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 &

If it's not executable you can run it this way:

$ sh file

The above has the advantage of not having to worry about chmod or PATH. It has the disadvantage of having to type "sh" all the time.

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

Have you noticed this:

ls           (gives multi-column output)
ls > file    (gives single-column output)
ls | prog    (gives single-column output)

You can detect if output is going to terminal, file or pipe, and adjust output accordingly.
Shell script to detect where output is going:


if [ -t 1 ]
then
 echo stdout
else
 echo pipe or file
fi

Test it:

prog
prog | cat
prog > file

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