Script

• Shell script: a file containing a script or list of commands that the shell can execute.
• A shell script file must be an executable file.
• Usually it has the extension .sh, although it's not necessary.
• Each line in the script is interpreted by the binary executing the script and executed.
• Any command recognized by the specified shell can be included in the script as such.
• We will discuss bash shell programming.

Comments

• Comments: any line starting with a # will be ignored except for the very first one specifying the shell to execute the script.
• Exception: the first line usually contains information about which shell must execute the script. This line starts with the #! symbols. Examples:
  #!/bin/sh
  #!/usr/bin/perl
  #!/usr/bin/python
• The first one will be executed by the shell "sh". The second is a Perl script. The last ones is a Python script.

Combining Commands

• Several commands on the same line: separated with ;
  cd .. ; make ; cd src
• Continuing a command: with the \
  echo "This is our first shell script \
  and we are writing a long line"
• Grouping commands with ()
  (cd .. ; make ) ; cd src
• This creates a separate process for each group of commands in separate parenthesis.

Positional Parameters

• The positional parameters are arguments that can be given in the command line when we run the script.
• $* contains all the positional parameters.
• These parameters are identified by $number:
• $0 is the name of the script file. $1 is the first argument in the command line, $2 is the second, etc.
• After 10, the number must be placed in braces: ${10} .
• You can find how many arguments the script has by the variable $#:
• The shift command promotes all the arguments by one number and discards the first one. The $2 becomes $1, $3 becomes $2, etc. The command is not reversible.

Command-line Expansion

• The operation of transforming an expression into a command before executing it. It replaces some patterns and wildcards and syntax elements with what they stand for.
• Tilde expansion. Replacing ~ with the home directory and ~user with the home directory of that user.
• Path expansion. Replacing the following wildcards in a path name or in a file name:
  * with anything ls libm*
  ? with a single character ls /usr/bin/?spell
• The above generates a list of all the paths/files that match the pattern.

Patterns

• Bracket expansion [ ]. Creating a word for every character between the brackets. We can also specify a range like 0-12 or a-z.
• Brace expansion { }. If a list between braces appears in the expression, a word is generated for every item in the list.
• Parameter and variable expansion. Replacing an expression starting with a $, like $PATH, $1, ${12}, with the value of that variable or parameter.

Examples

$ ls *old.txt
bold.txt   file_old.txt   mold.txt    to_hold.txt
$ ls ?old.txt
bold.txt   mold.txt
• Brackets expansion
  $ ls slides[12a].html slides[4-6].html
  slides1.html   slides2.html   slidesa.html   slides4.html  
  slides5.html   slides6.html
  $ mkdir memo[A-D]; ls memo*
  memo[A-D]
• Braces expansion
  $ mkdir memo{A,B,C,D} ; ls memo *
  memoA  memoB   memoC   memoD

Arithmetic Expansion

• Evaluating an arithmetic expression and replacing it with that value.
• All computations in a shell script are done with integers. Anything else is converted to 0.
• In assignments the expression must be placed within the following syntax: $[expression]
  $ a=$[2+3] ; echo $a 5
• Another possibility: with the command let
  $ let a=2+3 ; echo $a
  5
  $ declare -i b=7 ; let b=$b+1 ; echo $b
  8