Notes on Bash scripting session

[ccbaumler]

bash scripting by Oliver

bash flows

JPG to PNG conversion. How can this be automated?

Simple for loops on the command line.

$ ls Images/

images2.jpg images3.jpg images.jpg

$ rpm -qa ImageMagick
#checks the application

#to convert a single file
$ convert Images/images.jpg Images/images.png

$ ls Images/

images2.jpg images3.jpg images.jpg images.png

$ eom Images/*

$ for i in Images/*.jpg; do echo $i; done

images2.jpg 
images3.jpg 
images.jpg

$ for i in Images/*.jpg; do echo $(dirname $i)/$(basename $i .jpg).png; done

images2.png 
images3.png 
images.png

$ for i in Images/*.jpg; do convert $i $(dirname $i)/$(basename $i .jpg).png; done

$ ls Images/

images2.jpg images3.jpg images.jpg images.png images2.png images3.png

preamble of the for loop:
for is the built in command of the shell. Bash has variables just like other languages.
i is a user defined variable.
Images/*.jpg

; allows for the next command

body of the for loop:
do do the next command
$ return the current value of the variable
() will run the for loop across the sub-command
i the variable defined in the preamble
dirname returns the directory name
basename returns the base name/file name (when including .jpg at the end of the command the .jpg is removed by the basename command)

end of for loop:
;
done

eom -> https://man.archlinux.org/man/eom.1.en

ImageMagick -> https://anaconda.org/conda-forge/imagemagick

bash arrays

#To view large directory of many files

$ ls Images/ | less

#cause crashes in command line

$ ls *.xml 

#but this does not

$ echo *.xml

globbing with wildcards (*)

Everything in bash is a string (a flat string of characters). For good and bad.

Bash arrays allows for a bit more complexity in bash.

make is a command to make large software products.
https://www.gnu.org/software/make/manual/make.html

to build an empty array
$ args=()

#to add to the array
$ args+=(DEBUG=1)

$ args+=(Foo)

#but echo only returns the first variable
$echo $args
DEBUG=1

#to treat it like an array in bash include `()` where `[#]` is the location on the arrya
$ echo $(args[0])
DEBUG=1
$ echo $(args[1])
Foo

#to return the entire array from a specific point
$ echo ${args[0]}
DEBUG=1 Foo
$ echo ${args[1]}
Foo
$ echo ${args[0]}
DEBUG=1 Foo

$ args[0]=DEBUG=0

$ echo ${args[0]}
DEBUG=0 Foo

#create a filename that includes a string
$ mv imaged.jpg "pretty image.jpg"

$ mv imaged.jpg pretty image.jpg
mv: target 'image.jpg': No such file or directory

#to bypass the error of including a whitespace in the filename
$ files=(file1)
$ files+=(file 2)
$ echo ${files[0]}
file1 file 2

$ for i in ${files[@]}; do echo $i ; done
file1
file
2

$ for i in "${files[@]}"; do echo $i ; done
file1
file 2


$ files=(Images/*.jpg)

$ for i in "${files[@]}"; do echo $i ; done
images2.jpg 
images3.jpg 
images.jpg 
Pretty Image.jpg

Because bash uses strings, it will print each string but split them by whitespaces.

[ccbaumler]

Additional links for the bash from Nick:

And if you know regular expressions but struggle with sed/awk, this tool is a nice replacement: https://github.com/chmln/sd

This is a helpful reference if, like me, you easily forget arcane languages you only use occasionally: https://learnxinyminutes.com/

https://ucdavisdatalab.github.io/workshop_introduction_to_the_command_line/