Introduction to Shell

Shell gives you a quick and powerful way of having a programmable User Interface, where you can quickly write short programs to automate repetitive tasks.

I have had a changing collection of my own personal shell scripts moving with me from place to place and machine to machine since the 1980s. It is I think the only technology that I have found useful for that entire period.

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.

Shell program can have any extension (typically no extension). Edit a file, put some UNIX commands in it, perhaps strung together with some logic:

if condition
then
 rm f1
else
 rm f2
fi

Make it executable:

$ chmod +x file

Make sure it is in the PATH and just run it:

$ file
$ file &

Pipes and redirection in scripts

Remember pipes and redirection on command line.

Multi-line pipe:
Shell script can have multi-line pipe. Easier to read. e.g.:


cat file | 

    grep -i "http:.*dcu.ie"          | 

    sed -e "s|COMPAPP|computing|g"   | 
    sed -e "s|compapp|computing|g"   | 

    sort -u

sed explained later. Here it does a "search and replace" operation on text.

Two output streams:


UNIX separates "ordinary output" (standard output, stdout, 1>, >)
from "error output" (standard error, stderr, 2>)

prog > output 2> errors

Null output:

> /dev/null		to get rid of some unwanted output
			e.g. error/warning messages from compilation/search
			(redirects it into a non-existent file)

Redirect from within script:

To redirect script output to a file, from command line:
 
  script > file

To redirect script output to a file from within the script, put this at start of script:

  exec > file

Then run:

  script

Redirection

Arguments and returns


$*           all arguments to the Shell program
$1           1st argument, etc.
$0		name of prog
$#		no. of args

#                       comment

shift		shift args leftwards 
		this is useful if you want to remove some of the first args, then "shift" a couple of times,
		and then do "for i in $*" with the remaining args
		e.g. grep (switches) (string) file1 ... filen

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

$?		return code of last prog executed
		e.g. quiet grep: 
		 grep > /dev/null 
		and then check $?
		though grep may have -q (quiet) option anyway

Flow of control, test


# test if 1st argument = "0"

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