Section 1: Review of a few Basic UNIX =
Topics=20
Shell scripting involves chaining several UNIX commands =
together to=20
accomplish a task. For example, you might run the 'date' command =
and then=20
use today's date as part of a file name. I'll show you how to do =
this=20
below.=20
Some of the tools of the trade are variables, backquotes and =
pipes.=20
First we'll study these topics and also quickly review a few other =
UNIX=20
topics.=20
Variables=20
Page 1
I/O =
Redirection=20
- Topics covered: specifying the input or capturing the =
output=20
of a command in a file=20
- Utilities covered: wc, sort=20
- The wc command counts the number of lines, words, and =
characters in=20
a file
wc /etc/passwd
wc -l /etc/passwd
- You can save the output of wc (or any other command) with =
output=20
redirection
wc /etc/passwd > wc.file
- You can specify the input with input redirection
wc =
< /etc/passwd
- Many UNIX commands allow you to specify the input file by =
name or by=20
input redirection
sort /etc/passwd
sort < /etc/passwd
- You can also append lines to the end of an existing file =
with output=20
redirection
wc -l /etc/passwd >> wc.file
Page 2
Backquotes=20
=20
and execute it (right click and save in a file) #!/bin/sh
# Illustrates using backquotes
# Output of 'date' stored in a variable
Today=3D"`date`"
echo Today is $Today
Execute the script with sh backquotes.sh
The example above shows you how you can write commands into =
a file=20
and execute the file with a Bourne shell=20
Backquotes are very useful, but be aware that they slow down =
a=20
script if you use them hundreds of times=20
You can save the output of any command with backquotes, but =
be aware=20
that the results will be reformated into one line. Try this: =
LS=3D`ls -l`
echo $LS
Page 3
Pipes=20
- Topics covered: using UNIX pipes=20
- Utilities covered: sort, cat, head=20
- Pipes are used for post-processing data=20
- One UNIX command prints results to the standard output =
(usually the=20
screen), and another command reads that data and processes it =
sort /etc/passwd | head -5
- Notice that this pipe can be simplified
cat /etc/passwd =
| head -5
- You could accomplish the same thing more efficiently with =
either of=20
the two commands:
head -5 /etc/passwd
head -5 < /etc/passwd
- For example, this command displays all the files in the =
current=20
directory sorted by file size
ls -al | sort -n -r +4
- The command ls -al writes the file size in the fifth column, =
which=20
is why we skip the first four columns using +4.=20
- The options -n and -r request a numeric sort (which is =
different=20
than the normal alphabetic sort) in reverse order
Page 4
awk=20
Page 5
=
Section 2: Storing Frequently Used =
Commands in=20
Files: Shell Scripts
Shell Scripts =
- Topics covered: storing commands in a file and =
executing the=20
file=20
- Utilities covered: date, cal, last (shows who has =
logged in=20
recently)=20
- Store the following in a file named =
simple.sh
=20
and execute it #!/bin/sh
# Show some useful info at the start of the day
date
echo Good morning $USER
cal
last | head -6
Shows current date, calendar, and a six of previous logins=20
Notice that the commands themselves are not displayed, only =
the=20
results=20
To display the commands verbatim as they run, execute =
with sh -v simple.sh
Another way to display the commands as they run is with -x =
sh -x simple.sh
What is the difference between -v and -x? Notice that with =
-v you=20
see '$USER' but with -x you see your login name=20
Run the command 'echo $USER' at your terminal prompt and see =
that=20
the variable $USER stores your login name=20
With -v or -x (or both) you can easily relate any error =
message that=20
may appear to the command that generated it=20
When an error occurs in a script, the script =
continues=20
executing at the next command=20
Verify this by changing 'cal' to 'caal' to cause an error, =
and then=20
run the script again=20
Run the 'caal' script with 'sh -v simple.sh' and with 'sh -x =
simple.sh' and verify the error message comes from cal=20
Other standard variable names include: $HOME, $PATH, =
$PRINTER. Use=20
echo to examine the values of these variables
Page 6
Storing File =
Names in=20
Variables
- Topics covered: variables store strings such as file =
names,=20
more on creating and using variables=20
- Utilities covered: echo, ls, wc=20
- A variable is a name that stores a string=20
- It's often convenient to store a filename in a variable=20
- Store the following in a file named variables.sh
=20
and execute it #!/bin/sh
# An example with variables
filename=3D"/etc/passwd"
echo "Check the permissions on $filename"
ls -l $filename
echo "Find out how many accounts there are on this system"
wc -l $filename
Now if we change the value of $filename, the change is =
automatically=20
propagated throughout the entire script
Page 7
Scripting =
With sed=20
=20
to your home directory and notice how the word 'vBNS' appears in =
it=20
several times=20
Change 'vBNS' to 'NETWORK' with sed -e =
's/vBNS/NETWORK/g' < nlanr.txt
You can save the modified text in a file with output =
redirection sed -e 's/vBNS/NETWORK/g' < nlanr.txt > nlanr.new
Sed can be used for many complex editing tasks, we have only =
scratched the surface here
Page 8
=
Section 3: More on Using UNIX =
Utilities=20
Performing =
Arithmetic=20
- Topics covered: integer arithmetic, preceding '*' =
with=20
backslash to avoid file name wildcard expansion=20
- Utilities covered: expr=20
- Arithmetic is done with expr
expr 5 + 7
expr 5 \* 7
- Backslash required in front of '*' since it is a filename =
wildcard=20
and would be translated by the shell into a list of file names=20
- You can save arithmetic result in a variable=20
- Store the following in a file named a=
rith.sh
=20
and execute it #!/bin/sh
# Perform some arithmetic
x=3D24
y=3D4
Result=3D`expr $x \* $y`
echo "$x times $y is $Result"
Page 9
Translating =
Characters=20
- Topics covered: converting one character to another,=20
translating and saving string stored in a variable=20
- Utilities covered: tr=20
- Copy the file sdsc.txt<=
/A>=20
to your home directory=20
- The utility tr translates characters
tr 'a' 'Z' < =
sdsc.txt
- This example shows how to translate the contents of a =
variable and=20
display the result on the screen with tr=20
- Store the following in a file named tr1=
.sh
=20
and execute it #!/bin/sh
# Translate the contents of a variable
Cat_name=3D"Piewacket"
echo $Cat_name | tr 'a' 'i'
This example shows how to change the contents of a variable=20
Store the following in a file named tr2=
.sh