Lesson 9

Date: 3/26/2014
Linux shell scripting tutorial
Linux for Engineering and IT Applications


Looping with for statement

  • Script scr4.sh 
    #!/bin/bash
for i in red white blue 
do
        echo "$i is a color"
done

  • Script backup-lots.sh 
    #!/bin/bash
for i in 0 1 2 3 4 5 6 7 8 9 ; do
        cp $1 $1.BAK-$i
done
    Now create a file important_data with some numbers in it and then run ./backup-lots.sh important_data, which will copy the file 10 times with 10 different extensions. As you can see, the variable $1 has a special meaning--it is the first argument on the command-line. Note, watch for correct syntax: 
    for i in 0 1 2 3 4 5 6 7 8 9 
do
....
done
    another correct alternative: 
    for i in 0 1 2 3 4 5 6 7 8 9  ; do
....
done


    Now modify the script: 
    #!/bin/bash
if [ "$1" = "" ] ; then
        echo "Usage: backup-lots.sh <filename>"
        exit
fi
for i in 0 1 2 3 4 5 6 7 8 9 ; do
        NEW_FILE=$1.BAK-$i
        if [ -e $NEW_FILE ]; then
                echo "backup-lots.sh: **warning** $NEW_FILE"
                echo "                already exists - skipping"
        else
                cp $1 $NEW_FILE
        fi
done

  • Looping over glob expressions 
    #!/bin/bash
for i in *.txt ; do
        echo "found a file:" $i
done

    Note, the script above loops over all *.txt files in the current directory so you need to create several files with .txt extension in this exercise. 

    #!/bin/bash
for i in /usr/share/*/*.txt ; do
        echo "found a file:" $i
done


  • Breaking out of the loops and continuing 
    #!/bin/bash
for i in 0 1 2 3 4 5 6 7 8 9 ; do
        NEW_FILE=$1.BAK-$i
        if [ -e $NEW_FILE ]; then
                echo "backup-lots.sh: **error** $NEW_FILE"
                echo "                already exists - exitting"
                break
        else
                cp $1 $NEW_FILE
        fi
done
    which causes program execution to continue on the line after the done.

    The continue statement is useful for terminating the current iteration of the loop. 
    #!/bin/bash
for i in 0 1 2 3 4 5 6 7 8 9 ; do
        NEW_FILE=$1.BAK-$i
        if [ -e $NEW_FILE ] ; then
                echo "backup-lots.sh: **warning** $NEW_FILE"
                echo "                already exists - skipping"
                continue
        fi
        cp $1 $NEW_FILE
done


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