Shells

1
Shells

• Five major shells
• Bourne (sh)
• C shell (csh)
• Korn (ksh)
• Bourne Again (bash)
• T shell (tcsh)

2
Korn Shell

• Invented by David Korn of ATT Bell Labs in mid
  1980s
• Upwardly compatible with the Bourne shell
• Became a standard part of Unix Systems Labs (USL)
  SVR4
• Two major versions exist, ksh88 and ksh93
• ksh88 is on clyde
• ksh93 is also available as /usr/dt/bin/dtksh

3
Major Features

• .profile
• Command line editing
• History
• Environment file
• Random numbers
• Print
• Whence
• Built-in arithmetic
• Aliases

4

• Job control
• Additional built-in variables
• Additional pattern matching
• Additional options for test
• Typeset
• Read
• Parameter Expansion

5
.profile

• Kept in your home (login) directory
• Used for customizing the user interface of ksh
• Used to store
• Aliases
• Shell variables
• Options
• Location of environment file
• Only executed if ksh is a login shell

6
Options

• Options alter the the shell's behavior
• Over 20 options available - common ones are

• bgnice
• emacs
• ignoreeof
• markdirs
• monitor

• noclobber
• noglob
• trackall
• vi

• Options are set using "set -o optionname" and
  unset using "set o optionname"

7
Command Line Editing

• Enabled in two different ways
• Using the shell variable VISUAL
• VISUAL(whence vi)
• Using set option
• set -o vi
• Two different editors are available
• vi and emacs
• Note ksh does not use these editors, it has
  commands that emulate the functionality built in
  to the shell

8
Editing Commands in vi-mode

• CTRL-W - Erase previous word
• CTRL-V - "Quote" the next character
• ESC - Enter control mode (edit mode)
• Edit mode uses same movement commands as vi
• h, l, w,b 0,,, etc.
• Also uses same editing commands
• i, a, r, R, x, d(movement command)

9
File Name Completion in vi-mode

• File name completion is also supported
• Type the command an the beginning of the file
  name
• Touch ESC and the \
• ksh will attempt to complete the file name for
  you
• If no name matches, shell beeps and nothing
  happens
• If there is exactly one way to complete the name,
  the shell does it, if it's a directory, it adds a
  /
• If there are more than one match, the shell
  completes the longest common prefix
• Using instead of \ lists all matches

10
History

• This feature alone may make it worth moving to
  Korn shell (when combined with command line
  editing)
• History is stored in a file (normally
  .sh_history) in your home directory
• Location can be changed by defining HISTFILE
• Note if you want history kept for each subshell,
  you can define HISTFILE.sh_history
• Unfortunately, this leaves a bunch of history
  files laying around your directory that need
  cleaning
• Size is set by HISTSIZE - default is 128
• Please set to 50 in your .profile to conserve
  disk space

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History Example

• Given the command find / -name ribblestaff
  -print
• And assuming it was command 36
• C shell
• !36 (re-execute command 36 in history)
• !f ( re-execute the most recent command
  beginning with f)
• !fp (to print but not re-execute, makes it last
  in buffer)
• rira (to change "ribble" to "rabble")

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Same Thing Using Korn

• Korn shell
• r 36 (to re-execute command 36 in history)
• r f (to re-execute the most recent command
  beginning with f)
• fc f (to fix command that started with an
  f)or fc cmd_no.
• This will put you into the editor of your choice
  (defined by FCEDIT - default is ed!)
• When you leave the editor, the command will be
  executed
• fc l (lists history commands)
• ksh automatically aliases history to fc -l

13
Moving Around the History File

• Again, the vi commands work here
• k - move backwards one line (command)
• j - move forward one command
• nG - move to command n
• ?string - search backwards for string
• /string - search forwards for string
• n - repeat search in same direction
• N - repeat search in opposite direction
• The first two can be preceded by repeat counts
• 3k - move backwards 3 commands

14
Environment File

• Although predefined environmental variables will
  always be known to subshells, the shell must be
  explicitly told which other variables, aliases,
  and options should be communicated to subshells
• One way to do this is to put them in an
  environment file
• Do this by placing
• export ENV/environmentfile in your .profile and
  then putting your data in environmentfile

15
Random Numbers

• The Korn shell provides a random number generator
• To initialize it, you type RANDOM (or a
  specific number if you want a repeatable
  sequence)
• Each time you echo the RANDOM variable, you will
  get anew random number in the range 0 -32767

16
print

• In other shells, you usually use awk when you
  want formatted printing
• ksh provides print. It doesn't allow as much
  control over printing as awk but it does provide
  more control than echo
• print has a variety of formatting options and
  parameters

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print Formatting Options

• \a - announce, ring the bell
• \b - backspace
• \c - print line without newline (rest of args
  ignored)
• \f - formfeed
• \n - newline
• \r - carriage return
• \t - tab
• \v - vertical tab
• \Ox - the 8 bit ASCII character with octal value
  x
• \\ - backslash

18
print Parameters

• - process anything that follows as an
  argument if it begins with a -
• -R do not use the \ conventions, also anything
  except -n that follows is processed as an
  argument
• -n do not add trailing newlines to output
• -p send arguments via pipe to a co-process
• -r don't use the \ conventions
• -s direct the arguments to the history file
• -un redirect arguments to the file descriptor
  n (default is 1 or stdout)

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whence

• Tells you where a command resides
• Two options, -p and -v, control the format of the
  output
• whence alone prints the path to the command
• whence vi
• /bin/ucb/vi
• -v also lists the type of command
• whence -v vi
• vi is a tracked alias for /usr/ucb/vi
• -p does a path search even if the name is a
  reserved word or alias

20
Built-In Arithmetic

• In the Bourne shell, if you wanted to do math you
  had to use eval and command substitution
• ksh lets you do arithmetic using either the let
  command or (( ))
• (( )) good for True/False comparisons (( ))
  provides value of computation
• let a 68
• print a
• 14
• Note that there are no spaces in the expression.
  If you want to use spaces, you must single quote
  the expression

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• The (( )) is useful in as a conditional because
  it returns a 1 or 0
• if test ((minmaxtop))
• then
• print "Ooh, a big one!"
• else
• print "Guess not"
• fi
• ksh can also work with bases other than 10
• bn is the same as n to the base b
• 817 is an octal 17 (15 decimal)

22
Aliases

• ksh has aliases very similar to the C shell
• It adds the concept of tracked aliases
• These translate faster than normal aliases
• When used, the shell searches the path the first
  time and then remembers where the command was
  found
• Future uses of the alias go directly to the
  command
• You can also unalias in the Korn shell

23
Job Control

• Korn shell supports job control almost the same
  as C shell
• CTRL-Z places a job in the background, as does
  command
• fg brings a job back to the foreground
• Referring to background jobs is also similar
• n job n
• string job whose command begins with string
• ?string job whose command contains string
• most recently invoked background
  job
• same as
• - second-most recently invoked bg job

24
Additional Built-In Variables

• PS1, PS2, PS3, PS4 - prompt strings
• LOGNAME - your login name
• CDPATH - provides a path to use when performing
  cd
• Normally, cd dirname looks for a directory in
  your current working directory
• CDPATH provides a list of places to look for
  dirname
• EDITOR - pathname of your preferred editor,
  various programs refer to this and it sets the
  mode for command line editing

25

• VISUAL - similar to EDITOR, used if EDITOR not
  set
• FCEDIT - editor to use with fc to edit commands
  in the history file
• OLDPWD - previous directory before last cd
• SECONDS - number of seconds since the current
  shell was invoked

26
Additional Pattern Matching

• Korn shell supports additional pattern matching
  metacharacters
• ?(patternpattern) matches zero or one
  occurrence of any of the patterns
• phre?(drsxx) would match phred, phrer, phres,
  and phrexx, ?(x) would match null and 'x'
• (patternpattern) matches zero or more
  occurrences of any pattern
• phre(0-9) would match 'phre' or 'phre'
  followed by any number of digits

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• (patternpattern) matches one or more
  occurrences of any pattern
• phre(0-9) would match 'phre' followed by one
  or more digits, (x) would match 'x', 'xx', 'xxx'
  
• _at_(patternpattern) matches any occurrence of
  any pattern
• Most useful when used as a logical OR.
  phre_at_(ddydsak) would match phreddy or phreds or
  phreak. _at_(xyz) matches 'x' or 'y' or 'z'

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• !(patternpattern) matches zero or more
  occurrences of any pattern
• Not really a regular expression but it is handy.
  It matches any that is not a pattern.
• phre!(d0-5) would match anything that started
  with 'phre' but did not end with a 'd' or the
  digits 0-5. !(x) would match any ASCCI character
  except 'x'

29
Additional Options for test

• The Korn shell gives additional options for the
  test command
• -a the file exists
• -G file has same group as test process
• -L file is symbolic (soft) link
• -O file has same owner as test process
• -o option true if shell option is on (set)
• -S file is a socket
• -z string is zero length
• f1 -ef f2 the two files are linked (same inode)
• f1 -nt f2 true if f1 is newer than f2
• f1 -ot f2 true if f1 is older than f2

30
Typeset

• In the Korn shell, you can assign types to
  variables
• typeset -i count
• makes count an integer, if you tried to assign a
  string to it, the shell would give you an error
  message
• You can typeset with the following attributes
• -u make uppercase (forces string to uppercase)
• -l make lowercase
• -in require integers and store internally as
  integer n specifies the base
• -r make variable readonly
• -x export the variable

31

• -Ln left justify to remove leading spaces fill
  with spaces or truncate on right to n bytes
• -Rn right justify to remove trailing spaces fill
  with spaces or truncate on left to n bytes
• -Zn same as -R except pad with leading 0's
  instead of spaces
• typeset o is used to turn the option off

32
Read

• read functions much like it did in the Bourne
  shell
• However, it can be smart and send the prompt to
  stderr without special processing
• read variable?'prompt'
• Note the location of the ? and the tick marks.
  These are critical!
• This will send prompt to stderr if the input
  device is the terminal but if stdin has been
  redirected, no prompt will be generated.

33
Parameter Expansion

• var - uses the value of var
• var-value - if var is set (non-null) return
  it, otherwise return value
• varvalue - if var has been set, return it,
  otherwise set var to value and return it
• var?message - if var has been set, return it,
  otherwise print var and message (default is
  "parameter null or not set")
• varvalue - if var has been set, return
  value, otherwise return null

34
Substrings

• variablepattern - if pattern matches the
  beginning of variable's value, delete the
  shortest part that matches and return the rest
• variablepattern - if pattern matches the
  beginning of variable's value, delete the longest
  part that matches and return the rest
• variablepattern - if pattern matches the end
  of variable's value, delete the shortest part
  that matches and return the rest
• variablepattern - if pattern matches the end
  of variable's value, delete the longest part that
  matches and return the rest

35
Command Substitution

• In the Korn shell, command substitution is done
  with a different syntax
• The Bourne shell syntax is supported but
  discouraged
• (command arg1 arg2 ) is replaced with the
  result of the command
• An example might be in your .profile file when
  you set up your VISUAL or EDITOR variable
• EDITOR(whence vi)
• This would evaluate to
• EDITOR/bin/ucb/vi

36
Functions

• The Korn shell's function feature is an expanded
  version of that found in the Bourne shell
• A function is similar to a script except it is
  kept in the shell's memory. Because it is kept
  in memory, it runs faster than loading a script
• Functions allow you to create your own groups of
  commands, which can make shell programming much
  easier since they can be used analogously to
  subroutines
• Functions are not inherited by subshells unless
  you define them in the environment file with the
  typeset command typeset -fx functname

37
Syntax

• function functname
• shell command
• shell command
• You can delete a function using unset -f
  functname
• You can find out what functions are defined using
  the command (actually an alias) "functions"
• The shell will print the names and definitions of
  all functions

38

• There are two important differences between
  functions and scripts
• A functions does not run as a separate process
• It shares variables with the process that invoked
  it
• The present working directory is that of the
  caller
• If the function changes it, it will also be
  changed for the caller
• If a function has the same name as a script or
  executable program, the function takes precedence
• The return statement returns the exit status of
  the last command executed
• Functions can be recursive but you need to be
  careful

39
Autoloaded Functions

• The simplest place to define functions is in your
  .profile or environment file
• However, if you have a lot of definitions, the
  delay in reading these files each time you log in
  or invoking a subshell can become unacceptably
  slow
• Autoload lets you tell the shell to only read the
  function definition when it is invoked the first
  time
• "autoload functname" creates an undefined
  function, functname, that won't be loaded until
  the first time it is actually called

40
Finding Autoload Definitions

• The FPATH variable tells the shell where to
  search for autoload functions
• For example, if your .profile (or environment
  file) contains the following
• FPATH/funcs
• autoload my_function
• When you invoke the function my_function the
  first time, the shell will search the directories
  in FPATH for a file called my_function which has
  a definition of function my_function and load it
  into the shell's memory space

41
Example Functions

• function increment
• typeset sum sum is a local variable
• ((sum 1 1))
• return sum
• print -n "The sum is "
• increment 5
• print ? ? holds the return value of the
  last command executed
• print sum sum was local to function and
  no longer is defined

"typeset var" with no options makes var a
local variable in a function
42

• function square
• ((sq11))
• print "Number to be squared is 1."
• print "The result is sq."
• square 10
• Number to be squared is 10.
• The result is 100.