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Bash

https://linuxhandbook.com/tag/bash-beginner/September 16, 2021

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Count the total number of lines that exist in whatever file the user enters:
Convert mp4 files to mp3
Passing multiple arguments to a bash shell script
Download remote config
Replace values in json file
Pass default value
Special variables in Bash shell

Count the total number of lines that exist in whatever file the user enters:

#!/bin/bash
echo -n "Please enter a filename: "
read filename
nlines=$(wc -l < $filename)
echo "There are $nlines lines in $filename"

Convert mp4 files to mp3

bash

# From https://askubuntu.com/questions/837916/convert-mp4-to-mp3-using-shell-script

videos=$(find . -name '*.mp4')
sounds=()
sampleRate="48000"
for video in $videos; do
    sound=${video%.mp4}.mp3
    if [ ! -f "$sound" ]; then
        ffmpeg -i "$video" -vn -acodec libmp3lame -ac 2 -qscale:a 4 -ar "$sampleRate" "$sound"
        sounds+=("$sound")
    fi
done

echo "Converted ${#sounds[@]} vidoes"

for i in "${sounds[*]}"; do
    echo -e "$i\n"
done

Passing multiple arguments to a bash shell script

script.sh arg1 arg2

#!/bin/bash
cd ..
repo=$1
cd $repo
branch=${2:-'master'}
git co $branch
echo "start $repo($branch)"
git pull
yarn
yarn start

Download remote config

#!/bin/bash

url=${1:-'example.com'}
endpoint=https://$url/config/config.json
echo "⬇️  Download config from $endpoint"

mkdir -p public/config/
curl -o public/config/config-temp.json $endpoint

Replace values in json file

#!/bin/bash

DIRNAME=$(dirname $0)
gsed -i -r 's;"label": ".*";"label": "'$(git branch --show-current --no-color)'";' ${DIRNAME}/../public/versions.json

gsed is an non-interactive stream editor. -i: edit files in place, -r: use extended regular expressions

Pass default value

Default value handling is done by parameter of the form: ${parameter:[-=?+]word} such as:

${parameter:-word} to use a default value
${parameter:=word} to assign a default value (mutate original parameter)
${parameter:?word} to display an error if unset or null
${parameter:+word} to use an alternate value if parameter is set and not null

Special variables in Bash shell

Special Variable	Description
$0	The name of the bash script.
$1, $2...$n	The bash script arguments.
$$	The process id of the current shell.
$#	The total number of arguments passed to the script.
$@	The value of all the arguments passed to the script.
$?	The exit status of the last executed command.
$!	The process id of the last executed command.

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