3-bash-loops.md

layout	root	title	author	date	output
lesson	../..	Loops in Bash	Revised by Mark Schildhauer from Matt Jones 2014	June, 2016	html_document

Loops tutorial objective

• Learn why it's useful to use loops
• Practice basic loop syntax

Why loops

• Automate repetitive tasks compared to the GUI
• Increase efficiency
• Fewer commands, less likelihood for errors

A basic loop

$ for filen in paleo*
>    do echo $filen
> done

produces

paleo-mammals-v2.txt
paleo-mammals-v3.txt
paleo-mammals.txt

Running multiple commands in a loop

$ for file in paleo-mammals-v?*
> do
>        echo $file
>        wc -l $file
>        diff -q paleo-mammals.txt $file
> done

produces

paleo-mammals-v2.txt
      23 paleo-mammals-v2.txt
Files paleo-mammals.txt and paleo-mammals-v2.txt differ
paleo-mammals-v3.txt
      23 paleo-mammals-v3.txt
Files paleo-mammals.txt and paleo-mammals-v3.txt differ

Renaming files

• Imagine you have a lot of data files named similarly
• To rename all of them, try a loop like:

$ for filename in *.csv
> do
>    mv $filename orig-$filename
> done

Bash scripts

• Series of commands can be saved in a file and run at any time
• Useful to write a program that automates a task
• Start with #! syntax to indicate which program should be used for the script
  • e.g., #!/bin/bash to use the bash shell

A simple bash script

#!/bin/bash

# Example of a simple shell script
# Makes copy of files specified, with "backup-" 
# appended to original filename

PREFIX="backup-"
FILES=$@
# Above parameters of "$@" expand out 
# to positional parameters-- e.g. file1, file2

for file in $FILES
do
    cp $file $PREFIX$file
done

• Store above bash script in filename of your choosing, e.g. "batch-rename"
• Make "batch-rename" file executable: chmod 755 batch-rename
• Execute script: ./batch-rename *
• All files in current directory will now have a copy that is prefixed "backup-original-name"

Online resources

• http://www.tldp.org/LDP/Bash-Beginners-Guide/html/Bash-Beginners-Guide.html/