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Manual: sh(1)

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Please contact or Mt Xia for assistance with all your shell programming needs.


sh(1)									sh(1)



NAME

  sh, Rsh - The	Bourne shell, an interactive command interpreter and command
  programming language

SYNOPSIS

  sh [-ir] [+ |	-aefhkntuvx] [file] [argument ...] [-ccommand_string | -s]

  Rsh [-ir] [+ | -aefhkntuvx] [file] [argument ...] [-ccommand_string |	-s]

  The shell carries out	commands either	interactively from a keyboard or from
  a file.

DESCRIPTION

  The Bourne shell is a	command	programming language that executes commands
  read from a terminal or a file.  Rsh is a restricted version of the stan-
  dard command interpreter sh; Rsh is used to set up login names and execu-
  tion environments whose capabilities are more	controlled than	those of the
  standard shell.  This	allows you to create user environments that have a
  limited set of privileges and	capabilities.  (See Restricted Shell for more
  information.)

  A 
  script, a shell procedure, or	a command file.

  A simple command is a	sequence of words separated by spaces or tabs.	A
  word is a sequence of	characters that	contains no unquoted spaces or tabs.
  The first word in the	sequence (numbered as 0), usually specifies the	name
  of a command.	 Any remaining words, with a few exceptions, are passed	to
  that command.	 A space refers	to both	spaces and tabs.

  The value of a simple	command	is its exit value if it	ends normally, or
  (octal) 200 added to the signal number if it terminates due to a signal.
  For a	list of	status values, see the signal()	system call.

  A pipeline is	a sequence of one or more commands separated by	a | (vertical
  bar) or, for historical compatibility, by a ^	(circumflex).  In a pipeline,
  the standard output of each command becomes the standard input of the	next
  command.  Each command runs as a separate process, and the shell waits for
  the last command to end.  A filter is	a command that reads its standard
  input, transforms it in some way, then writes	it to its standard output.  A
  pipeline normally consists of	a series of filters.  Although the processes
  in a pipeline	(except	the first process) can execute in parallel, they are
  synchronized to the extent that each program needs to	read the output	of
  its predecessor.

  The exit value of a pipeline is the exit value of the	last command.

  A list is a sequence of one or more pipelines	separated by ; (semicolon), &
  (ampersand), && (two ampersands), or || (two vertical	bars) and optionally
  ended	by a ; (semicolon), an & (ampersand), a	|& (coprocess),	or a newline.
  These	separators and terminators have	the following effects:

  ;   Causes sequential	execution of the preceding pipeline; the shell waits
      for the pipeline to finish.

  &   Causes asynchronous execution of the preceding pipeline; the shell does
      not wait for the pipeline	to finish.

  &&  Causes the list following	it to be executed only if the preceding	pipe-
      line returns a 0 (zero) exit value.

  ||  Causes the list following	it to be executed only if the preceding	pipe-
      line returns a nonzero exit value.

      The cd command is	an exception; if it returns a nonzero exit value, no
      subsequent commands in a list are	executed, regardless of	the separator
      characters.

  The ;	and & separators have equal precedence,	as do && and ||.  The
  single-character separators have lower precedence than the double-character
  separators.  An unquoted newline character following a pipeline functions
  the same as a	; (semicolon).

  The shell treats as a	comment	any word that begins with a # character	and
  ignores that word and	all characters following up to the next	newline	char-
  acter.

  Shell	Flow Control Statements

  Unless otherwise stated, the value returned by a statement is	that of	the
  last simple command executed in the statement.

  for identifier [in word ...] do list done
      For each word, sets identifier to	word and executes the commands in
      list.  If	you omit in word ..., then the for command executes list for
      each positional parameter	that is	set.  Execution	ends when there	are
      no more words in the list.

  case word in [pattern	[ | pattern ] ...) list	;;] ...	esac
      Executes the commands in the list	associated with	the first pattern
      that matches word.  Uses the same	character-matching notation in pat-
      terns that you use for filename substitution (see	Filename Substitu-
      tion), except that you do	not need to match explicitly a / (slash), a
      leading .	(dot), or a . (dot) immediately	following a / (slash).

  if list then list [elif list then list] ... [else list] fi
      Executes the list	following the if keyword.  If it returns a 0 (zero)
      exit value, executes the list following the first	then.  Otherwise,
      executes the list	following elif (if there is an elif), and if its exit
      value is 0 (zero), executes the next then.  Failing that,	executes the
      list following the else.	If no else list	or then	list is	executed, the
      if command returns a 0 (zero) exit value.

  while	list do	list done
      Executes the list	following the while.  If the exit value	of the last
      command in the list is 0 (zero), executes	the list following do.	Con-
      tinues looping through the lists until the exit value of the last	com-
      mand in the while	list is	nonzero.  If no	commands in the	do list	are
      executed,	the while command returns a 0 (zero) exit value.

  until	list do	list done
      Executes the list	following the until.  If the exit value	of the last
      command in the list is nonzero, executes the list	following do.  Con-
      tinues looping through the lists until the exit value of the last	com-
      mand in the until	list is	0 (zero).  If no commands in the do list are
      executed,	the until command returns a 0 (zero) exit value.

  (list)
      Executes the commands in list in a subshell.

  { list; }
      Executes the commands in list in the current shell process; does not
      spawn a subshell.

  name () {  list;  }
      Defines a	function that is referenced by name.  The body of the func-
      tion is the list of commands between the braces.

  The following	reserved words are recognized only when	they appear without
  quotes as the	first word of a	command.

       if      esac
       then    case
       else    for
       elif    while
       fi      until
       do      done
       {  }

  Command Execution

  Each time the	shell executes a command, it carries out the substitutions
  discussed in the following text.  If the command name	matches	one of the
  built-in commands discussed in Built-In Commands, it executes	it in the
  shell	process.  If the command name does not match a built-in	command	but
  matches the name of a	defined	function, it executes the function in the
  shell	process.  The shell sets the positional	parameters to the parameters
  of the function.

  If the command name matches neither a	built-in command nor the name of a
  defined function and the command names an executable file that is a com-
  piled	(binary) program, the shell (as	parent)	spawns a new (child) process
  that immediately runs	the program.  If the file is marked executable but is
  not a	compiled program, the shell assumes that it is a shell script.	In
  this case, the shell spawns another instance of itself (a subshell), to
  read the file	and execute the	commands included in it	(note how this
  differs from the execution of	functions).  The shell also executes a com-
  mand enclosed	in parentheses in a subshell.  From the	perspective of an end
  user,	a compiled program is run in exactly the same way as a shell script.

  The shell normally searches for commands in two places in the	file system.
  The shell first looks	for the	command	in the current directory; if it	does
  not find the command there, it looks in the /usr/bin directory.  This
  search order is in effect if the PATH	environment variable is	not set	(or
  is set to :/usr/bin, as is the case by default on many systems).

  You can also give a specific pathname	when you invoke	a command, for exam-
  ple /usr/bin/sort, in	which case the shell does not search any directories
  other	than the one you specify in the	pathname.  If the command name con-
  tains	a / (slash), the shell does not	use the	search path (note that the
  restricted shell will	not execute such commands).  You can give a full
  pathname that	begins with the	root directory (as in /usr/bin/sort), or a
  pathname relative to the current directory, for example bin/myfile.  In
  this last case, the shell looks in the current directory for a directory
  named	bin and	in that	directory for myfile.

  You can change the particular	sequence of directories	searched by resetting
  the PATH variable (see Variables Used	by the Shell).

  The shell remembers the location in the search path of each executed com-
  mand (to avoid unnecessary exec commands later).  If the command was found
  in a relative	directory (one whose name does not begin with /), the shell
  must redetermine its location	whenever the current directory changes.	 The
  shell	forgets	all remembered locations whenever you change the PATH vari-
  able or execute the hash -r command (see Built-In Commands).



  Signals

  The shell ignores SIGINT and SIGQUIT signals for an invoked command if the
  command is terminated	with a	& (that	is, if it is running in	the back-
  ground).  Otherwise, signals have the	values inherited by the	shell from
  its parent, with the exception of signal 11 (see also	the built-in trap
  command in Built-In Commands).

  Initialization Files

  When you log in, the shell is	called to read your commands.  Before it does
  that,	however, it checks to see if a file named /etc/profile exists on the
  system, and if it does, it reads commands from it (this file sets variables
  needed by all	users).	 After this, the shell looks for a file	named .pro-
  file in your login directory.	 If it finds one, it executes commands from
  it.  Finally,	the shell is ready to read commands from your standard input.

  Filename Substitution

  Command arguments are	very often filenames.  You can automatically produce
  a list of filenames as arguments on a	command	line by	specifying a pattern
  that the shell matches against the filenames in a directory.

  Most characters in such a pattern match themselves, but you can also use
  some special pattern-matching	characters in your pattern.  These special
  characters are as follows:

  .   Matches any single character, except a newline character.

  *   Matches any string, including the	null string.

  ?   Matches any one character.

  [ ...]
      Matches any one of the characters	enclosed in brackets.

  [! ...]
      Matches any character other than those that follow the exclamation
      point within brackets.

  Inside brackets, a pair of characters	separated by a - (dash)	specifies a
  set of all characters	lexically within the inclusive range of	that pair
  according to the current collating sequence.	The LANG and LC_COLLATE
  environment variables	control	the collating sequence.

  The current collating	sequence groups	characters into	equivalence classes
  for the purpose of defining the endpoints of a range of characters.  For
  example, if the collating sequence defines the lexical order to be AaBbCc
  ... and groups uppercase and lowercase characters into equivalence classes,
  then all the following have the same effect:	[a-c], [A-C], [a-C], and
  [A-c].

  Pattern matching has some restrictions.  If the first	character of a
  filename is a	. (dot), it can	be matched only	by a pattern that begins with
  a dot.  For example, * (asterisk) matches the	filenames myfile and your-
  file,	but not	the filenames .myfile and .yourfile.  To match these files,
  use a	pattern	such as	the following:

       .*file


  If a pattern does not	match any filenames, then the pattern itself is
  returned as the result of the	attempted match.

  File and directory names should not contain the characters *,	?, [, or ],
  because this requires	quoting	those names in order to	refer to the files
  and directories.

  Shell	Variables and Command-Line Substitutions

  The shell has	several	mechanisms for creating	variables (assigning a string
  value	to a name).  Certain variables,	positional parameters and keyword
  parameters, are normally set only on a command line.	Other variables	are
  simply names to which	you or the shell can assign string values.

  Positional Parameters

  When you run a shell script, the shell implicitly creates positional param-
  eters	that reference each word on the	command	line by	its position on	the
  command line.	 The word in position 0	(the procedure name), is called	$0,
  the next word	(the first parameter) is called	$1, and	so on up to $9.	 To
  refer	to command-line	parameters numbered higher than	9, use the built-in
  shift	command	(see Built-In Commands).

  You can also assign values to	these positional parameters explicitly by
  using	the built-in set command (see Built-In Commands).

  When an argument for a position is not specified, its	positional parameter
  is set to null.

  Positional parameters	are global and can be passed to	nested shell scripts.

  User-Defined Variables

  The shell also recognizes alphanumeric variables to which string values can
  be assigned.	You assign a string value to a name, as	follows:

       name=string

  A name is a sequence of letters, digits, and underscores that	begins with
  an underscore	or a letter.  To use the value that you	have assigned to a
  variable, add	a $ (dollar sign) to the beginning of its name.	 Thus, $name
  yields the value string.  Note that no spaces	surround the = (equal sign)
  in an	assignment statement.  (Positional parameters cannot appear in an
  assignment statement;	they can only be set as	described earlier.) You	can
  put more than	one assignment on a command line, but remember:	the shell
  performs the assignments from	right to left.

  If you surround string with quotes, either " " (double) or ' ' (single),
  the shell does not treat spaces, tabs, semicolons, and newline characters
  within it as word delimiters but embeds them literally in the	string.

  If you surround string with double quotes, the shell still recognizes	vari-
  able names in	the string and performs	variable substitution; that is,	it
  replaces references to positional parameters and other variable names	that
  are prefaced by $ with their corresponding values, if	any.  The shell	also
  performs command substitution	(see Command Substitution) within strings
  that are surrounded by double	quotes.

  If you surround string with single quotes, the shell does no variable	or
  command substitution within the string.  The following sequence illustrates
  this difference:

  You enter:


       stars=*****
       asterisks1="Add $stars"
       asterisks2='Add $stars'
       echo $asterisks1


  The system displays:

       Add *****

  You enter:

       echo $asterisks2

  The system displays:

       Add $stars

  The shell does not reinterpret spaces	in assignments after variable substi-
  tution (see Interpretation of	Spaces).  Thus,	the following assignments
  result in $first and $second having the same value:

       first='a	string with embedded spaces'
       second=$first


  When you reference a variable, you can enclose the variable name (or the
  digit	designating a positional parameter) in { } (braces) to delimit the
  variable name	from any following string.  In particular, if the character
  immediately following	the name is a letter, digit, or	underscore and the
  variable is not a positional parameter, then the braces are required:

  You enter:

       a='This is a'
       echo "${a}n example"

  The system displays:

       This is an example

  You enter:

       echo "$a	test"

  The system displays:

       This is a test

  See Conditional Substitution for a different use of braces in	variable sub-
  stitutions.

  A Command's Environment

  All the variables (with their	associated values) that	are known to a com-
  mand at the beginning	of its execution constitute its	environment.  This
  environment includes variables that a	command	inherits from its parent pro-
  cess and variables specified as keyword parameters on	the command line that
  calls	the command.

  The shell passes to its child	processes the variables	that were named	as
  arguments to the built-in export command.  The export	command	places the
  named	variables in the environments of both the shell	and all	its future
  child	processes.


  Keyword parameters are variable-value	pairs that appear in the form of
  assignments, normally	before the procedure name on a command line (but see
  also the -k flag, discussed under the	set command in Built-In	Commands).
  Such variables are placed in the environment of the procedure	being called.


  For example, given the following simple procedure that echoes	the values of
  two variables	(saved in a command file named key_command):

       # cat key_command
       echo $a $b
       #


  the following	command	lines produce the output shown:

  You enter:

       a=key1 b=key2 key_command


  The system displays:

       key1 key2


  You enter:

    


  The system displays:

       tom john


  A procedure's	keyword	parameters are not included in the parameter count
  stored in $#.

  A procedure can access the values of any variables in	its environment; how-
  ever,	if it changes any of these values, these changes are not reflected in
  the shell environment.  They are local to the	procedure in question.	To
  place	these changes in the environment that the procedure passes to its
  child	processes, you must export these values	within that procedure.

  To obtain a list of variables	that were made exportable from the current
  shell, enter:

       export


  (You will also get a list of variables that were made	read only.) To get a
  list of name-value pairs in the current environment, enter:

       env


  Conditional Substitution

  Normally, the	shell replaces $variable with the string value assigned	to
  variable, if there is	one.  However, there is	a special notation that
  allows conditional substitution, depending on	whether	the variable is	set
  and is not null.  By definition, a variable is set if	it was assigned	a
  value.  The value of a variable can be the null string, which	you can
  assign to a variable in any one of the following ways:

       A=
       bcd=""
       Efg=''
       set '' ""


  The first three of these examples assign the null string to each of the
  corresponding	variable names.	 The last example sets the first and second
  positional parameters	to the null string and unsets all other	positional
  parameters.

  The following	is a list of the available expressions you can use to perform
  conditional substitution:

  ${ variable-string }
      If the variable is set, substitute the value of variable in place	of
      this expression.	Otherwise, replace this	expression with	the value of
      string.

  ${ variable:-string }
      If the variable is set and is not	null, substitute the value of vari-
      able in place of this expression.	 Otherwise, replace this expression
      with the value of	string.

  ${ variable=string }
      If the variable is set, substitute the value of variable in place	of
      this expression.	Otherwise, set variable	to string and then substitute
      the value	of the variable	in place of this expression.  You cannot
      assign values to positional parameters in	this fashion.

  ${ variable:=string }
      If the variable is set and is not	null, substitute the value of vari-
      able in place of this expression.	 Otherwise, set	variable to string
      and then substitute the value of the variable in place of	this expres-
      sion.  You cannot	assign values to positional parameters in this
      fashion.

  ${ variable?string }
      If the variable is set, substitute the value of variable in place	of
      this expression.	Otherwise, display a message of	the form:
	   variable:	   string

      and exit from the	current	shell, unless the shell	is the login shell.
      If you do	not specify string, the	shell displays the following message:
	   variable:	   parameter null or not set

  ${ variable:?string }
    
      in place of this expression.  Otherwise, display a message of the	form:
	   variable:	   string

      and exit from the	current	shell, unless the shell	is the login shell.
      If you do	not specify string, the	shell displays the following message:
	   variable:	   parameter null or not set

  ${ variable+string }
      If the variable is set, substitute the value of string in	place of this
      expression.  Otherwise, substitute the null string.

  ${ variable:+string }
      If the variable is set and not null, substitute the value	of string in
      place of this expression.	 Otherwise, substitute the null	string.

  In conditional substitution, the shell does not evaluate string until	it
  uses it as a substituted string, so that, in the following example, the
  shell	executes the pwd command only if d is not set or is null:

       echo ${ d:-`pwd`	}




  Variables Used by the	Shell

  The shell uses the following variables.  The shell sets some of them,	and
  you can set or reset all of them.

  CDPATH
      The search path for the cd (change directory) command.

  HOME
      The name of your login directory,	the directory that becomes the
      current directory	upon completion	of a login.  The login program ini-
      tializes this variable.  The cd command uses the value of	$HOME as its
      default value.  If you use this variable in your shell scripts rather
      than using the full pathname, your procedures run	even if	your login
      directory	is changed or if another user runs them.

  LANG
      Specifies	the locale of your system, which is comprised of three parts:
      language,	territory, and codeset.	 The default locale is the C locale,
      which specifies the value	English	for language, U.S. for territory, and
      ASCII for	codeset.  The locale specified for the LANG variable controls
      the language applied to messages.	 Unless	set explicitly,	the
      LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and LC_TIME
      variables	also derive their settings from	the locale set for LANG.

  LC_COLLATE
      Specifies	the collating sequence to use when sorting names and when
      character	ranges occur in	patterns.  The default value is	the collating
      sequence for American English.  If absent, the collating sequence	may
      be taken from the	LANG variable.	If both	LC_COLLATE and LANG are
      absent, the ANSI C collating sequence is used.

  LC_CTYPE
      Specifies	the character classification information to use	on your	sys-
      tem.  The	default	value is American English.

  LC_MESSAGES
      Specifies	the language that the system expects for user input of yes
      and no strings.  The default value is American English.

  LC_MONETARY
      Specifies	the monetary format for	your system.  The default value	is
      the monetary format for American English.

  LC_NUMERIC
      Specifies	the numeric format for your system.  The default value is the
      numeric format for American English.

  LC_TIME
      Specifies	the date and time format for your system.  The default value
      is the date and time format for American English.

  LOGNAME
      Your login name, marked readonly in the /etc/profile file.

  MAIL
      The pathname of the file used by the mail	system to detect the arrival
      of new mail.  If MAIL is set, the	shell periodically checks the modifi-
      cation time of this file and displays the	value of $MAILMSG, if this
      time changes and the length of the file is greater than 0	(zero).

      Set MAIL in your .profile	file.  The value normally assigned to it by
      users of the mail	or mailx commands is /var/spool/mail/$LOGNAME.

  MAILCHECK
      The number of seconds that the shell lets	elapse before checking again
      for the arrival of mail in the files specified by	the MAILPATH or	MAIL
      variables.  The default value is 600 seconds (10 minutes).  If you set
      MAILCHECK	to 0 (zero), the shell checks before each prompt.

  MAILPATH
      A	list of	filenames separated from one another by	a : (colon).  If you
      set this variable, the shell informs you of the arrival of mail in any
      of the files specified in	the list.  You can follow each filename	by a
      %	(percent sign) and a message to	be displayed when mail arrives.	 Oth-
      erwise, the shell	uses the value of MAILMSG or, by default, the message
      you have mail.

      When MAILPATH is set, these files	are checked instead of the file	set
      by MAIL.	To check the files set by MAILPATH and the file	set by MAIL,
      specify the MAIL file in your list of MAILPATH files.

  MAILMSG
      The mail notification message.  If you explicitly	set MAILMSG to a null
      string (MAILMSG=""), no message is displayed.

  NLSPATH
      Specifies	a list of directories to search	to find	message	catalogs.

  PATH
      An ordered list of directory pathnames separated by colons.  The shell
      searches these directories in the	specified order	when it	looks for
      commands.	 A null	string anywhere	in the list represents the current
      directory.

      PATH is normally initialized in the /etc/profile file, usually to
      :/usr/bin	(by definition,	a null string is assumed in front of the
      leading colon).  You can reset this variable to suit your	own needs.
      Thus, if you wish	to search your current directory last rather than
      first, you would enter:
	   PATH=/usr/bin:


      If you have a personal directory of commands (say, $HOME/bin) that you
      want searched before the standard	system directories, set	your PATH as
      follows:
	   PATH=$HOME/bin:/usr/bin:


      The best place to	set your PATH to something other than the default
      value is in your .profile	file (see The .profile File).  You cannot
      reset PATH if you	are executing commands under the restricted shell
      (Rsh).

  PS1 The string to be used as the primary system prompt.  An interactive
      shell displays this prompt string	when it	expects	input.	The default
      value of PS1 is $	followed by a space.

  PS2 The value	of the secondary prompt	string.	 If the	shell expects more
      input when it encounters a newline character in its input, it prompts
      with the value of	PS2.  The default value	of PS2 is > followed by	a
      space.

  IFS The characters that are internal field separators	(the characters	that
      the shell	uses during interpretation of spaces, see Interpretation of
      Spaces).	The shell initially sets IFS to	include	the space, tab,	and
      newline characters.

  SHACCT
      The name of a file that you own.	If this	variable is set, the shell
      writes an	accounting record in the file for each shell script executed.
      You can use accounting programs such as acctcom and acctcms to analyze
      the data collected.

  SHELL
      A	pathname whose simple part (the	part after the last /) contains	an r
      if you want the shell to become restricted when invoked.	This should
      be set and exported by the $HOME/.profile	file of	each restricted
      login.

  TIMEOUT
      A	number of minutes.  After the shell displays its prompt, you have
      TIMEOUT minutes to enter a command.  If you fail to do so, the shell
      exits; in	the login shell, such an exit is a logout.  Setting TIMEOUT
      to 0 (zero) inhibits automatic logout.

  Predefined Special Variables

  Several variables have special meanings; the following are set only by the
  shell:

  $#  The number of positional parameters passed to the	shell, not counting
      the name of the shell script itself.  The	$# variable thus yields	the
      number of	the highest-numbered positional	parameter that is set.	One
      of the primary uses of this variable is to check for the presence	of
      the required number of arguments.

  $?  The exit value of	the last command executed.  Its	value is a decimal
      string.  Most commands return 0 (zero) to	indicate successful comple-
      tion.  The shell itself returns the current value	of $? as its exit
      value.

  $$  The process number of the	current	process.  Because process numbers are
      unique among all existing	processes, this	string of up to	five digits
      is often used to generate	unique names for temporary files.  The fol-
      lowing example illustrates the recommended practice of creating tem-
      porary files in a	directory used only for	that purpose:
	   temp=$HOME/temp/$$
	   ls >$temp
		   .
		   .
		   .
	   rm $temp


  $!  The process number of the	last process run in the	background (using the
      &	terminator).  Again, this is a string of up to five digits.

  $-  A	string consisting of the names of the execution	flags (see Built-In
      Commands)	currently set in the shell.

  Command Substitution

  To capture the output	of any command as an argument to another command,
  place	that command line within ` ` (grave accents).  This concept is known
  as command substitution.  The	shell first executes the command or commands
  enclosed within the grave accents, and then replaces the whole expression,
  grave	accents	and all, with their output.  This feature is often used	in
  assignment statements:

       today=`date`


  This statement assigns the string representing the current date to the
  today	variable.  The following assignment saves, in the files	variable, the
  number of files in the current directory:


       files=`ls | wc -l`


  You perform command substitution on any command that writes to standard
  output by enclosing that command in grave accents.  You can nest command
  substitutions	by preceding each of the inside	sets of	grave accents with a
  \ (backslash):

       logmsg=`echo Your login directory is \`pwd\``


  You can also give values to shell variables indirectly by using the built-
  in read command.  The	read command takes a line from standard	input (usu-
  ally your keyboard), and assigns consecutive words on	that line to any
  variables named:

       read first middle last


  Thus,	read will accept the following input line

       Jane C. Chen


  and it will have the same effect as if you had entered

       first=Jane init=C. last=Chen


  The read command assigns any excess words to the last	variable.

  Quoting Mechanisms

  The following	characters have	a special meaning to the shell and cause ter-
  mination of a	word unless quoted:

       ; & ( ) | ^ < >   

  Using	' ' (single) and " " (double) quotes to	surround a string or a \
  (backslash) before a single character	enables	the character to stand for
  itself, instead of conveying special meaning to the shell.

  Within single	quotes,	all characters (except the single quote	character
  itself), are taken literally,	with any special meaning removed.  Thus,
  entering:

       stuff='echo $? $*; ls * | wc'


  results only in the literal string echo $? $*; ls * |	wc being assigned to
  the stuff variable; the echo,	ls, and	wc commands are	not executed, nor are
  the variables	$? and $* and the special character * expanded by the shell.

  To verify this you could export the variable stuff with the command export
  stuff, and then use the command printenv stuff to view it.  Note that	this
  is different from the	simple command echo $stuff.

  Within double	quotes,	the special meaning of certain characters ($, `, and
  ") does persist, while all other characters are taken	literally.  Thus,
  within double	quotes,	command	and variable substitution takes	place.	In
  addition, the	quotes do not affect the commands within a command substitu-
  tion that is part of the quoted string, so characters	there retain their
  special meanings.

  Consider the following sequence:

  You enter:

       ls *

  System displays:

       file1
       file2
       file3

  You enter:

       message="This directory contains	`ls * `	"
       echo $message

  System displays:

       This directory contains file1 file2 file3

  This shows that the *	special	character inside the command substitution was
  expanded.

  To hide the special meaning of $, `, and " within double quotes, precede
  these	characters with	a \ (backslash).  Outside of double quotes, preceding
  a character with \ (backslash) is equivalent to placing it within single
  quotes.  Hence, a \ (backslash) immediately preceding	the newline character
  (that	is, a \	(backslash) at the end of the line) hides the newline charac-
  ter and allows you to	continue the command line on the next physical line.

  Redirection of Input and Output

  In general, most commands do not know	or care	whether	their input or output
  is associated	with the keyboard, the display screen, or a file.  Thus, a
  command can be used conveniently either at the keyboard or in	a pipeline.

  Standard Input and Standard Output

  When a command begins	running, it usually expects that three files are
  already open:	standard input,	standard output, and standard error (some-
  times	called error output or standard	error output).	A number called	a
  file descriptor is associated	with each of these files as follows:

  File descriptor 0
      Standard input

  File descriptor 1
      Standard output

  File descriptor 2
      Standard error

  A child process normally inherits these files	from its parent; all three
  files	are initially assigned to the tty.  Conventionally, commands read
  from standard	input (0), write to standard output (1), and write error mes-
  sages	to standard error (2).	The shell permits them to be redirected	else-
  where	before control is passed to a command.	Any argument to	the shell in
  the form <file or >file opens	the specified file as the standard input or
  output, respectively.


  In the case of output, this process destroys the previous contents of	file,
  if it	already	exists and write permission is available.  An argument in the
  form >>file directs the standard output to the end of	file, thus allowing
  you to add data to it	without	destroying its existing	contents.  If file
  does not exist, the shell creates it.

  Such redirection arguments are subject only to variable and command substi-
  tution; neither interpretation of spaces nor pattern matching	of filenames
  occurs after these substitutions.  Thus, entering:

       echo 'this is a test' > *.ggg


  produces a 1-line file named *.ggg, and entering:

       cat < ?


  produces an error message, unless you	have a file named ? (a bad choice for
  a filename).

  Diagnostic and Other Output

  Diagnostic output from commands is normally directed to the file associated
  with file descriptor 2.  You can redirect this error output to a file	by
  immediately preceding	either output redirection symbol (> or >>) with	a 2
  (the number of the file descriptor).	Note that there	must be	no space
  between the file descriptor and the redirection symbol; otherwise, the
  shell	interprets the number as a separate argument to	the command.

  You can also use this	method to redirect the output associated with any of
  the first 10 file descriptors	(numbered 0 to 9) so that, for instance, if a
  command writes to file descriptor 9 (although	this is	not a recommended
  programming habit), you can capture that output in a file named savedata as
  follows:

       command 9> savedata


  If a command writes to more than one output, you can independently redirect
  each one.  Suppose that a command directs its	standard output	to file
  descriptor 1,	directs	its error output to file descriptor 2, and builds a
  data file on file descriptor 9.  The following command line redirects	each
  of these outputs to a	different file:

       command > standard 2> error 9> data


  Inline Input (Here) Documents

  Wh
  is any string	that contains no pattern-matching characters, the shell	takes
  the subsequent lines as the standard input of	command	until it reads a line
  consisting of	only eof_string	(possibly preceded by one or more tab charac-
  ters):

       command << eof_string

  The lines between the	first eof_string and the second	are frequently
  referred to as a here	document.  If a	- (dash) immediately follows the <<,
  the shell strips leading tab characters from each line of the	input docu-
  ment before it passes	the line to the	command.

  The shell creates a temporary	file containing	the input document and per-
  forms	variable and command substitution on its contents before passing it
  to the command.  It performs pattern matching	on filenames that are a	part
  of command lines in command substitutions.  If you want to prohibit all
  substitutions, quote any character of	eof_string:

       command << \eof_string


  The here document is especially useful for a small amount of input data
  that is more conveniently placed in the shell	script rather than kept	in a
  separate file	(such as editor	scripts).  For instance, you could enter:

       cat <<- xyz
       This message is shown on	the
       display with leading tabs removed.
       xyz


  This feature is most useful in shell scripts.	 Note that inline input	docu-
  ments	cannot appear within grave accents (command substitution).

  I/O Redirection with File Descriptors

  As discussed previously, a command occasionally directs output to some file
  associated with a file descriptor other than 1 or 2.	The shell also pro-
  vides	a mechanism for	creating an output file	associated with	a particular
  file descriptor.  For	example, if you	enter the following, where digit1 and
  digit2 are valid file	descriptors, you can direct the	output that would
  normally be associated with file descriptor digit1 to	the file associated
  with digit2:

    

  The default value for	digit1 and digit2 is 1 (standard output).  If, at
  execution time, no file is associated	with digit2, then the redirection is
  void.	 The most common use of	this mechanism is to direct standard error
  output to the	same file as standard output, as follows:

       command 2>&1


  If you want to redirect both standard	output and standard error output to
  the same file, enter:

       command > file 2>&1


  The order here is significant.  First, the shell associates file descriptor
  1 with file; then it associates file descriptor 2 with the file that is
  currently associated with file descriptor 1.	If you reverse the order of
  the redirections, standard error output goes to the display and standard
  output goes to file because at the time of the error output redirection,
  file descriptor 1 was	still associated with the display.

  You can also use this	mechanism to redirect standard input.  You could
  enter:

       digit1<&digit2

  where	digit1 refers to standard input	and digit2 refers to standard output,
  to cause both	file descriptors to be associated with the same	input file.
  Fo
  is 0 (standard input).  For commands that run	asynchronously (commands ter-
  minated by &), the default value of digit1 and digit2	is /dev/null.  Such
  input	redirection is useful for commands that	use two	or more	input
  sources.


  Summary of Redirection Options

  The following	can appear anywhere in a simple	command	or can precede or
  follow a command, but	they are not passed to the command:

  <file
      Use file as standard input.

  >file
      Use file as standard output.  Create the file if it does not exist;
      otherwise, truncate it to	0 (zero) length.

  >>file
      Use file as standard output.  Create the file if it does not exist;
      otherwise, append	the output to the end of the file.

  <<[-]eof_string
      Read as standard input all lines from eof_string up to a line contain-
      ing only eof_string or up	to an End-of-File character.  If any charac-
      ter in eof_string	is quoted, the shell does not expand or	interpret any
      characters in the	input lines; otherwise,	it performs variable and com-
      mand substitution	and ignores a quoted newline character (\newline).
      Use a \ (backslash) to quote characters within eof_string	or within the
      input lines.

      If you add a - (dash) to <<, then	all leading tabs are stripped from
      eof_string and from the input lines.

  < &digit
      Associate	standard input with file descriptor digit.

  > &digit
      Associate	standard output	with file descriptor digit.

  < &-
      Close standard input.

  > &-
      Close standard output.

  The restricted shell does not	allow the redirection of output.

  Interpretation of Spaces

  After	the shell performs variable and	command	substitution, it scans the
  results for internal field separators	(those defined in the IFS shell	vari-
  able,	see Variables Used by the Shell).  It splits the line into distinct
  words	at each	place it finds one of these characters.	 It retains explicit
  null arguments (""  '') and discards implicit	null arguments (those result-
  ing from parameters that have	no values).

  Built-In Commands


  :   Does nothing.  This null command returns a 0 (zero) exit value.

  . file
      Reads and	executes commands from file and	returns.  Does not spawn a
      subshell.	 The search path specified by PATH is used to find the direc-
      tory containing file.

  for, while, or until loop.
      If n is specified, resumes at the	nth enclosing loop.

  cd [directory]
      Changes the current directory to directory.  If no directory is speci-
      fied, the	value of the HOME shell	variable is used.  The CDPATH shell
      variable defines the search path for directory.  Alternative directory
      names appear in a	list, separated	from one another by a :	(colon).  A
      null pathname specifies the current directory, which is the default
      path.  This null pathname	can appear immediately after the = (equal
      sign) in the assignment or between the colon delimiters anywhere else
      in the path list.	 If directory begins with a / (slash), the shell does
      not use the search path.	Otherwise, the shell searches each directory
      in the path.  The	cd command cannot be executed by the restricted
      shell.

  echo [argument ...]
      Writes arguments to standard output.

  eval [argument ...]
      Reads arguments as input to the shell and	executes the resulting com-
      mands.  The eval command is most often used in command substitution.
      For example, the following command sets up the shell's TERM and TERMCAP
      variables	according to the type of tty the user is logged	in on:
	   eval	`tset -s vt100`


  exec [argument ...]
      Executes the command specified by	argument in place of this shell
      without creating a new process.  Input and output	arguments can appear
      and, if no other arguments appear, cause the shell input or output to
      be modified (not a good idea with	your login shell).  If this command
      is given from your login shell, you are logged out after the specified
      command has been executed.

  export [name ...]
      Marks the	specified names	for automatic export to	the environments of
      subsequently executed commands.  If you do not specify a name, the
      export command displays a	list of	all the	names that are exported	in
      this shell.  You cannot export function names.

  hash [-r] [name ...]
      For each name, finds and remembers the location in the search path of
      the command specified by name.  The -r flag causes the shell to forget
      all locations.  If you do	not specify the	flag or	any names, the shell
      displays information about the remembered	commands.  In this informa-
      tion, hits is the	number of times	a command has been run by the shell
      process.	The cost is a measure of the work required to locate a com-
      mand in the search path.	There are certain situations that require
      that the stored location of a command be recalculated (for example, the
      location of a relative pathname when the current directory changes).
      Commands for which that might be done are	indicated by an	* (asterisk)
      next to the hits information.  cost is incremented when the recalcula-
      tion is done.

  inlib	library_name
      This command is no longer	supported.  See	the loader(5) reference	page
      for information on using shared libraries.

  newgrp [-] [group]
      Changes the primary group	identification of the current shell process
      to group.	 If you	specify	-, newgrp changes the login environment	to
      the login	environment of the new group.  If you do not specify a group,
      newgrp changes the group identification to that specified	for the
      current user in the /etc/passwd file.  Thr newgrp	command	recognizes
      group names only;	it does	not recognize group ID numbers.

      Only a user with superuser authority can change the primary group	of
      the shell	to one to which	that user does not belong.

      Any active user-generated	shell is terminated when the newgrp command
      is used.

  pwd Displays the current directory.  See pwd for a discussion	of command
      options.

  read [name...]
      Reads one	line from standard input.  Assigns the first word in the line
      to the first name, the second word to the	second name, and so on,	with
      leftover words assigned to the last name.	 This command returns a	0
      (zero) unless it encounters an end of file.

  readonly [name...]
      Marks the	specified names	as read	only.  The values of these names can-
      not be reset.  If	you do not specify any names, the readonly command
      displays a list of all readonly names.

  rmlib	library_name
      This command is no longer	supported.  See	the loader(5) reference	page
      for information on using shared libraries.

  set [+ | -flag ...] [argument	...]
      Sets one or more of the following	flags:

      -a  Marks	for export all variables that are modified or changed.

      -e  Exits	immediately if a command exits with a nonzero exit value.

      -f  Disables filename substitution.

      -h  Locates and remembers	the commands called within functions as	the
	  functions are	defined.  (Normally these commands are located when
	  the function is executed; see	the built-in hash command.)

      -k  Places all keyword parameters	in the environment for a command, not
	  just those that precede the command name.

      -n  Reads	commands, but does not execute them.

      -t  Exits	after reading and executing one	command.

      -u  Treats an unset variable as an error when performing variable	sub-
	  stitution.

      -v  Displays shell input lines as	they are read.

      -x  Displays commands and	their arguments	as they	are executed.

      --  Does not change any of the flags.  This is useful in setting the $1
	  positional parameter to a string beginning with a - (dash).

      Using a +	(plus sign) rather than	a - (dash) unsets flags.  You can
      also specify these flags on the shell command line.  The $- special
      variable contains	the current set	of flags.

      Any arguments to set are positional parameters and are assigned, in
      order, to	$1, $2,	and so on.  If you do not specify flags	or arguments,
      set displays all names.

  $1 and
      assigning	the value of $2	to $1, of $3 to	$2, and	so on.	If there are
      more than	nine command line arguments, the tenth is assigned to $9 and
      any that remain are still	unassigned (until after	another	shift).	 If
      there are	nine or	fewer arguments, a shift unsets	the highest-numbered
      positional parameter.

      $0 is never shifted.  The	command	shift n	is a shorthand notation	for n
    

  test expression | [ expression ]
      Evaluates	conditional expressions.  See test for a discussion of com-
      mand options.

  times
      Displays the accumulated user and	system times for processes run from
      the shell.

  command once when the trap	is
      set and once when	the trap is taken).  The trap commands are executed
      in order of signal number.  Any attempt to set a trap on a signal	that
      was ignored on entry to the current shell	is ineffective.

    
      current values.  If command is a null string, this signal	is ignored by
      the shell	and by the commands it invokes.	 If n is 0 (zero), the com-
      mand is executed on exit from the	shell.	If neither a command or	a
      signal (n) is specified, trap displays a list of commands	associated
      with each	signal number.

  type [name ...]
      For each name specified, indicates how the shell interprets it as	a
      command name.

  ulimit [-HS] [-a | -c	| -d | -f | -h | -m | -n | -s |	-t] [limit]
      Displays or adjusts allocated shell resources.  There are	two modes for
      displaying the shell resource settings, which can	either be displayed
      individually or as a group.  The default mode is to display resources
      set to the soft setting, or the lower bound, as a	group.	To display
      the hard,	or upper bound,	limits,	use the	-h flag	as the only argument
      for this group. To display an individual soft limit, use the flag	that
      corresponds to the required resource on the command line.	 To display
      an individual hard limit,	use the	-h flag	along with the resource	flag.

      The setting of shell resources depends on	the effective user ID of the
      current shell.  The hard level of	a resource can be set only if the
      effective	user ID	of the current shell is	root.  If a user other than
      the superuser attempts to	set a resource's hard level, an	error occurs.
      By default, the superuser	sets both the hard and soft limits of a	par-
      ticular resource.	 Therefore, the	superuser should be careful in using
      the -S, -H, or default flag usage	of limit settings.  The	standard user
      can only set the the soft	limit of a resource.  Furthermore, the stan-
      dard user	can only expand	the soft limit up to the current hard limit
      setting.	To set a resource limit, choose	the appropriate	flag and the
      new resource limit value.	 The new resource limit	value should be	an
      integer.	You can	set only one resource limit at a time.	If more	than
      one resource flag	is specified, the results are undefined.  By default,
      ulimit with only a new value on the command line sets the	file size of
      the shell.  Therefore, use of the	-f flag	is optional.  You can use the
      following	flags with ulimit:

      -a  Sets or displays the address space for the shell.

      -c  Sets or displays the amount of core segment size for the shell.

      -d  Sets or displays the amount of data segment size for the shell.

      -f  Sets or displays the file size for the shell.

      -h  Sets or displays the current hard resource setting.

      -H  Sets or displays the hard resource limit (superuser only).

      -m  Sets or displays the memory allocation for the shell.

      -n  Sets or displays the maximum number of open file descriptors for
	  the shell.

      -s  Sets or displays the stack segment size for the shell.

      -S  Sets or displays the soft resource limit.

      -t  Sets or displays the CPU time	maximum	for the	shell.	(For more
	  information on resource settings, see	getrlimit(2).)

      -v  Sets or displays the virtual memory size.  Note:  This option	is
	  supported only if RLIMIT_VMEM	has been defined in
	  /usr/include/sys/resource.h.

      -w  Sets or displays the swap area size.	Note:  This option is sup-
	  ported only if RLIMIT_SWAP has been defined in
	  /usr/include/sys/resource.h.

      If no option is given, -f	is assumed.

  umask	[nnn]
      Sets your	file-creation mask to the octal	value nnn (see the umask()
      system call).  If	you omit nnn, umask displays the current value of the
      mask.

  unset	[name ...]
      For each name, removes the corresponding variable, built-in command, or
      function.	 The PATH, PS1,	PS2, MAILCHECK,	and IFS	variables cannot be
      unset.

  [:charclass:]


  This format instructs	the system to match any	single character belonging to
  charclass; the defined classes correspond to ctype() subroutines as fol-
  lows:

       alnum
       alpha
       cntrl
       digit
       graph
       lower
       print
       punct
       space
       upper
       xdigit

  Your locale might define additional character	properties, such as the	fol-
  lowing:

       [:vowel:]

  The preceding	character class	could be TRUE for a, e,	i, o, u, or y.	You
  could	then use [:vowel] inside a set construction to match any vowel.
  Refer	to The LC_CTYPE	Category section of the	locale file format reference
  page for more	information.

  Running the Shell

  The sh command can be	run either as a	login shell or as a subshell under
  the login shell.  Only the login command can call sh as a login shell.  It
  does this by using a special form of the sh command name:  -sh.  When
  called with an initial - (dash), the shell first reads and runs commands
  found	in the system .profile file and	your $HOME/.profile, if	one exists.
  It then accepts commands as described	in the following discussion of flags.

  Once logged in and working under a login shell, you can call sh with the
  command name sh.  This command runs a	subshell, a second shell running as a
  child	of the login shell.

  Restricted Shell


  The restricted shell,	Rsh, is	used to	set up login names and execution
  environments whose capabilities are more controlled than those of the	stan-
  dard shell.  The actions of Rsh are identical	to those of sh,	except that
  the following	are not	allowed:

    ++  Changing	directory (with	the cd command).

    ++  Setting the value of PATH or SHELL.

    ++  Specifying pathnames or command names containing	/.

    ++  Redirecting output (with	> and >>).

  A restricted shell can be invoked in one of the following ways:  (1) Rsh is
  the filename part of the last	entry in the /etc/passwd file; (2) the
  environment variable SHELL exists and	Rsh is the filename part of its
  value; (3) the shell is invoked and Rsh is the filename part of argument 0
  (zero); (4) the shell	is invoked with	the -r flag.

  When a command to be run is determined to be a shell script, Rsh invokes sh
  to run the command.  Thus, it	is possible to provide the end user with
  shell	scripts	that have access to the	full power of the standard shell,
  while	imposing a limited menu	of commands; this scheme assumes that the end
  user does not	have write and execute permissions in the same directory.

  The preceding	restrictions are enforced after	the .profile file is inter-
  preted.  Therefore, the writer of the	.profile has complete control over
  user actions by performing set-up actions and	leaving	the user in an
  appropriate directory	(probably not the login	directory).  An	administrator
  can set up a directory of commands in	/usr/rbin that the Rsh command can
  invoke.

  When called with the name -rsh or -Rsh, Rsh reads the	user's .profile	from
  $HOME/.profile.  It acts as the standard sh while doing this,	except that
  an Interrupt causes an immediate exit	instead	of a return to command level.

  The system administrator should be aware that	use of Rsh does	not imply
  that the system is secure.  A	secure system implements a system-wide frame-
  work to protect the system against unauthorized activity.  Rsh is not
  designed to implement	this type of system security.

FLAGS

  The following	flags are interpreted by the shell only	when you call it.
  Note that unless you specify either the -c or	-s flag, the shell assumes
  that the next	argument is a command file (shell procedure).  It passes any-
  thing	else on	the command line to that command file (see Positional Parame-
  ters).

  -c command_string
      Runs commands read from command_string.  The shell does not read addi-
      tional commands from standard input when you specify this	flag.

  -i  Makes the	shell interactive, even	if input and output are	not from a
      terminal.	 In this case, the shell ignores the SIGTERM signal (so	that
      kill 0 does not stop an interactive shell) and traps a SIGINT (so	that
      you can interrupt	wait).	In all cases, the shell	ignores	the SIGQUIT
      signal.  (See the	sigaction() system call	and kill for more information
      about signals.)

  -r  Creates a	restricted shell (the same as running Rsh).

  -s  Reads commands from standard input.  Any remaining arguments specified
      are passed as positional parameters to the new shell.  Shell output is
      written to standard error, except	for the	output of built-in commands
      (see Built-In Commands).

  The remaining	flags and arguments are	discussed in the description of	the
  built-in set command (see Built-In Commands).

NOTES

   1.  If a command is executed, and a command with the	same name is
       installed in a directory	in the search path before the directory	where
       the original command was	found, the shell executes the original com-
       mand.  Use the hash command to correct this situation.

   2.  When the	shell encounters the >>	characters, it does not	open the file
       in append mode; instead,	the shell opens	the file for writing and
       seeks to	the end.

   3.  Failure (nonzero	exit status) of	a special command preceding a || sym-
       bol prevents the	list following || from executing.

   4.  XPG/4 and SVR4 compliance
       To make your shell environment XPG/4 compliant, you must	set the	value
       of the environment  to "xpg4", by typing:
	    BIN-SH=xpg4; export	BIN-SH
       When you	do that, the Bourne shell automatically	invokes	the XPG/4
       compliant shell,	which is currently the Korn shell.

       The syntax for the C shell is:
	    setenv BIN-SH xpg4
       If you do not set or unset BIN-SH, the normal Bourne shell runs.	To
       unset BIN-SH, type:
	    unset  BIN-SH

       The syntax for the C shell is:
	    unsetenv BIN-SH xpg4

       To make your shell environment SVR4 compliant, you must set the value
       of the environment  to "svr4", by typing:
	    BIN-SH=svr4; export	BIN-SH
       When you	do that, the Bourne shell automatically	invokes	the SVR4 com-
       pliant shell.

       The syntax for the C shell is:
	    setenv BIN-SH svr4

       The SVR4	version	of the Bourne shell must have been installed by	the
       system administrator or you get an error	message.
       If you do not set or unset BIN-SH, the normal Bourne shell runs.	To
       unset BIN-SH, type:
	    unset  BIN-SH

       The syntax for the C shell is:
	    unsetenv BIN-SH svr4

RETURN VALUES

  For information about	exit values, see the following sections: The Shell
  Commands, Predefined Special Variables, Built-In Commands, and FLAGS.

FILES

  $HOME/.profile
	     User profile.

  /etc/passwd
	     Contains user information.

RELATED	INFORMATION

  Commands:  acctcms(8), acctcom(8), cd(1), csh(1), echo(1),
  env(1)/printenv(1), ksh(1), login(1),	mail(1)/binmail(1), mailx(1)/Mail(1),
  pwd(1), test(1).

  Functions:  fcntl(2),	exec(2), fork(2), pipe(2), sigaction(2), stat(2),
  umask(2).

  Routines:  ulimit(3).

  Files: null(7).

  Miscellaneous:  loader(5).

-
Manual: sh(1)
-
 

Introduction
Table of Contents
Chapter 1
Chapter 2a
Chapter 2b
Chapter 3
Chapter 4a
Chapter 4b
Chapter 5a
Chapter 5b
Chapter 6a
Chapter 6b
Chapter 6c
Chapter 7a
Chapter 7b
Chapter 8
Chapter 9a
Chapter 9b
Manual: ar(1)
Manual: cb(1)
Manual: cc(1)
Manual: expr(1)
Manual: f77(1)
Manual: ftp(1)
Manual: ksh(1)
Manual: lint(1)
Manual: sh(1)
Manual: test(1)
Manual: time(1)


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