1. Unix Scripting Dave Yearke
Session 2
March 13, 2008

2. Tangent: Text Editors
nano or pico – Really easy to use, not much power, but good for starting scripting.
Emergency exit: Control-X
vi – Preferred editor for Unix admins, powerful but learning curve involved.
Emergency exit: Escape key followed by :q!
emacs – Incredibly powerful, menu bar available in window mode, text mode can be challenging.
Emergency exit: Control-X Control-C
Other choices: gedit (X/GNOME), dtpad (X/CDE), vim/gvim (many including Windows)

3. Basic Shell Statements
Variable assignment:
name="george"
count="5"
today=`date`
Variable unassignment:
unset name
Do basic math on a variable:
counter=`expr $counter + 1`
Read from standard input (usually console):
read answer
Print text:
echo "Your name is $name"

4. Basic Shell Statements
Conditional Execution:
if .. elif .. else .. fi
case word in pattern) .. ;; ... esac
Loops and Iteration:
for name in words; do .. done
while list do; .. done

5. if … fi The “if” statement provides conditional execution of commands:
count=5
if [ $count -eq 0 ]; then
echo "Nothing left to do" fi
name="Herbert"
if [ "$name" != "George" ]; then
echo "You're not George. I will name you George."

6. if … fi
When doing text comparisons, always put variables inside double-quotes:
name=""
if [ $name != "George" ]; then
echo "You're not George. I will name you George." fi
“test: argument expected” is an error you may see a lot of at first. Because variable substitution happens early in the process, the above reduces to:
if [ != "George" ]; then
To be safe, put all variables in double quotes. It won’t hurt. Don’t use single quotes, variables aren’t expanded in them.

7. Test
Test operators (located inside the square brackets) are described in the man page for “test”, not “sh”.
Common String Test operators:
Numeric String
Equality $a –eq $b "$a" = "$b"
Not Equal $a –ne $b "$a" != "$b"
Greater Than $a –gt $b
Less Than $a –lt $b
Empty String -z "$a"
Non-Empty String -n "$a"

8. Test Common File Test operators: Joining test operators:
File Exists -f "$a"
Directory Exists -d "$a"
Symlink Exists -h "$a"
File is Readable -r "$a"
File is Writeable -w "$a"
File is Executable -x "$a“
Negate any of the above ! –f "$a"
Joining test operators:
File Exists and is readable:
if [ -f "$a" –a –r "$a" ]; then
File is a file or a directory:
if [ -f "$a" –o –d "$a" ]; then

9. if … elif … else … fi
The “if” statement can provide alternative actions if the condition isn’t met:
read response
if [ "$response" = "yes" ]; then
echo "We want to do that."
elif [ "$response" = "no" ]; then
echo "We don't want to do that."
else
echo "Huh?" fi

10. case … esac
The “case” statement can evaluate alternate results for the same condition, and can also use file matching wildcard patterns:
read response
case "$response" in
[Yy]*)
echo "We want to do that." ;;
[Nn]*)
echo "We don't want to do that." ;; *)
echo "Huh?" ;;
esac

11. Getting Loopy The “for” loop: The “while” loop:
for file in a b c d e; do
echo "$file"
done
The “while” loop:
done="no"
while [ "$done" = "no" ]; do
read magicword
if [ "$magicword" = "plugh" ]; then
done="yes" fi

12. I/O Redirection One of the most powerful features of Unix
Three standard I/O channels:
0 – Standard Input (stdin)
1 – Standard Output (stdout)
2 – Standard Error Output (stderr)
Redirect a file into a command using stdin:
grep SENS < myreport
Redirect command output into a file using stdout:
ps –ef > processes
Redirect input and output using stdin and stdout:
grep SENS < infile > outfile

13. I/O Redirection Redirect stdout and stderr into the same file:
grep SENS reports/* > out 2>&1
Redirect stdout and stderr into different files:
grep SENS reports/* > out 2> badstuff
Print to stderr instead of stdout:
echo "It all went wrong!" 1>&2

14. Pipes
Pipes take the output of one command and makes it the input of another command:
Print just the year from the date command:
date | awk -e '{print $6;}'
date | cut –c 25-28
date "+%Y"
Show how many processes are running:
ps -e | wc -l
Show how many processes I’m running:
ps -ef | grep yearke | wc -l
ps –fu yearke | wc –l
The concept of the pipe is one of the fundamental underpinnings of Unix.

15. Semi-Practical Example 2.1
What side of the Node am I in?
#!/bin/sh cd
homedir=`pwd`
case "$homedir" in
/nsm/home/*)
side="Natural Sciences and Math" ;;
/eng/home/*)
side="Engineering School" ;; *)
side="Unknown" ;;
esac
if [ "$side" = "Unknown" ]; then
echo "I'm sorry, I cannot determine your affilation."
else
echo "Your affiliation is the $side part of SENS." fi

16. Semi-Practical Example 2.2
Examine some files in the current directory:
#!/bin/sh
counter=0
for file in t*; do
if [ -f "$file" -a -w "$file" ]; then
counter=`expr $counter + 1` fi
done
echo "There are $counter writeable files starting with 't'."

17. Homework
Write a script that runs on a Solaris 10 or Linux system (the unix.eng timeshares are great for this) and counts the number of files and directories under the “/etc” directory. You do not need to traverse subdirectories and enumerate files and directories under them, just the count under the top-level of “/etc” will be sufficient.

18. Links Current Solaris Bourne Shell man page:
An Introduction to the Unix Shell:
The Unix Programming Environment:

19. Next Time Argument Clinic Child/Parent issues (environment)
A lurking “gotcha” in loops