'&$%CSE 303:Concepts and Tools for Software DevelopmentHal PerkinsAutumn 2008Lecture 3— I/O Redirection, Shell ScriptsCSE303 Autumn 2008, Lecture 3 1'&$%Where are We• A simple view of the system: files, users, processes, shell• Lots of small useful programs; more to come• An ever-more- com plicated shell defi nition:– Filename expansion– Command-line editing– History ex pansion– I/O redirection– Programming constructs– VariablesCSE303 Autumn 2008, Lecture 3 2'&$%Simple view of input/output• Old news: Programs take an array of strings as arguments• Also: Programs return an integer (convention: 0 for “success”)The shell also sets up 3 “streams” of data for the program to access:• stdin a.k.a. 0: an input stream• stdout a.k.a. 1: an output stream• stderr a.k.a. 2: another output streamThe default shell behavior uses the keyboard for stdin and the shellwindow for stdout and stderr.Examples:ls prints files stdout and “No match” to stderr.mail takes message body from stdin (waiting for C-d (“end of file”)to stop taking input).CSE303 Autumn 2008, Lecture 3 3'&$%File RedirectionUsing arcane characte rs, we can tell the shell to use files instead of thekeyboard/screen (Bash Manual, Section 3.6):• redirect input: cmd < file• redirect output, overwriting file: cmd > file• redirect output, appending to file: cmd >> file• redirect error output: cmd 2> file• redirect output and e rror output to file: cmd &> file• ...Examples:• How I ge t the histories for the web page.• ls uses stdout and stderr.• Using cat to copy information to/from files.CSE303 Autumn 2008, Lecture 3 4'&$%Pipescmd1 | cmd2Change the stdout of cmd1 and the stdin of cmd2 to be the same,new stream!Very powerful idea:• In the shell, larger comm and out of smaller com mands• To the user, combine small programs to get more usefulness– Each program can do one thing and do it well!Examples:• foo --help | less• djpeg me.jpg | pnmscale -xysize 100 150 | cjpeg >me thumb.jpgCSE303 Autumn 2008, Lecture 3 5'&$%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 = xCSE303 Autumn 2008, Lecture 3 6'&$%Combining CommandsCombining s impler c ommands to form more com plicated ones is veryprogramming-like. In addition to pipes, we have:• cmd1 ; cmd2 (sequence)• cmd1 || cmd2 (or, using int result – the “exit status”)• cmd1 && cmd2 (and, like or)• cmd1 ‘cmd2‘ (use output of cmd2 as input to cmd1). (Veryuseful for your homework. Note cmd2 surrounded by backquotes,not regular quotes)– Useless example: cd ‘pwd‘.– Non-useless example: mkdir ‘whoami‘A‘whoami‘.Note: Previous line’s exit status is in $?.CSE303 Autumn 2008, Lecture 3 7'&$%Non-alphabet soupList of characters with special (before program/built-in runs) meaningis growing: ‘ ! % & * ~ ? [ ] " ’ \ > < | $ (and we’re notdone).If you ever want these characte rs or (space) in something like anargument, you nee d som e form of es caping; each of " ’ \ haveslightly different meaning.CSE303 Autumn 2008, Lecture 3 8'&$%Toward Scripts...A running shell has a state, i.e., a current• working directory• user• collection of aliases• history• ...In fact, next time we will learn how to extend this state with new shellvariables.We learned that source can execute a file’s contents, which can affectthe shell’s state.CSE303 Autumn 2008, Lecture 3 9'&$%Running a scriptWhat if we want to run a bunch of commands 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 thingsCSE303 Autumn 2008, Lecture 3 10'&$%Writing a script• Make the first line exactly: #!/bin/bash• Give yourself “exe cute” permission on the file• Run itNote: The shell consults the first line:• If a shell-program is there, launch it and run the script• Else if it’s a “real executable” run it (more later).Example: listhomeCSE303 Autumn 2008, Lecture 3 11'&$%Accessing argumentsThe script accesses the arguments with $i to get the ithone (name ofprogram is $0).Example: make thumbnail1Also very useful for homework: shift (manual Section 4.1)Example: countdownWe would like optional arguments and/or usage messages. Need:• way to find out the number of arguments• a conditional• some stuff we already haveExample: make thumbnail2CSE303 Autumn 2008, Lecture 3 12'&$%More expressionsbash expressions can be:• math or string tests (e.g., -lt)• logic (&&, ||, !) (if you use double-brackets)• file tests (very common; see Pocket Guide)• math (if you use double-parens)Gotcha: parens and brackets must have spaces before and after them!Example: dcdls (double cd and ls) can check that arguments aredirectories.Exercise: script that replaces older file with newer oneExercise: make up your ownCSE303 Autumn 2008, Lecture 3 13'&$%Review• The shell runs programs and builtins, interpreting specialcharacters for file names, history, I/O redirection.• Some builtins like if support rudimentary programming.• A script is a program to its user, but is written using shellcommands.So the shell language is okay for interaction and “quick-and-dirty”programs, making it a strange beast.For both, shell variables are extremely useful.CSE303 Autumn 2008, Lecture 3 14'&$%Variablesi=17 # no spacessetecho $iset | grep iecho $iunset iecho $if1=$1(The last is very useful in scripts before shifting, e .g., see homework.)Enough for your homework (arithmetic, c onditionals, shift, variables,redirection, ...)Gotcha: using undefined variables (e.g., because of typo) doesn’t fail(just the empty string).CSE303 Autumn 2008, Lecture 3
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