Intro to Bash Scripting

Class Description
The class: a four week course on Tues evenings covering basic and intermediate scripting in the bash shell. We will examine a file with about 15 lines of code together each night, modify the code, run it, and come up with our own scripts. We will also get to know the "Advanced Bash Scripting Guide" [1] and the Gnu "Bash Reference Manual" [2] in some depth and learn to research and solve our own programming problems. User projects are encouraged -- bring your problems and we will solve them together!

I am a fifth year Ph D student from Berkeley in Demography, writing an anthropology of a small lumber town in Oregon. I have worked as a programmer in Linux for almost 10 years, and I am currently employed part time as a software project manager at Portland State.

The first class target: Oct 28, 5:00 to 6:30, 2008. I will probably have to take off a week and delay a class in the middle of the sequence.

Some helpful links to get us started:

http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html

http://tldp.org/LDP/abs/html/

http://www.gnu.org/software/bash/manual/bashref.html

http://www.intuitive.com/wicked/wicked-cool-shell-script-library.shtml

Class Outline
Day One (2008-10-28): What is a "script" and what is a "variable". We will write a simple script in nano, with comments, a "shebang" line, appropriate permissions, and simple output. We work on the idea of a variable, using shell expansion to assign output to variables, interpolating variables, and exporting environment variables. We will also examine the output and input streams ("stdin", "stdout", "stderr"). We will "comment-out" code. Finally we will talk about style, including indentation, variable names, trickiness, and comment-first scripting.

Day Two (2008-11-04): "For-loops", "word splitting", and printf. We will explore the for-loop in all its glory, going over lists stored in variables. This will require a discussion of how Bash automatically splits strings into words and how we can control this through quoting syntax. We will also make interesting formatting commands with the printf command.

Day Three (2008-11-11): Conditionals ("if/ then" statements). We will show how to write "if" and "case" statements, and incorporate pattern matching and "file tests" into our scripts.

Day Four (2008-11-25): File input, "while-read", and useful scripting. We will investigate how to get an input line from a file, parse it into useable pieces, and do interesting things with that data (like write a simple email "spambot").

Class Approach
Each class period I will have a file of code that is on the wiki. We will go over it together, showing you how to look up questions using online documentation. You will type examples from this wiki page, with modifications to make your own scripts, run them from the command line, and fix bugs.

Every day instructions
Each new class, make a directory classX, cd to it, and do your work there; then if you want to send yourself the files via email you have the option. Open two terminals and cd to this directory in both.

Each time we cover a script, open a file in your favorite editor named classX-scriptY.sh where X stands for the class (1, 2, 3, 4), and Y stands for the script we are working on in class. Type all the code in. Save the file. In your other terminal window, chmod 0700 the script, then try to run it.

Each day please also open the three links above to the various pieces of bash documentation. As we have questions we will try to look up the answers here rather than relying on me. (Teach a person to fish...)

Note that this class will only cover the tip of the iceberg for Bash scripting; to learn more -- follow the bash links above, type in the code you find while you read the explanations, and experiment.

Note also that this class is a work in progress, and I might jump into the wiki to make a change to the notes or the code for next time. Feel free to suggest changes or ask questions, especially if you buy me dinner after class.

First day -- scripts and variables
What is a shell script anyway? It is ...

... a file of text ...

... full of unix commands, variables, and control structures ...

... that usually executes from top to bottom ...

... using variables to hold data ...

... and loops and conditionals to do fancy programming stuff...

... with a way to read input and write output ...

... that probably has some "side-effects".

Why learn shell? Old-school Unix style? Class?

Script 1
Following just prints my name. Run it as is, but then change it to print your name.


 * 1) !/bin/bash
 * 2) script 1 wsprague@nospam.org 2008-10-29
 * 3) Script just prints my name
 * 1) Script just prints my name

FIRSTNAME='Webb' LASTNAME='S.' echo "Hello, $FIRSTNAME $LASTNAME"

Edit it by running the following: nano class1-script1.sh, then save it (ctrl-O in nano), then chmod 0700 class1-script1.sh, and run as ./class1-script1.sh

in the other terminal window.

Script 2
This script takes a parameter from the command line and uses it as the name: "variable expansion". Note the expansion, but also how different quotes or lack thereof have different effects.


 * 1) !/bin/bash
 * 2) script 2 wsprague@nospam.org 2008-10-29
 * 3) Prints names from command line
 * 1) Prints names from command line

NAME=$1 echo "Hello, $NAME." echo 'Hello, $NAME.' echo Hello, $NAME. echo Hello, "$NAME". echo Hello, \"$NAME\". echo "Hello, ${NAME}with text." echo "Hello, $NAMEwith text."

try from the command line:

./class1-script2.sh ./class1-script2.sh Foobar ./class1-script2.sh "Foobar Smith" ./class1-script2.sh Foobar Smith

Script 3
This script does some basic math, and then outputs it using variable expansion. Check out the quoting. Also note what happens when we try to do math on weird input.


 * 1) !/bin/bash
 * 2) Script adds two numbers from command line
 * 1) Script adds two numbers from command line

LEFT=$1 RIGHT=$2 RES=$(( $LEFT + $RIGHT )) echo $(( $LEFT + $RIGHT )) echo '$(( $LEFT + $RIGHT ))' echo "$(( $LEFT + $RIGHT ))" echo "$LEFT + $RIGHT = $RES." echo '$LEFT + $RIGHT = $RES.' # Why does this do what it does?

try from the command line:

./class1-script3.sh 100 10 ./class1-script3.sh 'one hundred' 'ten' ./class1-script3.sh 34

Script 4
This script does some "shell expansion" using the unix command "date", which gives a formatted string of the date; use "date --help" to see more).


 * 1) !/bin/bash
 * 2) Script 4.  Script calculates the year if we give it years from now.
 * 1) Script 4.  Script calculates the year if we give it years from now.

YEAR=$( date +'%Y' ) YEARS_FORWARD=$1 echo "Start at year $YEAR, finish at year $(($YEAR + $YEARS_FORWARD))"

Try from the command line

./class1-script4.sh ./class1-script4.sh 10 ./class1-script4.sh "ten"

Script 5

 * 1) !/bin/bash
 * 2) Script 5 takes a filename, strips any spaces from it, and moves the original file
 * 3)    to the new name.
 * 1)    to the new name.

RES=$( echo $1 | sed 's/ /_/g' ) echo "After stripping of spaces, \"$1\" looks like \"$RES\"" # Why do we escape the quotes here? echo mv "$1" "$RES" # try this same script without "echo" here, after touching a file with a name that you put into the parameters of the script

Try from the command line:

touch "a file with spaces in the name" ./class1-script5.sh "a quoted filename with spaces" ./class1-script5.sh an unquoted filename with spaces ././class1-script5.sh a_filename_wo_spaces

Discussion
Coding style -- variable names, comments, indentation, trickiness.

Comment first design.

"Commenting out" code.

Second day -- for-loops
What is a for-loop? It is a bash construct that repeatedly grabs one item from a sequence of data separated by whitespace, does something with that piece of the sequence, until there is no more data.

Here is the paradigm:

for VARIABLE in SEQUENCE; do # bunch of statements in here that are done repeatedly # referencing $VARIABLE # more statements done

Finger exercises
Edit a few files with nano, find them with ls, and delete them with rm

Run the command `seq 1 10` from the command line

Run the command `printf "hello %s, my name is %s, I am %i years old" Tarzan Jane 32`

Run the command `factor 144` from the command line

Script 0

 * 1) !/bin/bash
 * 2) script 0 wsprague@nospam.org 2008-10-29
 * 3) Basic for loop with seq and printf
 * 1) Basic for loop with seq and printf

START=$1 FINISH=$2 FULL_SEQ=$( seq $START $FINISH ) COUNT=0

printf "Start = %i, finish = %i\n" $START  $FINISH for X in $FULL_SEQ; do   echo "touching file.$X" touch "file.$X" COUNT=$(( $COUNT+1 )) done printf "finished working on %i files\n" $COUNT

Save this as class2.script0.sh, then chmod 0700 class2.script0.sh. Run as follows:

./class2.script0.sh 1 10 ./class2.script0.sh 4 14

Examine your directory to see the new files and their names. Those are from this script.

Script 1

 * 1) !/bin/bash
 * 2) script 1 wsprague@nospam.org 2008-10-29
 * 3) Do some fancy formatting with printf,
 * 4)    by calculating the first 10 "orders of magnitude", and printing them
 * 1)    by calculating the first 10 "orders of magnitude", and printing them

MAX=7 SEQ=$( seq 0 $MAX) echo $SEQ for I in $SEQ; do   OUT=$(( 10 ** $I )) printf "The %3i order of magnitude = %i.\n" $I $OUT done

Run this as

./class2.script2.sh

Modify it to make "MAX" store the number from $1.

Script 2
The following factors a number and creates a bunch of files.


 * 1) !/bin/bash
 * 2) script 2 wsprague@nospam.org 2008-10-29
 * 3) Takes a name and a number as parameters,
 * 4)    factors the number, touches all files "$name.$number".
 * 1)    factors the number, touches all files "$name.$number".

NAME=foobar # use $1 NUMBER=12  factor $NUMBER | sed 's/^[0-9]*://g' ) printf "Factors working on: %s\n" "$FACTORS" echo

TEST=1 COUNT=0 for SUFFIX in $FACTORS; do   TEST=$(($TEST * $SUFFIX)) COUNT=$(($COUNT + 1)) touch "$NAME.$SUFFIX" printf "Touched: prefix = %s, suffix = %i\n" $NAME $SUFFIX done printf "\nFinished working on %i files\n" $COUNT

Run this script as

./class2.script2.sh

Then modify it to set "NAME" and "NUMBER" from the command line.


 * 1) !/bin/bash
 * 2) script 3 wsprague@nospam.org 2008-10-29
 * 3) Generate times tables by using a nested loop.
 * 1) Generate times tables by using a nested loop.

START=$1 FINISH=$2 INCREMENT=1 # try changing this SEQ=$(seq $START $FINISH )

printf "     " for x in $SEQ; do   printf "%4i " $x done
 * 1) Print top row

printf "\n    " for x in $SEQ; do   printf "_____" done
 * 1) Print top row separator

echo

for x in $SEQ; do   printf "\n%4i| " $x for y in $SEQ; do       printf "%4i " $(( $x * $y )) done; done; printf "\n"
 * 1) Fill in each row with left label and cell result

Run this as

./class2-script3.sh 1 12

Discussion
Debugging loops with echo statements

Precalculating things like sequences

Code style -- when to add whitespace between sections and stanzas

Script 1 -- is a number greater than 10

 * 1) some conditional code goes here

Script 2 -- do we have command line parameters (finally!)
...

Script 3 -- filter output from a sequence (if, then, elif, else)
...

Script 4 -- using case
...

Discussion
...

Fourth day -- while-read and functions
We are going to read a file with email addresses (and other information like "valid") and send spam to all of them.

This is going to be a prelude to the command line social networking application we will write in the sequel to this class -- "Bash application development with SQL databases"

Script 1 -- simple while-read
Note that you can memorize the idiom, then figure out what each piece means.

Script 2 -- while-read with complex processing
...

Script 3 -- simple functions
...

Stuff
C=1 while read FOO ; do                # files and dirs  that exist in cwd if -e $FOO ; then echo "Got existing: $FOO" fi
 * 1) Take each line of stdin into FOO
 * 2)  and do something with it

case $FOO in       $PATTERN) echo "got a $PATTERN: $FOO";;        *) echo "else" > /dev/null;; esac C=$(( $C + 1 )) done echo "Evaluated $C files"

PSQL="/opt/local/lib/postgresql83/bin/psql" CMD=" select zcta, astext(centroid(the_geom)), 'blah blah' from zips order by zcta " $PSQL postgis_pdx_2008 -F ' ' -A -t -c "$CMD" | while read ZCTA POINT; do    echo "$ZCTA: $POINT.  Yippee!" done
 * 1) Nifty thing that could read a database and send out emails