This document provides an introduction to scripting in Linux shells. It covers basics like: 1) Writing a simple "Hello World" script. 2) Redirecting input/output streams between programs and files. 3) Using pipes to connect the output of one program to the input of another. 4) Working with variables, conditions, loops and functions in shell scripts. 5) An assignment example of writing a script to automatically create 1000 user accounts.

1. Scripting 101

2. My First Sony ^H^H^H hellscript
#!/bin/bash
echo “Hello World!”
$ sh script.sh
 Hello World

3. Output redirection
There are 3 file descriptors: Examples:
stdin, stdout and stderr.
1. ls -l > ls-l.txt
Basically you can: 2. grep da * 2> grep-errors.txt
 redirect stdout to a file 3. grep da * 1>&2
 redirect stderr to a file 4. grep * 2>&1
 redirect stdout to a stderr 5. rm -f $(find / -name core) &>
/dev/null
 redirect stderr to a stdout
 redirect stderr and stdout to a file
 redirect stderr and stdout to
stdout
 redirect stderr and stdout to stderr
1 'represents' stdout and 2 stderr.

4. Pipes
Pipes can redirect output from one command to
another
Example:
$ cat /etc/passwd | grep „whoami‟

5. Variables
STR=“Hello World!”
OF=“/var/my-backup-$(date +%Y%m%d).tgz”
#!/bin/bash
STR=“Hello World!”
echo $STR
#!/bin/bash
OF=“/var/my-backup-$(date +%Y%m%d).tgz”
tar -cZf $OF /home/me/

6. Local variables
#!/bin/bash
HELLO=Hello
function hello {
local HELLO=World
echo $HELLO
}
echo $HELLO
hello
echo $HELLO

7. Conditions
if [expression]; Examples:
then
code if 'expression' is true. #!/bin/bash
if [ "foo" = "foo" ]; then
fi echo “expression
evaluated as true”
fi
#!/bin/bash
if … then T1="foo"
else T2="bar"
if [ "$T1" = "$T2" ]; then
elseif echo “expression
evaluated as true”
else
echo “expression
evaluated as false”
fi

8. Loops
3 kinds of loops Examples:
#!/bin/bash
 for loop for i in `seq 1 10`; do
echo $i
 while loop Done
 until loop
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo “The counter is “ $COUNTER
let COUNTER=COUNTER+1
done

9. Assignment:
Write a linux shell script to automagically
create 1000 user accounts. Make sure their
default password is “Hello123”
(hint: you cannot set the pasword with the –p
option….)

10. Useful files and commands
nano, joe, vi (editors)
/etc/passwd
useradd
passwd
rm -rf