Exploring Bash

Witty Subtitle Goes Here

Mark Drago

Vice-President of LILUG

Outline

Simple Command-Line Programs
Pipes, Redirection, STDOUT, STDIN
Short Interactive Scripts
Introduction to Scripting
Advanced Uses of Scripts
Fun Examples

Prerequisites

Some knowledge of command-line environment
- cd, ls, cp, mv, etc.
Elementary knowledge of programming is nice
- conditionals, looping, functions, etc.
There will be sections for advanced users too
- Newbies: feel free to ask questions
- Gurus: stay with me, there's some cool stuff

Simple Command-Line Programs

This slide is meant to get some building blocks in place
ls - lists files in current directory
cat - prints contents of a file out to the screen
wc - counts characters, words, and lines in files
grep - searches lines of text for a string and prints matching lines
ps - prints information about currently running processes (programs)

STDIN and STDOUT

Most command-line programs accept data on 'STDIN' and print output on 'STDOUT'
STDOUT - like speaking into a pipe where someone is listening on the other end
STDIN - like putting your ear on the end of a pipe which has someone talking into the other end of it
Now, we can connect programs with, you guessed it, pipes

Pipes

cat datafile | wc
- Yeah, so what?
Well, now we can connect anything into wc and it will count it
ls | wc -l
- count files & directories in current directory
ps -e | wc -l
- count the total number of processes running on the system

Redirection

The greater than sign: '>' allows you to redirect STDOUT to a file
cat datafile > datafile2 results in a copy of datafile
wc datafile > datafile3 stores output of wc in datafile3
The less than sign: '<' allows you to suck a file in through STDIN
wc < datafile is equivalent to cat datafile | wc

Advanced use of Pipes

Lets say you want to get a list of the 10 processes that have the most threads running
This may be an unusual need, but a similar command could be used to get the top 10 of lots of things
A script like the following would work very well:
ps -eL | awk '{print $5}' | uniq -c | sort -rn | head
- ps: prints out the processes
- awk: prints out the 5th column
- uniq: prints the number of occurences of each process
- sort: sorts the results in reverse numerical order
- head: prints out only the first 10 results.

Scripting Stuff

You can make little scripts that perform common tasks that don't necessarily require any in-depth programming knowledge
A script is just a text file with a few commands listed in them
A few things you have to do to get the script to execute
- Add '#!/bin/bash' to the top of the file, this tells Linux what interpreter to use to run the script
- You'll have to make the script executable with: chmod u+x filename
Short trip into guru land...

First Script

This script is just a few commands
It's purpose is to count the number of running apache threads and store the number in a file in our home direcory
Yeah, I don't know why you'd want to do that either.
It's actually possible to shrink the whole script down to one command:
ps -eL | grep httpd | wc -l > ~/num_httpd_procs

#!/bin/bash

#put a list of all threads into /tmp/all_procs
ps -eL > /tmp/all_procs

#filter out everything that has 'httpd' in it and put in /tmp/httpd_procs
cat /tmp/all_procs | grep httpd > /tmp/httpd_procs

#count all lines in /tmp/httpd_procs and store count in /tmp/num_httpd_procs
cat /tmp/httpd_procs | wc -l > /tmp/num_httpd_procs

#copy num_httpd_procs to desktop
cp /tmp/num_httpd_procs /home/mdrago

#remove temporary files
rm /tmp/num_httpd_procs
rm /tmp/httpd_procs
rm /tmp/all_procs

Picking Up the Pace - Variables

Bash offers many features that make it much more than just a 'batch' script
Variables can be used to remember values and strings
When a variable is being assigned a value, use it like this:
var=10
When you are referring to the value stored in the variable, prefix the name with a dollar sign: echo $var
var1=10; var2=20; echo $var1 $var2

Picking Up the Pace - Loops

Bash also provides looping constructs

dir="/home/mdrago/Desktop/BashTalk/Scripts"

for file in `ls $dir`; do
  wc $dir/$file
done

You can use C-Style 'for' loops in bash also

for ((i=0; i <= 10; i++)); do
  echo $i
done

Picking Up the Pace - Conditionals

Conditionals (if-then-else statements) can be tricky in bash because you have to be exact with spacing, etc.
```
var=10
if [ $var -eq 10 ]; then
  echo 'var equals 10'
fi
```
Gurus: explanation for strict spacing requirements

Useful File Tests

There are also many useful tests that can be done against files, the following command tests if a file exists

file='/home/mdrago/filename'
if [ -e $file ]; then
  echo "$file exists"
else
  echo "$file does not exist"
fi

Here's a short list of other useful file tests:
- -f: file is a regular file
- -d: file is a directory
- -w: file has write permissions

Putting It All Together

Goal: Resize photos taken from a digital camera
ImageMagick includes command-line programs to manipulate pictures
- mogrify: resizes, rotation, color reduction, etc.
- montage: composites multiple images together
- animate: makes an animation from multiple images
- convert: a kind of grab bag

This should resize all files in the current directory to 800x600

for file in `ls *.jpg`; do
  convert "$file" -resize 800x600 small-$file
done

Resize Pictures v0.1

#!/bin/bash

dir="Thumbnails-$1"

mkdir -p $dir

for file in `ls *.jpg`; do
  convert "$file" -resize $1 $dir/$file
done

This script has a few features
- It lets us choose the dimensions for the picture
- It creates a 'thumbnails' directory labeled with the size of the pictures
- But, it is not very user-friendly. Lets see if we can help that.

Using `dialog` to Resize Pictures

dialog is a program that creates interfaces in the console that the user can interact with.
It prints what the user enters so scripts can get at the information.
The dialog command we use in this script is:
dialog --ok-label 'Resize' --inputbox 'Enter Desired Image Size' 8 30 '800x600' 2> $tempfile

#!/bin/bash

tempfile='/tmp/dialog'

dialog --ok-label 'Resize' --inputbox 'Enter Desired Image Size' 8 30 '800x600' 2> $tempfile

exitstatus=$?

size=`cat $tempfile`

if [ $exitstatus -eq 0 ]; then
    dir="Thumbnails-$size"
    mkdir -p $dir

    for file in `ls *.jpg`; do
        convert "$file" -resize $size $dir/$file
    done
fi

Using `zenity` to Resize Pictures

dialog uses the 'curses' library to generate a user interface
zenity uses GTK+ to do the same thing
- kdialog uses the QT toolkit
Nautilus (the Gnome File Manager) has a few nice features that can aid scripts also.
Using zenity and nautilus we're able to extend the desktop with bash shell (or any other kind of) scripts.
Placing a script in the ~/.gnome2/nautilus-scripts directory results in it being seen in the right-click menu in nautilus

#!/bin/bash

size=`zenity --entry --title='ResizePics' --text='Enter the desired size for th\e selected pictures.' --entry-text='800x600'`

dir="Thumbnails-$size"
mkdir -p $dir

for file in $@
do
    #get just the filename
    shortfile=`basename "$file"`

    #convert to smaller size and store in $dir
    convert -size $size "$file" -resize $size "$dir/$shortfile"

done

Further use of `zenity`

One big problem with this script is that it takes a long time to finish.
It would be nice if we could notify the user that work is getting done.
We can use zenity to show a progress meter
The command given to zenity to use a progress meter is the following:
zenity --progress --title="ResizePics" --text="Resizing Pictures to $size"
You can then 'echo' a percentage into zenity and it will display it.

#!/bin/bash

size=`zenity --entry --title='ResizePics' --text='Enter the desired size for the selected pictures.' --entry-text='800x600'`

dir="Thumbnails-$size"

mkdir -p $dir

num_done=0
total=$#

(for file in $@
do

    #get just the filename
    shortfile=`basename "$file"`

    #convert to smaller size and store in $dir
    convert -size $size "$file" -resize $size "$dir/$shortfile"

    num_done=$(($num_done+1))

    percent=$(($num_done*100/total))

    echo $percent

done) | zenity --progress --title="ResizePics" --text="Resizing Pictures to $size"

The End

Any Questions?
References:
- Advanced Bash Scripting Guide
  www.tldp.org/LDP/abs/html/
- dialog man page
- zenity man page
- builtin man page (all about bash builtins)
- ps man page
- ...other various man pages
S5: A Simple Standards-Based Slide Show System
- www.meyerweb.com/eric/tools/s5/

LILUG / December 14th, 2004

Exploring Bash: Witty Subtitle Goes Here

Exploring Bash

Witty Subtitle Goes Here

Mark Drago

Vice-President of LILUG

Outline

Prerequisites

Simple Command-Line Programs

STDIN and STDOUT

Pipes

Redirection

Advanced use of Pipes

Scripting Stuff

First Script

Picking Up the Pace - Variables

Picking Up the Pace - Loops

Picking Up the Pace - Conditionals

Useful File Tests

Putting It All Together

Resize Pictures v0.1

Using `dialog` to Resize Pictures

Using `zenity` to Resize Pictures

Further use of `zenity`

The End

LILUG / December 14th, 2004

Exploring Bash: Witty Subtitle Goes Here

Exploring Bash

Witty Subtitle Goes Here

Mark Drago

Vice-President of LILUG

Outline

Prerequisites

Simple Command-Line Programs

STDIN and STDOUT

Pipes

Redirection

Advanced use of Pipes

Scripting Stuff

First Script

Picking Up the Pace - Variables

Picking Up the Pace - Loops

Picking Up the Pace - Conditionals

Useful File Tests

Putting It All Together

Resize Pictures v0.1

Using dialog to Resize Pictures

Using zenity to Resize Pictures

Further use of zenity

The End

Using `dialog` to Resize Pictures

Using `zenity` to Resize Pictures

Further use of `zenity`