'&$%CSE 303:Concepts and Tools for Software DevelopmentDan GrossmanSpring 2005Lecture 3— I/O Redirection, Shell Scripts EmacsDan Grossman CSE303 Spring 2005, Lecture 3 1'&$%Where are We• A simple view of the system: files, users, processes, shell• Lots of small useful programs; more to come• An ever-more- complicated s hell definition:– Filename expansion– History expansion– Command-line editing– I/O redirection– Programming constructs– VariablesDan Grossman CSE303 Spring 2005, Lecture 3 2'&$%Simple view of input/output• Old news: Programs take an array of strings as arguments• Also: Programs return an integer (convention: 0 for “success”)The shell also sets up 3 “streams” of data for the program to access:• stdin: an input stream• stdout: an output stream• stderr: another output streamThe default shell behavior uses the ke yboard for stdin and the shellwindow for stdout and stderr.Examples:ls prints files stdout and “No match” to stderr.mail takes m ess age body from stdin (waiting for C-d to end the file).Dan Grossman CSE303 Spring 2005, Lecture 3 3'&$%File RedirectionUsing arcane characters, we can tell the shell to use files instead of thekeyboard/screen:• redirect input: cmd < file• redirect output, overwriting file: cmd > file• redirect output, appending to file: cmd >> file• redirect output and e rror output to file: cmd >& file• ...Examples:• How I put the histories on the web page.• ls uses stdout and stderr.• Mailing a file’s contents.Dan Grossman CSE303 Spring 2005, Lecture 3 4'&$%Pipescmd1 | cmd2Change t he stdout of cm d1 and the stdin of cmd2 to be the same,new stream!Very powerful idea:• In the shell, larger command out of smaller com mands• To the user, combine small programs to get more usefulness– Each program can do one thing and do it well!Examples:• foo --help | less• djpeg me.jpg | pnmscale -xysize 100 150 | cjpeg >me thumb.jpg• your homework... (with grep, commonly used in pipes)Dan Grossman CSE303 Spring 2005, Lecture 3 5'&$%cat and redirectionJust to show there is some math underlying all this nonsense, here aresome fun and useless equivalences (like 1 · y = y):• cat y = cat < y• x < y = cat y | x• x | cat = xDan Grossman CSE303 Spring 2005, Lecture 3 6'&$%Combining CommandsCombining s impler com mands to form m ore c omplicated ones is veryprogramming-like. In addition to pipes, we have:• cmd1 ; cmd2 (sequence)• cmd1 || cmd2 (or, using int result – the “exit status”)• cmd2 && cmd2 (and, like or)• cmd1 ‘cmd2‘ (use output of cmd2 as input to cmd1 ).– Useless example: cd ‘pwd‘.– Non-useless example: mkdir ‘whoami‘.Note: Previous line’s exit status is in $?.Dan Grossman CSE303 Spring 2005, Lecture 3 7'&$%Non-alphabet soupList of characters with special (before program/built-in runs) meaningis growing: ‘ ! % & * ~ ? [ ] " ’ \ > < | $ (and we’re notdone).If you ever want these characters or (space) in som ething like anargument, you nee d some form of escaping ; each of " ’ \ haveslightly different meaning.Dan Grossman CSE303 Spring 2005, Lecture 3 8'&$%Toward Scripts...A running shell has a state, i.e., a current• working directory• user• collection of aliases• history• ...In fact, ne xt time we w ill learn how to extend this state with new shellvariables.We learned that source can execute a file’s contents, which can affectthe shell’s state.Dan Grossman CSE303 Spring 2005, Lecture 3 9'&$%Running a scriptWhat if we want to run a bunch of com mands without changing ourshell’s state?Answer: start a new shell (sharing our stdin, stdout, stderr), run thecommands in it, and exit.Better answer: Automate this process.• A shell script as a program (user doesn’t even know it’s a script).• Now we’ll want the shell to end up being a programming language• But it will be a bad one except for simple thingsDan Grossman CSE303 Spring 2005, Lecture 3 10'&$%Writing a script• Make the first line exactly: #!/bin/csh• Give yourself “exec ute” permission on the file• Run itNote: The shell consults the first line:• If a shell-program is there, launch it and run the script• Else if it’s a “real executable” run it (more later).Example: listhomeDan Grossman CSE303 Spring 2005, Lecture 3 11'&$%Accessing argumentsThe script accesses the arguments with $i to get the ithone.Example: make thumbnail1We would like optional arguments and/or usage mes sages. Need:• way to find out the number of arguments• a conditional• some stuff we already haveExample: make thumbnail2Dan Grossman CSE303 Spring 2005, Lecture 3 12'&$%More expressionstcsh expres sions c an be math (+, *, -, ...), logic (&& , ||, ! ), or filetests.Example: dcdls (double cd and ls) can check that arguments aredirectories.Exercise: Do make thumbnail3.Exercise: script that replaces older file with newer oneExercise: make up your ownDan Grossman CSE303 Spring 2005, Lecture 3 13'&$%Review• The shell runs programs and builtins, interpreting specialcharacters for filenames , history, I/O redirection.• Some builtins like if support rudimentary programming.• A script is a program to its use r, but is written using shellcommands.So the shell language is okay for interaction and “quick-and-dirty”programs, making it a strange beast.For both, shell variables are extremely useful.Note: enough already for your homework except for grep, but askquestions!Dan Grossman CSE303 Spring 2005, Lecture 3 14'&$%Variablessetset i = 17setecho $iset | grep iset iecho $iunset iecho $iDan Grossman CSE303 Spring 2005, Lecture 3
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