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joplin notion extension/linux/Bash Basics.md
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joplin notion extension/linux/Bash Basics.md
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Bash Basics
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https://devhints.io/bash
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**Note**
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- for executing commands if uses single quotes then it will pass literal text , double quotes allows commands and variables to be execured
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- for loop
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```bash
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for arg in "$@"; do
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echo "$arg"
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done
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for (( i=0;i<$filesCount;i++)); do
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singleFile="${filesArray[$i]}"
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#here I replace ALL the spaces in the file's name, by underscores.
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trimmed=${singleFile// /\_}
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mv "${singleFile}" "${trimmed}"
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echo "renamed ${singleFile} as ${trimmed} !"
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done
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```
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- if elif else
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set of if else statements are terminated with fi
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```bash
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# String
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if [[ -z "$string" ]]; then
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echo "String is empty"
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elif [[ -n "$string" ]]; then
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echo "String is not empty"
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else
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echo "This never happens"
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fi
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```
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- Logical operators
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```bash
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# Logical AND and OR
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if [[ $varA == 1 && ($varB == "t1" || $varC == "t2") ]]; then
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# NOT
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if ! [[ $var == 2 || $var == 30 || $var == 50 ]] ; then
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do something
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fi
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if [[ ! ($var == 2 || $var == 30 || $var == 50) ]] ; then
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do something
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fi
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```
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- Functions
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```bash
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myfunc() {
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echo "hello $1"
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}
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# Same as above (alternate syntax)
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function myfunc {
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echo "hello $1"
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}
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myfunc "John"
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# Returning values
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myfunc() {
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local myresult='some value'
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echo "$myresult"
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}
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result=$(myfunc)
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```
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- Geting filename and extemsion
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```bash
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# First, get file name without the path:
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filename=$(basename -- "$fullfile")
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# or
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filename="${FILEPATH##*/}"
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extension_only="${filename##*.}"
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filename_with_noext="${filename%.*}"
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```
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- array
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```bash
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# Declaring an array
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arr=()
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arr1=("hey" "there" "how")
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arr2=("er" "yu")
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declare -a arr3=()
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# Declare associative array to be able to have keys rather than index
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declare -A asscoArr
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# array elements is "${arr1[@]}"
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for ele in "${arr1[@]}";do
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echo $ele
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done
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# Output
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hey
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there
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how
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# array index/keys is "${!arr1[@]}"
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for i in "${!arr1[@]}";do
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echo $i
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done
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# Accessing elements through index
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"${arr1[2]}"
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# getting length of array
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${#arr1[@]}
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```
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- shift
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To shift arguments to left so that first argument is lost
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- C like switch case in bash
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```bash
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#!/bin/bash
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# Loop through all the arguments
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for arg in "$@"; do
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case $arg in
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-a|--optionA)
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echo "Option A detected!"
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# handle option A
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;;
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-b|--optionB)
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echo "Option B detected!"
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# handle option B
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;;
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*)
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# Handle positional arguments or invalid flags
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;;
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esac
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done
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```
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- pasing variable arguments
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```bash
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# Define your arguments in an array
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passargs=(-x '*/node_modules/*')
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# Use the array with the command
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zip -r archive.zip . "${passargs[@]}"
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# following method wont work
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passargs="'-x */node_modules/*'"
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zip -r $ARCHIVE_NAME $FILES_TO_ARCHIVE $passargs
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```
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- convert string which r separted by spaces to array
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using declare will resolve some bugs related to delimiters
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```bash
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# arrow looks like following
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arr1=('hey' 'there')
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# if
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str="hey there"
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declare -a arr=($str)
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# now
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# arr is ('hey' 'there')
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```
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- extract parent directory
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```bash
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// Output : /home/username
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echo "$PWD"
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// Output : /home
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echo "${dirname $PWD}"
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```
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### Examples
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- Find non common elements from 2 array
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```bash
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distinctive_files(){
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# dir1=$1
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# dir2=$2
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# arr1=( $( ls $dir1 ) )
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# arr2=( $( ls $dir2 ) )
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declare -a arr1=("er" "rt")
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declare -a arr2=("er" "rt" "ty")
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# echo "arr1: ${arr1[@]}"
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# echo "arr2: ${arr2[@]}"
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declare -A all_Elements
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common_Files=()
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distinct_Files=()
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for elem in "${arr1[@]}";do
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# echo "elem arr1: $elem"
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((all_Elements[$elem]++))
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# echo "arr1 all_Elements: ${all_Elements[$elem]}"
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done
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for elem in "${arr2[@]}";do
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# echo "elem arr2: $elem"
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((all_Elements[$elem]++))
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# echo "arr2 all_Elements: ${all_Elements[$elem]}"
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done
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# echo "Non common elements:"
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for key in "${!all_Elements[@]}" ; do
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if [ ${all_Elements[$key]} -eq 1 ]; then
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distinct_Files+=($key)
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fi
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# echo "key: $key"
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done
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echo "${distinct_Files[@]}"
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# Calling function
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disti=$(distinctive_files)
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```
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- Pass 2 arrays as arguments in bash
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```bash
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# by passing number of elements of each array
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myFunc(){
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local n1=$1
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local n2=$2
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shift 2
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local arr=("$@")
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arr1=()
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arr2=()
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i=0
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for ele in "${arr[@]}"; do
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echo "ele: $ele"
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if [[ i -ge $n1 ]]; then
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arr2+=($ele)
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else
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arr1+=($ele)
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fi
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((i++))
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done
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echo "arr1 ${arr1[@]}"
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echo "arr2 ${arr2[@]}"
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}
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declare -a arr1=("er" "rt")
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declare -a arr2=("er" "rt" "ty")
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(added_zipped_files ${#arr1[@]} ${#arr2[@]} ${arr1[@]} ${arr2[@]})
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# by passing reference(from bash 4.3 released in year 2016)
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demo_multiple_arrays() {
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local -n _array_one=$1
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local -n _array_two=$2
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printf '1: %q\n' "${_array_one[@]}"
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printf '2: %q\n' "${_array_two[@]}"
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}
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array_one=( "one argument" "another argument" )
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array_two=( "array two part one" "array two part two" )
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demo_multiple_arrays array_one array_two
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```
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- Append certain character
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```bash
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#!/bin/bash
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# Check if a file name is provided as an argument
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if [ $# -ne 1 ]; then
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echo "Usage: $0 <filename>"
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exit 1
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fi
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# Check if the file exists
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if [ ! -f "$1" ]; then
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echo "File '$1' not found."
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exit 1
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fi
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# Append semicolon to each line in the file
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while IFS= read -r line; do
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echo "$line;"
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done < "$1" > "$1.tmp"
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# Replace the original file with the modified one
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mv "$1.tmp" "$1"
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echo "Semicolons appended to each line in '$1'."
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```
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- to display ram of top processes based on ram
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```sh
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#!/bin/zsh
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echo "Command PSS (MB)"
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echo "================================================="
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smem -c "command pss" |
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awk '{
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cmd=$1; pss=$2; uss=$3; rss=$4;
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pssTotal[cmd]+=pss; ussTotal[cmd]+=uss; rssTotal[cmd]+=rss;
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} END {
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for (cmd in pssTotal) {
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# Convert from KB to MB
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pssMB = pssTotal[cmd] / 1024;
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printf "%-35s %10.2f MB\n", cmd, pssMB;
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}
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}' | sort -k 2 -nr | head -n 10
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```
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- random string generation
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random 16 character alphanumeric string
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`tr -dc 'A-Za-z0-9' < /dev/urandom | head -c 16; echo ''`
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pure bash
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```sh
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# Define allowed character set
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chars='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
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length=16
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result=''
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for ((i=0; i<length; i++)); do
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result+="${chars:RANDOM%${#chars}:1}"
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done
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echo "$result"
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```
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for have single quotes in literal in bash
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```bash
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ffmpeg -i 'The Witcher 3 In the Eternal Fire'\''s Shadow p1.mp4' -o ...
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```
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