Sample Shell programs

norm - set permissions as open as possible

chmod  u+rwx,go+rx-w  $*

hide - as hidden as possible

chmod  u+rwx,go-rwx  $*

semihide - just open enough as needed for Web

directories:

chmod  u+rwx,g-rwx,o+x-rw  $*

files (web pages, images, etc):

chmod  u+rwx,g-rwx,o+r-xw  $*

"norm" could of course replace all 3 if you don't mind granting more access than strictly necessary.

lsifexists - silent ls

If we "ls" a list of files - some of which exist and some do not - we get error messages.
e.g. Type this in a Web directory:

 ls -l *html *css *js *php

But what if one of these groups does not exist.
Can we make a silent ls that does not complain about non-existing files?

Here is a quiet version using a for loop and an if statement:

for i in $*
do
        if test -f $i
        then
         ls -l $i
        fi
done

In fact, we could just use regular ls and redirect stderr to null.

rmifexists - silent rm

Let's say every day we run a cleanup script that removes old editor .bak files:
rmifexists *.bak
If there are none today, or in this directory, this will give error messages.
Can we make a silent rm that does not complain about non-existing files?

for i in $*
do
        if test -f $i
        then
         rm $i
        fi
done

In fact, rm -f will do the job with no warnings
rm

Question

Why not:

        if test -f $*
        then
         rm $*
        fi

Recursive rm

rm -r (recursive rm)
Don't type:
rm -rf /

"Trying out some Deadly Linux Commands".
Includes typing rm -rf / and other scary commands, including the hilarious: mv / /dev/null
See more videos of people typing rm -rf /

A Steam for Linux shell script included a highly dangerous rm -rf line in 2015.
This led, depending how it was run, to entire user filesystems being destroyed.

wipe - clean up editor backup files

rmifexists *%
rmifexists .*% 

rmifexists  *~
rmifexists .*~

rmifexists  *.bak
rmifexists .*.bak

rmifexists  *.BAK
rmifexists .*.BAK

Easier than having to point-and-click each one. Especially if do this for multiple directories.
Safer than typing "rm *bak" every day. One day you will type "rm * bak"
In general, if you regularly type some command that would be dangerous if you make a typo, it would be better to debug it once and put it in a script and never type it directly again.

Exercise

Write a recursive wipe.

Command-line image processing

We need libjpeg utilities.
Last time I checked, at DCU this was:

Installed on PCs in labs.
Installed on student.computing.dcu.ie (ssh).

To make 1/4 size versions of 10,000 JPEGs without ever opening an image editor (or doing any work):

for i in *jpg do djpeg -scale 1/4 -bmp $i > temp.bmp cjpeg temp.bmp > small.$i done

JPEG needs to be decoded to a BMP (bitmap), then be re-sized, then re-coded back to JPEG.
Can do this in one line, leaving out the temporary file: Pipe result of djpeg into input of cjpeg.

Extract images from PDFs

I was once given an archive of thousands of scanned historical images. They were all inside PDFs.
I automatically extracted all the JPEGs from the PDFs using pdfimages
Last time I checked, at DCU this was:

Installed on PCs in labs.
Installed on student.computing.dcu.ie (ssh).

for i in *pdf # i = x.pdf do x=`basename "$i" ".pdf"` # get root filename x (without .pdf bit) pdfimages -j $i $x # extracts images to x-nnn.jpg done

# extract images and save as default PPM:
pdfimages file.pdf prefix

# extract images and save as JPEGs:
pdfimages -j file.pdf prefix

# sometimes JPEGs do not extract well, save all images as PNG:
pdfimages -png file.pdf prefix

# just list all images without extract:
pdfimages -list file.pdf  

pdfimages -help