CSCI 330 The UNIX System

1. CSCI 330The UNIX System Unit VII Bash Shell Basics

2. UNIX Command Interpreters • common term: shell • families: • Bourne shell • developed as part of original, commercial UNIX • C shell • developed as part of free, academic UNIX • standard: • every UNIX system has a “Bourne shell compatible” shell CSCI 330 - The UNIX System

3. Bourne shell family • sh: original Bourne shell • written 1978 by Steve Bourne • part of commercial UNIX • ash: Almquist shell • BSD-licensed replacement of sh • bash: Bourne-again shell • GNU replacement of sh • dash: DebianAlmquist shell • scripting shell in Ubuntu • ksh: Korn shell, Bell Labs improved sh • zsh: superset of all others • busybox: all-in-one utility for small systems CSCI 330 - The UNIX System

4. C shell family • csh • written in 1979 by Bill Joy • part of BSD UNIX • tcsh: TENEX csh • part of TENEX operating system • was original shell for Mac OS X CSCI 330 - The UNIX System

5. bash: Bourne-again Shell • default command interpreter for GNU/Linux • implements Bourne shell syntax • Bourne shell scripts run without modification • many improvements over Bourne shell • command line usability • many built-in commands • improved I/O redirection CSCI 330 - The UNIX System

6. bash Shell Basics • Invocation • Customization • variables • prompt and aliases • Startup and initialization • Command line behavior • history • quoting • redirections and pipe • background • directory stacks • eval command CSCI 330 - The UNIX System

7. Invoking bash • as login shell • specified in /etc/passwd • on the command line: % bash • with file as argument • file is shell script % bash scriptfile CSCI 330 - The UNIX System

8. Customizations • via command line options • rarely done • instead: process initialization file • .profile if login session shell • .bashrc if invoked from command line • via variables • terminal settings • set path to find executables • define custom prompt • define command aliases CSCI 330 - The UNIX System

9. Variables • shell remembers values in variables • 2 types of variables • environment • valid for complete login session • shell • valid for each shell invocation • to display variable’s value % echo $variable CSCI 330 - The UNIX System

10. Predefined Environment Variables “printenv” displays all environment variables CSCI 330 - The UNIX System

11. Predefined Shell Variables CSCI 330 - The UNIX System

12. Setting shell variables Syntax: varname=value Example: speed=fast echo "Today we go: $speed" Note: no spaces CSCI 330 - The UNIX System

13. Example: PATH variable • PATH lists a set of directories • shell finds commands in these directories • on Ubuntu: % echo $PATH /usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games % PATH=$PATH:~/bin % echo $PATH /usr/local/bin:/usr/sbin:/usr/bin: /sbin:/bin:/usr/games:/home/student/bin CSCI 330 - The UNIX System

14. bash Shell Prompt • can be set via “PS1” shell variable Example: % PS1="$USER > " z036473 > • Secondary prompts: PS2, PS3, PS4 CSCI 330 - The UNIX System

15. bash Shell Prompt • special “PS1” shell variable settings: \w current work directory \h hostname \u username \! history event number \d date \t time \a ring the “bell” Example: % PS1="\u@\h-\!: " ege@turing-22: CSCI 330 - The UNIX System

16. Shell Aliases • Allows you to assign a name, or alias to command(s) • use alias like any other command • can rename existing commands • to check current aliases: % alias • to set alias: % alias ll="ls –al" • to remove alias: % unaliasll CSCI 330 - The UNIX System

17. To Create an Alias • via the default startup files: • ~/.profile or ~/.bashrc • via a text file • enter alias definitions into text file • execute text file via “source” or “.” command • reads and executes content of file in current shell CSCI 330 - The UNIX System

18. Startup & Shutdown Files • ~/.profile • /etc/profile • ~/.bash_profile • ~/.bash_login • ~/.bashrc • /etc/bash.bashrc • options: • --norc don’t run initialization files • -l run as login shell • ~/.bash_logout executed for login shell executed for non-login shell CSCI 330 - The UNIX System

19. Customizations • startup • terminal settings • set path to find executables • define custom prompt • define command aliases • shutdown • save command history • remove temporary files CSCI 330 - The UNIX System

20. Command line behavior • history • sequence • substitution • quoting • background • i/o redirection and pipe CSCI 330 - The UNIX System

21. Shell History • shell keeps a record of previously entered commands so that they can later be: • re-executed • edited • Commands are saved • per session • per user • Size of history can be set via shell variables: HISTSIZE=500 HISTFILESIZE=100 CSCI 330 - The UNIX System

22. Shell History • Each previously run command gets a sequential event number • To view the history buffer: Syntax: history [-c] [count] • If no options are supplied, list all Useful options: -c clear history CSCI 330 - The UNIX System

23. Shell History • You can re-execute history events: • By the event number % !5 • By the number relative to current event % !-3 • By the text it contains % !ls CSCI 330 - The UNIX System

24. Command line editing • UP ARROW: move back one command in history list • DOWN ARROW: move forward one command • LEFT and RIGHT ARROW: move into command • BACKSPACE and DELETE: Remove information • TAB: complete current command or file name CSCI 330 - The UNIX System

25. Command Sequence • allows series of commands all at once • commands are separated by a semicolon ; Example: % date; pwd; ls CSCI 330 - The UNIX System

26. Command Substitution • command surrounded by back quotes ` is replaced by its standard output • newlines in the output are replaced by spaces Examples: % echo `date` % ls -l `which passwd` % var=`whoami`; echo $var CSCI 330 - The UNIX System

27. Command Substitution • second form of command substitution: $(command) Examples: % echo "User $(whoami) is on $(hostname)" User ege is on turing % echo "Today is" $(date) Today is Fri Jul 17 08:06:28 CDT 2009 CSCI 330 - The UNIX System

28. Quoting • allows to distinguish between the literal value of a symbol and the symbols used as code • used to distinguish between the literal symbol and the symbol’s use as a metacharacter or wild card characters • done via the following symbols: • Backslash (\) • Single quote (‘) • Double quote (“) CSCI 330 - The UNIX System

29. Backslash (\) • also called the escape character • preserve the character immediately following it • For example: to create a file named “tools>”, enter: % touch tools\> CSCI 330 - The UNIX System

30. Single Quote (‘) • protects the literal meaning of metacharacters • protects all characters within the single quotes • exception: it cannot protect itself Examples: % echo 'Joe said "Have fun"' Joe said "Have fun" % echo 'Joe said 'Have fun'' Joe said Have fun CSCI 330 - The UNIX System

31. Double Quote (“) • protects all symbols and characters within the double quotes, expect for: • $ (dollar sign) • ` (back quote) • \ (backslash) Examples: % echo "I've gone fishing" I've gone fishing % echo "your home directory is $HOME" your home directory is /home/ege CSCI 330 - The UNIX System

32. Quoting Examples: % echo “Hello Ray" Hello Ray % echo “Hello $USER" Hello a132436 % echo “It is now `date`” It is now Mon Feb 25 10:24:08 CST 2008 % echo “you owe me \$500” you owe me $500 CSCI 330 - The UNIX System

33. Output Redirection (>) Syntax: command > file Sends output of command to file, instead of to terminal Examples: % ls > listing % cat file > filecopy CSCI 330 - The UNIX System

34. Input Redirection (<) Syntax: command < file Command will read (take input) from file, instead of from terminal Example: % tr [a-z] [A-Z] < listing CSCI 330 - The UNIX System

35. Examples: Output / Input • Redirecting input and output: % tr [A-Z] [a-z] < r.in > r.out • Output of command becomes input to next: % ls > temp.txt; wc < temp.txt • Eliminate the middleman: pipe % ls | wc CSCI 330 - The UNIX System

36. Build the file ‘usage-status’ from the output of the ‘date’, ‘ls’, and ‘du’ commands Appending Output • Syntax: command >> file adds output of command at the end of file • If file does not exist, shell creates it • Examples: % date > usage-status % ls -l >> usage-status % du -s >> usage-status CSCI 330 - The UNIX System

37. Here Document • read input for current source, uses “<<“ symbol Syntax: command << LABEL • reads following lines until line starting with “LABEL” Example: % wc -l << DONE > line one > line two > DONE 2 CSCI 330 - The UNIX System

38. File Descriptor • positive integer for every open file • process tracks its open files with this number 0 – standard input 1 – standard output 2 – standard error output • bash uses file descriptor to refer to a file CSCI 330 - The UNIX System

39. Redirection syntax • Output: >or1> filename 2> filename • Input: <or0< • Combining outputs: 2>&1 or &> or >& Example: % cat hugo > /tmp/one 2>&1 or: cat hugo &> /tmp/one CSCI 330 - The UNIX System

40. Summary: Redirections and Pipe CSCI 330 - The UNIX System

41. Shell Job Control • Foreground job: • a job that has our immediate attention • user has to wait for job to complete • Background job: • a job that the user does not wait for • it runs independently of user interaction Examples: • compiling long programs • solving complex equations • searching for particular kinds of files • backing up large number of files CSCI 330 - The UNIX System

42. Shell Background Jobs • to execute command in the background, put & after it Example: % date; cd projects; cc gets.c –o gets & [1] 26432 % CSCI 330 - The UNIX System PID Job number

43. Managing jobs • Unix shells allow users to: • make jobs execute in the background, • move jobs from background to foreground • display jobs • command “jobs” lists your active jobs • each job has number, used with “%” to refer to job • move job to foreground • fg %1 • once a job finishes it will display exit status CSCI 330 - The UNIX System

44. Directory Stacks • To keep track of multiple directories using stack structure • To maintain a directory stack, we use the following three commands: • pushd • popd • dirs CSCI 330 - The UNIX System

45. The commands: pushd • pushd directory to push directory on top of stack • pushd (with no argument) to travel between two top stack entries • popd (with no argument) to remove the top directory and changes to the next stack entry • dirs: to display all directory entriesin stack CSCI 330 - The UNIX System

46. “eval” Command Synatx:eval [ arg … ] • arguments are read and concatenated into a single command, command is then executed • performs all shell substitutions, and then executes the command line Example: command="ls" options="-al" eval $command $options CSCI 330 - The UNIX System

47. Summary • common features of UNIX command interpreters • next: shell script programming CSCI 330 - The UNIX System