The UNIX Shell

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.

The Shell Metacharacters

• Wildcard characters such as *, ?, [, ], !

• Redirection characters such as >, <

• The pipe character, |

• Command substitution characters ` `

• The $ as a variable prefix

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 date output 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.

Examples of Shell Behavior

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.

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!

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.

Quoting

• Quoting protects most metacharacters 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.out foo “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.

Quoting

Be Careful Quoting Special CharactersLike “, ‘ and \

• Try these:• echo ‘ ‘ ‘• echo “ “ “• echo ‘ “ ‘• echo “ ‘ “• echo ‘ \ ‘• echo “ \ “

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

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.

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.

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.

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.

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 filelist List files on screen and in a file.

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.

Command Substitution• Allows command arguments to be obtained from standard output of another command.

• In command1 `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.)

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

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