Useful Bash utils and tips

_bash_utils_tips

### Useful Bash command and tips ###

array-syntax.sh

# bash array
${arr[*]}         # All of the items in the array
${!arr[*]}        # All of the indexes in the array
${#arr[*]}        # Number of items in the array
${#arr[0]}        # Length of item zero

bash-argument.sh

usage() { echo "Usage: $0 [-s <45|90>] [-p <string>]" 1>&2; exit 1; }
while getopts ":s:p:" o; do
    case "${o}" in
        s)  s=${OPTARG}
            ((s == 45 || s == 90)) || usage
            ;;
        p)  p=${OPTARG}
            ;;
        *)  usage
            ;;
    esac
done
shift $((OPTIND-1))
if [ -z "${s}" ] || [ -z "${p}" ]; then
    usage
fi
echo "s = ${s} and p = ${p}"
$# = total number of arguments / $1 is the first argument / $0 is the script itself.
!# = last argument

bash-ifs.sh

# bash - IFS
for loop splits when it sees any whitespace like space, tab, or newline. So, you should use IFS (Internal Field Separator):
IFS=$'\n'       # make newlines the only separator
for j in $(cat ./file_wget_med)    
do
    echo "$j"
done

default-var-value.sh

# default shell variable value
The := syntax
echo ${granslam=Maria Sharapova} NOTE: will not work with parameter.
echo ${granslam:-Maria Sharapova}
echo ${granslam:=Maria Sharapova}
if $granslam is empty, set assign Maria Sharapova
u=${1:-root} if “$1” not set, then use root
${varName:?Error varName is not defined or is empty}

detect-source-script.sh

# How to detect if a script is being sourced
[[ $_ != $0 ]] && echo "Script is being sourced" ||  echo "Script is a subshell"

download-jdr.sh

# download JAVA from oracle
wget --no-check-certificate -c --header "Cookie: oraclelicense=accept-securebackup-cookie" http://download.oracle.com/otn-pub/java/jdk/8u162-b12/0da788060d494f5095bf8624735fa2f1/jdk-8u162-linux-x64.rpm

execute-var-command.sh

# Execute variable as command
TIME_CMD='date +\%b" "\%d" "\%X" "\%Y'
echo "`eval $CMD` - INFO"

get-filename.sh

# get filename without extension
${FILENAME%.*} => bash_hackers.txt

# get file extension
${FILENAME##*.} => bash_hackers.txt

# get directory name
${PATHNAME%/*} => /hom/bash/bash_hackers.txt

# get filename
${PATHNAME##*/} => /hom/bash/bash_hackers.txt

# get script name without basename
_self="${0##*/}"; echo "$_self is called"

get-my-ip.sh

# Get my IP address
curl https://ifconfig.co/

is-function.sh

# test function exists
isFunction() { [[ "$(declare -Ff "$1")" ]]; }
Usage:
isFunction some_name && echo yes || echo no

mkdir-mode.sh

# mkdir with mode
_mkdir() {
   local d="$1"               # get dir name
   local p=${2:-0755}      # get permission, set default to 0755
   [ $# -eq 0 ] && { echo "$0: dirname"; return; }
   [ ! -d "$d" ] && mkdir -m $p -p "$d"
}

parse-html.sh

# Parse HREF URL 
shopt -s nocasematch    #Dont care about the character case
text='<a href="http://www.google.com/here2;i=!mfo1iu489fn1o2jlk21m4098mdoi">"test link"</a><br>'
regex='(<a\ +href=\")([^\"]+)(\">)'
[[ $text =~ $regex ]] && echo ${BASH_REMATCH[2]}

pipe-status.sh

# pipe status
<command> | tee out.txt; test ${PIPESTATUS[0]} -eq 0
echo ${PIPESTATUS[*}} all status in commands

remove-all-spaces.sh

# remove all spaces
echo "     This is a test" | sed -e 's/^[ \t]*//'

remove-first-2-col.sh

# remove first 2 columns using cut command
You can do it with cut:
cut -d " " -f 3- input_filename > output_filename
Explanation:
cut: invoke the cut command
-d " ": use a single space as the delimiter (cut uses TAB by default)
-f: specify fields to keep
3-: all the fields starting with field 3
input_filename: use this file as the input
> output_filename: write the output to this file.
awk '{$1=""; $2=""; sub("  ", " "); print}' input_filename > output_filename
awk: invoke the awk command
$1=""; $2="";: set field 1 and 2 to the empty string
sub(...);: clean up the output fields because field 1 & 2 will still be delimited by " "
print: print the modified line
input_filename > output_filename: same as above.

remove-non-printable-chars.sh

# remove non-printable chars
JENKINS_VER=$(tr -dc '[[:print:]]' <<< "${JENKINS_VER}")

rename-filename-to-lower.sh

# rename all *.txt filenames to lowercase
for file in *.txt; do
  /bin/mv "$file" "${file,,}"
done

screen-cmd.sh

# terminate a screen session
screen -X -S [session # you want to kill] kill

# start screen session
/usr/bin/screen -d -m -S android_emu ./emulator -avd ${DEVICE_NAME} -gpu host -wipe-data -no-boot-anim -no-audio -noaudio -netfast -timezone America/Los_Angeles >> emulator.log 2>&1

set_external_env.sh

# If variable is set outside of this script, use what is set instead to allow overrides of param store
function set_environment_variable {
  local ENV_VAR=$1
  if test -z ${!ENV_VAR};
  then
    CMD="export $1=$2"
    eval $CMD
  else
    echo "${ENV_VAR} is already set in environment"
  fi
}

function get_db_params {
  pattern='^([[:print:]]+)://([[:print:]]+):([[:digit:]]+)/([[:print:]]+)'
  if [[ $1 =~ $pattern ]]; then
          echo $BASH_REMATCH
          proto=${BASH_REMATCH[1]}
          host=${BASH_REMATCH[2]}
          port=${BASH_REMATCH[3]}
          dbname=${BASH_REMATCH[4]}
          set_environment_variable PGHOST $host
          set_environment_variable PGPORT $port
          set_environment_variable POSTGRES_DB $dbname
  else
    echo "ERROR failed to parse url - $1"
    exit
  fi
}

string-variable.sh

${parameter:-defaultValue}
${parameter:=defaultValue}
${parameter:?”ErrorMsg”}
${#var} - length of string
${var%pattern}
${var#Pattern} - Remove from $var the shortest part of $Pattern that matches the front end
${var##Pattern} - Remove from $var the longest part of $Pattern that matches the front end
${var%Pattern} - Remove from $var the shortest part of $Pattern that matches the back end
${var%%Pattern} - Remove from $var the longest part of $Pattern that matches the back end
${var:pos} - Variable var expanded, starting from offset pos.
${var:pos:len} - Expansion to a max of len characters of variable var, from offset pos
${var/Pattern/Replacement} - First match of Pattern, within var replaced with Replacement.
${var//Pattern/Replacement} - Global replacement. All matches of Pattern, within var replaced.
${var/#Pattern/Replacement} - If prefix of var matches Pattern, then substitute Replacement for Pattern.
${var/%Pattern/Replacement} - If suffix of var matches Pattern, then substitute Replacement for Pattern.
${!varprefix*}, ${!varprefix@} - Matches names of all previously declared variables beginning with varprefix.
numeric validation = [[ $var == ${var//[^0-9]/} ]] || echo “Not numeric”

time_stamp.sh

# find N days ago.
# yesterday
YES_DAT=$(date --date=' 1 days ago' '+%Y%d%m')

# any day 
ANY_YES_DAT=$(date --date=' $1 days ago' '+%Y%d%m')

whoami.sh

echo "$(whoami):$(id -g -n $(whoami))"