Loops

• Suppose you have been doing a repetitive task running the same set of commands for a project

• For example, you need to create 100 new directories numbered from 0 to 99

  • You would need to run, mkdir 0, mkdir 1,…, all the way to mkdir 99

• In this case, you can use something called bash loops

  • Loops iterate a set of commands for as many arguments given

    • The command in the previous scenario would be mkdir NUMBER, and arguments would be NUMBER (1 through 99)

Example: for Loops

# On command line, type each line below and hit enter
mkdir test
cd test
for NUMBER in {1..99}; do # hit enter, NUMBER is the list 1 to 99 
    mkdir -p $NUMBER # yours might not be tabbed, hit enter, to repeat make directory command
done # end for loop, hit enter

• ls and you should see 100 folders made

• Cleanup by cd ../; rm -r test

Example: “traditional for loop” way

for ((a=1; a <= 3; a++))
do
    echo $a
done
# => 1
# => 2
# => 3

• They can also be used to act on files. This will run the command cat on file1 and file2.

for Variable in file1 file2
do
    cat "$Variable"
done

• or the output from a command, this will cat the output from ls.

for Output in $(ls)
do
    cat "$Output"
done

• can also accept patterns, like this to cat all the Markdown files in current directory

for Output in ./*.markdown
do
    cat "$Output"
done

Example: while Loops

while [ true ]
do
    echo "loop body here..."
    break
done
# => loop body here...