Shell script is powerful if you could remember all the weird syntax. Here is the example to show how to use Array in Shell script. Note: readarray only works in Bash 4.x, check your version: bash –version

• Array in Shell script 1

  string='Paris, France, Europe';
  echo $string
  readarray -td, array <<<"$string"; declare -p array;
  ## declare -a array=([0]="Paris" [1]=" France" [2]=$' Europe\n')
  
  country='Paris/Frence/Europe/Sweden/Croatia';
  echo $country
  readarray -td/ array <<<"$country"; declare -p array;
  for item in "${array[@]}" 
  do
    echo "$item" 
  done      
  

• count number of argument

  echo "$#"
  var="string"
  

• array in bash

  arr=(                            
  "dog"                            
  "cat"                            
  "rat"                            
  )                                
  for var in ${arr[*]}             
  do                               
  echo "var=$var"                  
  done                             
  

• if statement, one liner

  var="$#" && if [ "$var" -eq 2 ]; then echo "two arguments"; else echo "not two argument"; fi
  

• Iterate all files with one line shell command

  for name in $(ls *); do echo $name; done
  

• String matches

  for name in $(ls *); do if [[ $name == *"txt"* ]]; then echo $name; else echo ""; fi 
  

• Use regex match =~

  if [[ $name =~ "txt" ]]; then echo $name; else echo ""; fi
  

• Repeat shell command with watch If you want to get constantly update information or stream data shell commands then you can use watch repeats your commands For Exmple, I want to monitor my system disk usage

  watch df -lah  # default 2 seconds interval
  

  watch shell commands with pipe, e.g. ps aux | grep --color httpd with double quotes

  watch "ps aux | grep --color httpd"
  

• Check a variable is empty or not in shell script, check empty string

  if test -z "$1" 
  then
     echo "\$1=$1 is empty"
  else
     echo "\$1=$1 is not empty"
  fi
  # or
  if [ -z "$1" ]
  then
     echo "\$1=$1 is empty"
  else
     echo "\$1=$1 is not empty"
  fi
  

• grep content of file with specified file name extension, show full file name

  grep --include="*.java" -Hnris pattern $PWD
  

• cat and grep

  cat /tmp/file.txt | grep string
  

• grep with color

  cat /tmp/file.txt | grep --color string  
  

• grep print NUM line of trailing context file after matching lines, two line after

  cat file | grep -A 'dog'
  

• grep print NUM lines of leading context file before matching lines, two line before

  cat file | grep -B 'dog'
  

• grep inversion

  # find all non java file  
  grep -v -E '.java$'       
  

• grep string that contains "txt", grep matches string

  if echo $name | grep -q "txt"; then echo $name; else echo ""; fi  
  

• grep string its suffix is "txt", e.g. "file.txt"

  if echo $name | grep -q "txt$" then echo $name; else echo ""; fi  
  

• grep string its prefix is "file", e.g. "file.txt"

  # grep -q => no output                                           
  if echo $name | grep -q "^file" then echo $name; echo echo ""; fi
  

• grep in POSIX basic and extended regular expression

  # basic regex                                                    
  ps aux | grep --color 'wai\|filewatcher'                         
  # extended regex                                                 
  ps aux | grep --color -E 'wai|filewatcher'                       
  

• kill all mysqld in MacOS, combine grep and awk

  ps aux | grep --color mysqld | awk '{print $2}' | line 'x -> kill -9 x'
  

• grep colo escape sequence color

  grep -E --color=always -Hnris 'begin|end' *.tex | more -R
  

• awk get the next line after match

  whois bbc.co.uk | awk -F: '/Registrar:/ && $0 != "" {getline; print $0}'
  # => British Broadcasting Corporation [Tag = BBC]
  

• awk as filter, match line contains root or www

  cat /etc/passwd | awk '/root|www/{print}'
  

• sed on MacOS is BSD version, it does not implement regular expression extension: GNU regex Extension

• Character class in GNU regex Extension.

  \w	[a-bA-Z_]	word	echo "dog cat" | sed 's/\w/X/g' => "XXX XXX"
  \W		non whitespace	echo "dog cat" | sed 's/\W/X/g' => "dogXcat"
  \s		whitespace	echo "dog cat" | sed 's/\s/X/g' => "dogXcat"
  §		non whitespace
  \b		match a word boundary	echo "dog cat" | sed 's/\b/X/g' => "XdogX XcatX"
  \<		match the beginning of a word
  \>		match the end of a word

• \d is NOT in the GNU regex expression, but it is in POSIX regex expression Extension
• gsed can be installed on your MacOS which is GNU sed.

# does not work on MacOS or FreeBSD
echo "dog cat " | sed -E 's/\s*//g'  => "dog cat "
echo "dog cat " | gsed   's/\s*//g'  => "dogcat"
# it seems gsed does not need flag -E

• Add more here

• What is that?

• POSIX Extend Regular Extend
• What is that?

• rysnc file with specified extensions such png, svg etct.

  • include and exclude files.

  # rsync svg, png and jpg files excluding other files.
  rsync -rvzu --include='*'{svg,png,jpf} --exclude "*"  source  dest
  # rsync html file excluding other files.
  rsync -rvzu --include='*.html' --exclude "*"  source  dest
  # rsync folder
  rsync -artv folder  source dest
  
  # rsync local folder with remote folder
  rsync -artv local_folder user@xfido.com:/var/www/html
  

• ports installation is not working.

cd /usr/ports/redis && make install clean BATCH=yes

• Add follwoing two lines to your ~/.profile, it works in FreeBSD.

  eval $(ssh-agent -s)
  ssh-add ~/.ssh/myprivate_id_rsa
  

• MacOS MacOS Public Key
• FreeBSD FreeBSD Public Key

• If the path is directory, the time will be changed if file is added or deleted.
• See filewatcher.hs or watchDir in AronModule.hs

modificationTime :: FileStatus -> EpochTime