https://www.slackwiki.com/index.php?action=history&feed=atom&title=Automating_tasks_with_BASH 2026-04-08T01:00:54Z Revision history for this page on the wiki MediaWiki 1.40.0 https://www.slackwiki.com/index.php?title=Automating_tasks_with_BASH&diff=75&oldid=prev 2009-06-02T03:18:06Z

<p>Copy from old</p> <p><b>New page</b></p><div>[[Category:Tips]]<br /> Often you will want to automate certain commands with BASH. The easiest way to do this is by using file globbing with wildcards otherwise called expansion. For example say I have a directory with the following files in it<br /> <br /> $ ls<br /> file1.txt file2.txt file3.txt<br /> <br /> If I type<br /> <br /> $ ls *.txt<br /> file1.txt file2.txt file3.txt<br /> <br /> The Shell will expand *.txt to match any file that matches the pattern of any number of characters followed by a .txt it then sends this list to the ls command. The ls command takes that list as a set and displays the directory listing for each one. Now most commands will work with file globbing but sometimes there are problems. For example if I wish to convert a heap of .jpeg files to 800x600 resolution with one line:<br /> <br /> $ ls *.jpeg<br /> img_2992.jpeg img_2994.jpeg img_2996.jpeg img_2998.jpeg img_3000.jpeg img_3002.jpeg<br /> img_2993.jpeg img_2995.jpeg img_2997.jpeg img_2999.jpeg img_3001.jpeg<br /> <br /> $ convert *.jpeg -resize 800x600 *.jpeg<br /> <br /> $ ls *.jpeg<br /> img_2992.jpeg img_2998.jpeg img_3002-10.jpeg img_3002-16.jpeg img_3002-3.jpeg img_3002-9.jpeg<br /> img_2993.jpeg img_2999.jpeg img_3002-11.jpeg img_3002-17.jpeg img_3002-4.jpeg img_3002.jpeg<br /> img_2994.jpeg img_3000.jpeg img_3002-12.jpeg img_3002-18.jpeg img_3002-5.jpeg<br /> img_2995.jpeg img_3001.jpeg img_3002-13.jpeg img_3002-19.jpeg img_3002-6.jpeg<br /> img_2996.jpeg img_3002-0.jpeg img_3002-14.jpeg img_3002-2.jpeg img_3002-7.jpeg<br /> img_2997.jpeg img_3002-1.jpeg img_3002-15.jpeg img_3002-20.jpeg img_3002-8.jpeg<br /> <br /> Of course that was not what we expected. There is an easier solution to this and that is to use BASH's for loop command.<br /> <br /> $ for var1 in *.jpeg; do convert $var1 -resize 800x600 $var1; done<br /> <br /> $ ls *.jpeg<br /> img_2992.jpeg img_2994.jpeg img_2996.jpeg img_2998.jpeg img_3000.jpeg img_3002.jpeg<br /> img_2993.jpeg img_2995.jpeg img_2997.jpeg img_2999.jpeg img_3001.jpeg<br /> <br /> What this command does is loop through all the .jpeg files in the directory and for each file it stores the name of that file in the variable $var1 which is then used as two arguments to convert, the input and output file names respectively.<br /> <br /> Basically it looks to the system like this<br /> <br /> convert img_2992.jpeg -resize 800x600 img_2992.jpeg<br /> convert img_2993.jpeg -resize 800x600 img_2993.jpeg<br /> convert img_2994.jpeg -resize 800x600 img_2994.jpeg<br /> convert img_2995.jpeg -resize 800x600 img_2995.jpeg<br /> convert img_2996.jpeg -resize 800x600 img_2996.jpeg<br /> convert img_2997.jpeg -resize 800x600 img_2997.jpeg<br /> convert img_2998.jpeg -resize 800x600 img_2998.jpeg<br /> convert img_2999.jpeg -resize 800x600 img_2999.jpeg<br /> convert img_3000.jpeg -resize 800x600 img_3000.jpeg<br /> convert img_3001.jpeg -resize 800x600 img_3001.jpeg<br /> convert img_3002.jpeg -resize 800x600 img_3002.jpeg<br /> <br /> You can play with this by echoing the output of the expansion to the console.<br /> <br /> $ for var2 in *.txt; do echo $var2 ;done<br /> file1.txt<br /> file2.txt<br /> file3.txt<br /> <br /> $ echo *.txt<br /> file1.txt file2.txt file3.txt<br /> <br /> Those are similar expansions but not exactly the same.<br /> <br /> Note: var1 and var2 are random variable names that I have picked. You can use other variable names like images or textfiles.</div>

Erik