The UNIX Shell

1. The UNIX Shell

2. The Shell • Program that constantly runs at terminal after a user has logged in. • Prompts the user and waits for user input. • Interprets command line and makes arrangements for its execution. • Generally waits for command to complete execution (locking • the terminal). • A variety of shells to choose from: Bourne, Korn, C, Bash, Tcsh • Killed on logging out.

3. The Shell Metacharacters • Wildcard characters such as *, ?, [, ], ! • Redirection characters such as >, < • The pipe character, | • Command substitution characters ` ` • The $ as a variable prefix

4. Examples of Shell Behavior cat chap* Shell expands * to match all filenames in the current directory that begin with chap. date > foo Shell sees the > first, opens the file foo and connects the dateoutput to it. who | sort Shell understands the strings on either side of the | as two separate programs and connects them. ls`cat foo` Shell first runs cat and supplies the output as arguments to ls. echo $HOME Evaluates $HOME as a variable before running echo.

5. Wildcards • Set of metacharacters used in an expression to match multiple but • similar filenames. • * means any sequence of characters (including none) • ? means a single character • [ ] means any characters specified in the bracket • ! means not • - means a range of characters or numbers • Shell creates list of filenames before allowing command to run. • Expansion can be prevented by quoting and escaping. • Filenames must not contain wild-card characters.

6. Some Wildcard Examples ls *.lst Lists all files with extension .lst. rm ??* Removes all files with at least 2 characters. cp *.[ch] cprogs Copies all files with .c or .h extension into cprogs. rm *[!a-zA-Z]* Removes files not containing at least one letter. cat note[0-1][0-9] Displays files note00, … note19 rm * .o Removes all files then removes all files with .o extension (an error). Watch out!

7. Escaping (Using a \ Before a Character) • Escaping reverses the usual meaning of metacharacter following it. • Example: rm \* removes a file named * • Can also protect itself. Example: echo \\ displays a \ • Protects space and [Enter]. Example: cd My\ Documents • changes to the directory named My Documents • Inconvenient to use when command line contains too many • metacharacters that need to be escaped.

8. Quoting Quoting • Quoting protects mostmetacharacters from interpretation by the • shell. Example: echo “*” prints a * • More convenient than escaping when protecting a group of • metacharacters. • Quoted string understood as a single argument by shell and C • programs. (a.outfoo “My Documents” has 2 arguments and not 3.) • Double quotes and single quotes are not equivalent. Example: • echo “$SHELL”is not the same as echo ‘$SHELL’ • Quoting doesn’t protect the \ metacharacter. Escaping is required.

9. Be Careful Quoting Special Characters Like “, ‘ and \ • Try these: • echo ‘ ‘ ‘ • echo “ “ “ • echo ‘ “ ‘ • echo “ ‘ “ • echo ‘ \ ‘ • echo “ \ “

10. File Handling in the Shell • A file is opened by accessing it by its pathname. • Opening returns a file descriptor (an integer). • Subsequent read/write operations on the file use the descriptor. • Kernel allocates lowest available number as descriptor. • First three descriptors (0, 1 and 2) are always allocated: • 0 is standard input, 1 is standard output, 2 is standard error

11. Standard Input • Uses file descriptor 0. • By default, assigned to the user’s terminal keyboard. • Can also be obtained from standard output of another program • (redirection). • Commands using redirection continue to read descriptor 0 and • have no knowledge of this manipulation.

12. Standard Output • Uses file descriptor 1. • By default, assigned to the user’s display terminal. • Can also be used as input to another program (redirection). • Commands using redirection continue to write to descriptor 1 • and have no knowledge of this manipulation.

13. Standard Error • Uses file descriptor 2. • By default, file is assigned to the user’s display terminal. • Standard error output can’t be used as input to another program • but error output can be redirected to a file (redirection). • Commands using redirection continue to write error messages to • descriptor 2 and have no knowledge of this manipulation.

14. Redirection Using < , >, and >> • Redirection is a shell feature for manipulating command input and output. • Most commands designed to write output to standard output. • Most commands designed to take input from standard input when • used without a filename as argument. • Shell can assign these files to disk files through redirection: • wc prog.c > countfile • wc prog.c >> countfile • wc < prog.c • wc prog.c > countfile 2> errorfile • Commands themselves have no knowledge of the reassignment.

15. Redirection Using a Pipe | • Connects standard output on one command to standard input of another • Example: who | wc –l Displays # users on system • No temporary file is created. • Used to solve many text manipulation problems. • Commands have no knowledge that they are reading from or writing • to a pipe. • tee command duplicates its input and sends to both file and standard out • Example: ls | tee filelistList files on screen and in a file.

16. Two More Special Files to Redirect Output • /dev/null basically sends out nowhere. File size is always zero. • Examples: cat myfile > /dev/null Command does nothing! • find / -name myfile –print 2> /dev/null • This command hunts for file named myfile • starting at root and lists it. Send all error • messages nowhere. • /dev/tty is the file indicating your terminal (not the same file as • standard output or standard error!) • Example: who > /dev/tty Lists users at your terminal.

17. Command Substitution • Allows command arguments to be obtained from standard output • of another command. • Incommand1 `command2`, command2 is run first and its • standard output used as arguments to command1. • Example: echo Today is `date` • Command enclosed by ` ` (backquotes) must write to standard output. • Convenient mechanism for running commands whose arguments are • known only at runtime. Example: • echo “There are `ls | wc –l` files in this directory.” • (Use double quotes around echo, not single quotes.)

18. Shell Variables • Users define shell variables as variable=value. Referenced with $. • Environment variables such as SHELL, HOME, and PATH are shell • variables. Example: echo $HOME • Variables have no type and need no declaration before being used. • All shell variables initialized to null (x=“” or x=‘’ or x=). • To remove a shell variable, use unset. Example: unset x • To prevent changes to a shell variable, use readonly. Example: • readonly pi

19. Using Shell Variables • Set a variable x to 5 and display the value: • x=5 ; echo $x (no whitespace on either side of =) • x = 5 is illegal; x is interpreted as a command to run. • Can be set from command substitution: • Example: directory=`pwd` • Useful for storing pathnames that are repeatedly used in • a shell script: • Example: addressbook=$HOME/data/addressbook