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

• Flow of control
• test

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