joplin-notes/joplin notion extension/linux/Bash Basics.md

Bash Basics https://devhints.io/bash

Note

• for executing commands if uses single quotes then it will pass literal text , double quotes allows commands and variables to be execured  

• for loop

    for arg in "$@"; do
    	echo "$arg"
    done
  
    for (( i=0;i<$filesCount;i++)); do
    	singleFile="${filesArray[$i]}"
    	#here I replace ALL the spaces in the file's name, by underscores.
    	trimmed=${singleFile// /\_}
    	mv "${singleFile}" "${trimmed}"
    	echo "renamed ${singleFile} as ${trimmed} !"
    done
  

• if elif else set of if else statements are terminated with fi

    # String
    if [[ -z "$string" ]]; then
      echo "String is empty"
    elif [[ -n "$string" ]]; then
      echo "String is not empty"
    else
      echo "This never happens"
    fi
  
  

• Logical operators

  # Logical AND and OR
  if [[ $varA == 1 && ($varB == "t1" || $varC == "t2") ]]; then
  
  # NOT
  if ! [[ $var == 2 || $var == 30 || $var == 50 ]] ; then
  	do something
  fi
  if [[ ! ($var == 2 || $var == 30 || $var == 50) ]] ; then
  	do something
  fi
  
  
  

• Functions

    myfunc() {
    	echo "hello $1"
    }
  
    # Same as above (alternate syntax)
    function myfunc {
    	echo "hello $1"
    }
  
    myfunc "John"
  
  
    # Returning values
    myfunc() {
    	local myresult='some value'
    	echo "$myresult"
    }
  
    result=$(myfunc)
  
  
  
  

• Geting filename and extemsion

  # First, get file name without the path:
  
   filename=$(basename -- "$fullfile")
   # or
   filename="${FILEPATH##*/}"
   extension_only="${filename##*.}"
   filename_with_noext="${filename%.*}"
  
  

• array

  # Declaring an array
  arr=()
  arr1=("hey" "there" "how")
  arr2=("er" "yu")
  declare -a arr3=()
  
  # Declare associative array to be able to have keys rather than index
  declare -A asscoArr
  
  # array elements is "${arr1[@]}"
  for ele in "${arr1[@]}";do
  	echo $ele
  done
  
  # Output
  hey
  there
  how
  
  # array index/keys is "${!arr1[@]}"
  for i in "${!arr1[@]}";do
  	echo $i
  done
  
  # Accessing elements through index
  "${arr1[2]}"
  
  # getting length of array
  ${#arr1[@]}
  
  

• shift To shift arguments to left so that first argument is lost

• C like switch case in bash

  #!/bin/bash
  
   # Loop through all the arguments
   for arg in "$@"; do
     case $arg in
   	-a|--optionA)
   	  echo "Option A detected!"
   	  # handle option A
   	  ;;
   	-b|--optionB)
   	  echo "Option B detected!"
   	  # handle option B
   	  ;;
   	*)
   	  # Handle positional arguments or invalid flags
   	  ;;
     esac
   done
  

• pasing variable arguments

  # Define your arguments in an array
    passargs=(-x '*/node_modules/*')
  
    # Use the array with the command
    zip -r archive.zip . "${passargs[@]}"
  
  
    # following method wont work
  
    passargs="'-x */node_modules/*'"
    zip -r $ARCHIVE_NAME $FILES_TO_ARCHIVE $passargs
  

• convert string which r separted by spaces to array using declare will resolve some bugs related to delimiters

  # arrow looks like following
  arr1=('hey' 'there')
  # if
  str="hey there"
  declare -a arr=($str)
  # now 
  # arr is ('hey' 'there')
  

• extract parent directory

  // Output : /home/username
  echo "$PWD"
  
  // Output : /home
  echo "${dirname $PWD}"
  
  
  

Examples

• Find non common elements from 2 array

  distinctive_files(){
  
    # dir1=$1
    # dir2=$2
  
    # arr1=( $( ls $dir1 ) )
    # arr2=( $( ls $dir2 ) )
  
    declare -a arr1=("er" "rt")
    declare -a arr2=("er" "rt" "ty")
  
    # echo "arr1: ${arr1[@]}"
    # echo "arr2: ${arr2[@]}"
  
    declare -A all_Elements
    common_Files=()
    distinct_Files=()
  
    for elem in "${arr1[@]}";do
        # echo "elem arr1: $elem"
        ((all_Elements[$elem]++))
        # echo "arr1 all_Elements:  ${all_Elements[$elem]}"
    done
  
    for elem in "${arr2[@]}";do
        # echo "elem arr2: $elem"
        ((all_Elements[$elem]++))
        # echo "arr2 all_Elements:  ${all_Elements[$elem]}"
    done
  
    # echo "Non common elements:"
    for key in "${!all_Elements[@]}" ; do
        if [ ${all_Elements[$key]} -eq 1 ]; then
  
            distinct_Files+=($key)
        fi
         # echo "key: $key"
    done
  
    echo "${distinct_Files[@]}"
  
    # Calling function
    disti=$(distinctive_files)
  
  

• Pass 2 arrays as arguments in bash

  # by passing number of elements of each array
  myFunc(){
    local n1=$1
    local n2=$2
    shift 2
    local arr=("$@")
    arr1=()
    arr2=()
    i=0
    for ele in "${arr[@]}"; do
       echo "ele: $ele"
       if [[ i -ge $n1 ]]; then
           arr2+=($ele)
       else
           arr1+=($ele)
       fi
       ((i++))
    done
  
    echo "arr1 ${arr1[@]}"
    echo "arr2 ${arr2[@]}"
  
  
  }
  
  declare -a arr1=("er" "rt")
  declare -a arr2=("er" "rt" "ty")
  
  (added_zipped_files ${#arr1[@]} ${#arr2[@]} ${arr1[@]} ${arr2[@]})
  
  
  
  # by passing reference(from bash 4.3 released in year 2016)
  demo_multiple_arrays() {
      local -n _array_one=$1
      local -n _array_two=$2
      printf '1: %q\n' "${_array_one[@]}"
      printf '2: %q\n' "${_array_two[@]}"
  }
  
  array_one=( "one argument" "another argument" )
  array_two=( "array two part one" "array two part two" )
  
  demo_multiple_arrays array_one array_two
  
  

• Append certain character

  #!/bin/bash
  
    # Check if a file name is provided as an argument
    if [ $# -ne 1 ]; then
    	echo "Usage: $0 <filename>"
    	exit 1
    fi
  
    # Check if the file exists
    if [ ! -f "$1" ]; then
    	echo "File '$1' not found."
    	exit 1
    fi
  
    # Append semicolon to each line in the file
    while IFS= read -r line; do
    	echo "$line;"
    done < "$1" > "$1.tmp"
  
    # Replace the original file with the modified one
    mv "$1.tmp" "$1"
  
    echo "Semicolons appended to each line in '$1'."
  
  

• to display ram of top processes based on ram

  #!/bin/zsh
  
  echo "Command                                  PSS (MB)"
  echo "================================================="
  
  smem -c "command pss" |
  	awk '{
  		cmd=$1; pss=$2; uss=$3; rss=$4;
  		pssTotal[cmd]+=pss; ussTotal[cmd]+=uss; rssTotal[cmd]+=rss;
  	} END {
  		for (cmd in pssTotal) {
  			# Convert from KB to MB
  			pssMB = pssTotal[cmd] / 1024;
  			printf "%-35s %10.2f MB\n", cmd, pssMB;
  		}
  	}' | sort -k 2 -nr | head -n 10
  

• random string generation random 16 character alphanumeric string tr -dc 'A-Za-z0-9' < /dev/urandom | head -c 16; echo '' pure bash

# Define allowed character set
chars='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
length=16
result=''

for ((i=0; i<length; i++)); do
  result+="${chars:RANDOM%${#chars}:1}"
done

echo "$result"

 

for have single quotes in literal in bash

ffmpeg -i 'The Witcher 3 In the Eternal Fire'\''s Shadow p1.mp4' -o ...