What is Shell

• It is a program that accepts your commands (one line at a time)

• Executes it.

• A shell is a full fledged programming language.

• Slightly strange syntax because it has to double up as a command processor.

How many shells?

• bash: Bourn Again Shell (Default on Gnu/Linux)
• tcsh: Default shell on BSDs
• zsh: A powerful new shell.

My shell?

• Check your password entry

$ grep ppk /etc/passwd
ppk:x:1000:1000:Piyush P Kurur,,,:/home/ppk:/usr/bin/zsh

The classical hello world program.

$ echo "Hello world"

Redirecting and piping

• cmd > filetowrite
• cmd < inputfromfile
• cmd >> appendtofile
• cmd1 | cmd2 | cmd3

$ who | wc -l

Some filters and transformers

• grep: search for a pattern in the file (or stdin)
• sed : basic stream editor.

$ who | grep james | sed 's/james/007/'

Shells support

• loops
• conditionals
• cases.

Variables

• foo=bar : Careful no space after =
• $foo : the value of the variable foo

Funny characters

$ echo *

$ echo \* ; echo 'The * is not a problem here'
$ foo=bar
$ echo "The value of foo is $foo" "$foo"-bar

Quotes

• single quotes: protects the string from shell interpretation

• double quotes: protects but variables are substituted

• back quote or back tick: Run command and substitute the value.

Let us write a count down program.

for i in `seq 10 -1 0`
do
    clear
    banner $i
    sleep 1
done