===== tasks =====
==== dummy ====
  * [[list files within a specific range]]

===== dummy =====
==== other pages in this wiki ====
  * [[bash scripting]]
  * [[shell scripting notes]]

==== useful links ====
  * https://explainshell.com/ - useful for understanding complex shell commands
  * https://mywiki.wooledge.org/ArithmeticExpression - Arithmetic Expansion
  * https://misc.flogisoft.com/bash/tip_colors_and_formatting - escape characters, color prompts, echo colored strings
  * http://ezprompt.net/ - Easy bash prompt generator

==== documentation links ====
  * ignoredups, erasedups - https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html search for ignoredups .

==== what is the difference between "ls > dirlist 2>&1" and "ls 2>&1 > dirlist"? ====
The command
<code>
ls > dirlist 2>&1
</code>
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command
<code>
ls 2>&1 > dirlist
</code>
directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.

To conclude, the order of redirections is significant.

Ref:- https://www.gnu.org/software/bash/manual/html_node/Redirections.html -> 5th paragraph.

===== getopts =====
==== useful articles ====
  * https://sookocheff.com/post/bash/parsing-bash-script-arguments-with-shopts/ - Parsing bash script options with getopts

===== Shell =====
==== what is the use of histappend? ====
See
  * https://unix.stackexchange.com/questions/6501/why-would-anyone-not-set-histappend-in-bash

===== code snippets =====
==== getopts shift OPTIND ====
<code>
shift "$((OPTIND-1))"
</code>

Notes:
  * The quotes are important for reasons explained in https://unix.stackexchange.com/questions/214141/explain-the-shell-command-shift-optind-1/

==== check if an integer is in an interval ====
<code>
    current_minute=`date +'%M'`
    clean_up_start_minute=40
    clean_up_end_minute=50
    if (( $current_minute >= $clean_up_start_minute )) &&
       (( $current_minute < $clean_up_end_minute )) ;
    then
        /* do something */
    fi
</code>

<code>
$ (( 5 >= 15 )) && (( 5 < 30 ))  && echo 1 || echo 0
0

$ (( 15 >= 15 )) && (( 15 < 30 ))  && echo 1 || echo 0
1

$ (( 20 >= 15 )) && (( 20 < 30 ))  && echo 1 || echo 0
1

$ (( 30 >= 15 )) && (( 30 < 30 ))  && echo 1 || echo 0
0

$ (( 35 >= 15 )) && (( 35 < 30 ))  && echo 1 || echo 0
0
</code>
==== check if an integer is equal to another integer ====
tags | equality
<code>
    current_minute=`date +'%M'`
    clean_up_minute=15
    if [ "$current_minute" -eq "$clean_up_minute" ];
    then
        /* do something */
    fi
</code>

It works for both positive and negative integers.
<code>
$ [ 15 -eq 2 ] && echo 1 || echo 0
0

$ [ 15 -eq 15 ] && echo 1 || echo 0
1

$ [ -15 -eq 15 ] && echo 1 || echo 0
0

$ [ -15 -eq -15 ] && echo 1 || echo 0
1
</code>

But not for floating point numbers
<code>
$ [ 1.5 -eq 2 ] && echo 1 || echo 0
bash: [: 1.5: integer expression expected
0

$ [ 15 -eq 2.0 ] && echo 1 || echo 0
bash: [: 2.0: integer expression expected
0

$ [ 2.0 -eq 2.0 ] && echo 1 || echo 0
bash: [: 2.0: integer expression expected
0
</code>
===== Scripting =====
==== echo a string with multiple spaces ====
Put the variable in double quotes to prevent field splitting
<code>
echo "$a"
</code>
Example:
<code>
$ a='^  foo|'

$ echo $a
^ foo|

$ echo "$a"
^  foo|
</code>

Ref:- https://unix.stackexchange.com/questions/273660/how-do-i-echo-a-string-with-multiple-spaces-in-bash-untouched/273663

search tags | string concatenation multiple spaces

==== stackoverflow links I came across ====
  * https://stackoverflow.com/questions/18668556/how-can-i-compare-numbers-in-bash
      * check if a number is greater than something
      * compare numbers

==== add an element to an array ====

<code>
appliances=("AC" "TV" "Mobile" "Fridge" "Oven" "Blender")
appliances+=("Dish Washer")
for appliance in "${appliances[@]}"
do
     echo $appliance
done
</code>

<code>
languages=("PHP" "MySQL" "Bash" "Oracle")
languages[${#languages[@]}]="Python"
for language in "${languages[@]}"
do
     echo $language
done
</code>

<code>
fruits=("Banana" "Mango" "Watermelon" "Grape")
fruits=(${fruits[@]} "Jack Fruit")
for fruit in "${fruits[@]}"
do
     echo $fruit
done
</code>

<code>
men=("John" "Watson" "Micheal")
women=("Lisa" "Ella" "Mila")
people=(${men[@]} ${women[@]})

for person in "${people[@]}"
do
     echo $person
done
</code>

Ref:-
  * https://linuxhint.com/bash_append_array/ - well written but the information density is low. So I just summarized the points here and renamed the variables a bit.
  * https://stackoverflow.com/questions/1951506/add-a-new-element-to-an-array-without-specifying-the-index-in-bash