This lesson continues the review of shell scripting.
Objectives important to this lesson:
read vs. tr
wc
expr
local variables
aliases
Concepts:
The course lecture for this week continues examination of more
scripting commands.
One of the first assignments I had as a programmer was to scan
a data stream to identify what kind of stream it was. If I could
identify it, I could parse out the data and present it on a
computer screen. It was good that the streams in question all
came from a mainframe, and they were the result of transactions
that had been written by different programmers. That meant that
the streams were all probably different from each other in some
detectable way. I had to detect the nature of the stream because
I would not know what transaction had been started.
That relates to the first topic for this week. When you need
to parse out data for display or processing, it helps to know
what to look for.
The first problem this week looks for breaks in the data stream
in two similar but different ways. The read command
typically reads STDIN until the user presses the Enter key. Go
over the article I gave you a link for in the last sentence. No,
really do it. I am referring to an example in it in the
discussion below.
The article explains
some of the ways you can use and modify the default behavior of
read. Consider some of the goodies in example 3.1 on that site:
IFS - The Internal Field Separator defines
what characters(s) can separate words in the data stream we
are using. It can be set for a specific line if necessary.
When you are reading data from ordinary text files, the
default setting works well. It is any whitespace character.
When you are reading from a comma delimited
file, like a CSV, you need to set IFS=','.
To set it back, you can enter IFS=" ", and
read will once again expect that any
whitespace character can separate words. It this example, the
IFS is changed inside a loop, which causes that action to be
forgotten when the loop ends. Nice.
exec - this opens a
filehandle to a named file to read it (note the <) exec {file_descriptor}<"./file.csv" At the end of the code module,
he/she closes the file with a redirection
operator, pointing to &- which can be read as "file close" exec {file_descriptor}>&-
declare - The author creates an array with
a declare statement. declare -a input_array
The author opens a while loop, and sets
the value of IFS to a comma. He/She then reads each line of
the file (-u) into an array (-a), and while doing so, outputs
the first and third fields of each line in the array.
while IFS="," read -a input_array -u
$file_descriptor
do echo "${input_array[0]},${input_array[2]}"
done
The use of an array to hold words gives us a
second method to access them. Another way is shown in our text.
The question shows us a use of the trap
command, putting a newline character on the screen every time a
space is trapped by the loop. Does this do the same thing? Test
it and see.
You should also have an idea what the wc
command does, since it can be used several different ways.
Consult Help in the Linux version of your choice, and you may
find there is a syntax you don't know.
The second problem for this week uses the expr
command. Among other things, it can tell the shell to consider a
string as number. You need to
do this from time to time for validation of data. If you need to
do math with a value in a variable, it may not happen if that
variable is considered as a string instead of as an integer or a
floating point number. Problem 2 asks you to test two lines of
code, and to walk through them, explaining what happens when
they execute. This is good practice for troubleshooting more
complicated programs. The video below describes the general use
of expr.
The third problem is easier to deal with if you remember the
word local. All variables in a shell script are local. What does
that mean? The question to research here is what must be done?
There is more than one answer.
The fourth problem is really just troubleshooting. What does
the alias command do in general? What does it
do in the example in the text? How does that affect your
experience when you test it?
Assignment: Answer all problems in this week's
homework. TEST THE CODE. I will require an explanatory
screen image for each problem.
