo "User exists  ${ovpnuser}"
fi