Introduction to Shell programming

Unix/Linux command-line - One-liner "programs" at the command prompt (e.g. prog piped to prog).

Shell programming - Text file of multiple Unix/Linux commands (e.g. "if" condition then execute command, else other command).

The combination of one-liner command-line plus multi-line Shell programs gives you a programmable User Interface, where you can quickly write short programs to automate repetitive tasks.

Shell program = an interpreted program (i.e. not compiled). Also known as a "Shell script" or "batch file" of UNIX commands.
Like .BAT files on Windows command line.

How to make a Shell program

Put it in a directory that is in the PATH.
In this course we will put it in $HOME/bin
It can have any extension (typically no extension).
Put valid UNIX commands in it.
Make it executable:
```
 $ chmod +x file
```
Run it by typing its name:
```
$ file
$ file &
```

Alternative ways of working

Pass program as arg to shell:
```
 $ sh prog 
```
Advantage: Does not have to be executable. Does not have to be in PATH.
Disadvantage: Have to type "sh" all the time. Have to be in same directory (or else type more complicated path to program).
Use file extension:
```
$ prog.sh
```
Advantage: Can easily see what the file is from a directory listing.
Disadvantage: Have to type the .sh
Could be used when no one ever types the name. (e.g. The script is only ever called by a program.)

I don't use either of these.

Arguments and returns

A Shell program is different to something typed on the command-line.
It can have arguments, and can exit at any point.


$1     1st command-line argument
$2     2nd command-line argument
...

$*     all arguments

#      comment 


exit		exit the Shell script
exit 0		exit with a return code that other progs can query

$?		return code of last prog executed

if, test and flow of control


# test if 1st argument = "0"

if test "$1" = "0"
then
 echo "yes"
else
 echo "no - first argument is $1"
fi