4/6262: COMP Programming UNIX
Lecture 10 shell, midterm review February 22, 2006
announcements
midterm
Wed. March 1, in class till todays lecture close book 1 piece of cheat sheet, letter size, double sided
UNIX
the 3 levels of UNIX
Kernel Shell
Tools and apps
Kernel: schedules tasks and manages storage; Shell: connects and interprets users' commands, calls programs from memory, and executes them; Tools and applications: offer additional functionality to the OS
shell
A program, command line interpreter An interface between an user and the raw operating system It facilitates
Multitasking Piping I/O redirection Wildcards Etc.
shell operation
When invoked
1.
2. 3.
read a start-up file (contains some initialization information, typically located in the users home directory) Displays a prompt and waits for a user command Ctrl-D on its own terminates the shell, otherwise the shell executes user command and returns to step 2
commands
Executable files
Stand alone programs that are invoked by the shell Stored in files with executable permissions the shell locates the program and execute it Examples: ls, sort, etc. man
Built-in commands
executed by the shell internally Examples: echo, cd, etc. help
command sequence, grouping,
script
a file that contains shell commands give it execute permission run by typing its name useful for storing commonly used sequences of commands
script
when a script is run, the system
determines which shell the script was written for: the first line of the script is just a #: the script is interpreted by the shell from which it was executed as a command the first line is of the form #! pathName: the executable program pathName is used to interpret the script neither of above: the script is interpreted by a Bourne shell executes the shell, using the script as its standard input
standard I/O, redirection, pipes
for each program, the shell establish 3 I/O channels redirection filters
shell pipes variables
hold data in a string format shell replaces $variableName with the value of variableName to create a variable, simply assign it a value local and environment variables
when a shell invokes a subshell, the child shell gets a copy of its parent shells environments variables, but not local variables use shell command (export in Bourne shell) to make a local variable environment
shell variables
predefined
$HOME $PATH $SHELL $TERM
special built-in
$$ process ID of the shell $0 the name of the shell script $1..$9 $n refers to the nth command line argument $* a list of all the command line arguments
metacharacters
characters that receive special processing by the shell Examples
> (>>) output redirection < input redirection * (?, []) file substitution wildcard | pipe ; sequence commands () group commands `` command substitution # comments (ignore everything after) $ expands the value of a variable && conditional execution & runs a command in the background \ prevents special interpretation of the next character
substitution
wildcard
*, ?, [] treated as a pattern is replaced by an alphabetical...