Scripting

quick overview of for, if, and while

for VAR in LIST
do
COMMANDS
done

for (( INITIALIZE; TEST; INCREMENT ))
do
COMMANDS
done

if COMMAND
then
COMMANDS
elif COMMAND
then
COMMANDS
else
COMMANDS
fi

when if’s COMMAND succeeds (i.e., exit status of 0), do then COMMANDS

test (also written [ ]) provides a lot of useful tests

use ! for negation, || for or, && for and

while COMMAND
do
COMMANDS
done

while also treats an exit status of 0 as true

pgplot example

conditional to check for directory (-d)

for loop

use of variable inside a string

" " allow shell substitutions (e.g., variables), ' ' do not

#!/bin/bash, chmod +x

exercise: pgplot extensions

compute number of files instead of hard-coding
(ls pgplot* | wc -l)

generate a file that lists the copied files in order
(> filename to initialize, echo "…" >> filename inside loop)

thumbnail example

processing arguments

$# is the variable for the number of arguments

doesn’t count $0

$0 is the command

$1, $2, … are the arguments

$@ is the list of all the arguments (not including $0)

if [ $# -eq 0 ], for arg in $@

redirect stdout to stderr (1>&2)

exit

terminates the script, takes a number as an argument to use as the exit status

$? is the variable for the most recent exit status

by convention, an exit status of 0 means success, and anything else means some kind of failure

parameter substitution

${parameter#word} deletes shortest match for word from the beginning of parameter (## for longest match)

${parameter%word} deletes shortest match for word from the end of parameter (%% for longest match)

${parameter/pattern/string} replaces first match for pattern in parameter with string

(use ${parameter//pattern/string} to replace all matches)

if pattern begins with #, it must match at the beginning of parameter

if pattern beings with %, it must match at the end of parameter

exercise: thumbnail extension

check that image file exists
(conditional to check for file: -f, continue)