bash.hlp

bash帮助手册

       tions specified by the command (see REDIRECTION below).

       If the reserved word !  precedes a pipeline, the exit  status  of  that
       pipeline	 is  the  logical  NOT of the exit status of the last command.
       Otherwise, the status of the pipeline is the exit status	 of  the  last
       command.	 The shell waits for all commands in the pipeline to terminate
       before returning a value.

       If the time reserved word precedes a pipeline, the elapsed as  well  as
       user  and  system  time consumed by its execution are reported when the
       pipeline terminates.  The -p option changes the output format  to  that
       specified  by  POSIX.   The  TIMEFORMAT variable may be set to a format
       string that specifies how the timing information should	be  displayed;
       see the description of TIMEFORMAT under Shell Variables below.

       Each  command in a pipeline is executed as a separate process (i.e., in
       a subshell).

   Lists
       A list is a sequence of one or more pipelines separated by one  of  the
       operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or
       <newline>.

       Of these list operators, && and || have equal precedence, followed by ;
       and &, which have equal precedence.

       A  sequence  of	one or more newlines may appear in a list instead of a
       semicolon to delimit commands.

       If a command is terminated by the control operator &,  the  shell  exe-
       cutes  the command in the background in a subshell.  The shell does not
       wait for the command to finish, and the return status is	 0.   Commands
       separated  by  a	 ; are executed sequentially; the shell waits for each
       command to terminate in turn.  The return status is the exit status  of
       the last command executed.

       The  control operators && and || denote AND lists and OR lists, respec-
       tively.	An AND list has the form

	      command1 && command2

       command2 is executed if, and only if, command1 returns an  exit	status
       of zero.

       An OR list has the form

	      command1 || command2


       command2	 is  executed  if and only if command1 returns a non-zero exit
       status.	The return status of AND and OR lists is the  exit  status  of
       the last command executed in the list.

   Compound Commands
       A compound command is one of the following:

       (list) list  is	executed  in  a	 subshell.   Variable  assignments and
	      builtin commands that affect  the	 shell's  environment  do  not
	      remain in effect after the command completes.  The return status
	      is the exit status of list.

       { list; }
	      list is simply executed in the current shell environment.	  list
	      must  be	terminated with a newline or semicolon.	 This is known
	      as a group command.  The return status is	 the  exit  status  of
	      list.   Note  that unlike the metacharacters ( and , { and } are
	      reserved words and must occur where a reserved word is permitted
	      to  be  recognized.   Since they do not cause a word break, they
	      must be separated from list by whitespace.

       ((expression))
	      The expression is evaluated according  to	 the  rules  described
	      below  under ARITHMETIC EVALUATION.  If the value of the expres-
	      sion is non-zero, the return status is 0; otherwise  the	return
	      status is 1.  This is exactly equivalent to let "expression".

       [[ expression ]]
	      Return  a	 status	 of  0 or 1 depending on the evaluation of the
	      conditional expression expression.  Expressions are composed  of
	      the  primaries  described	 below	under CONDITIONAL EXPRESSIONS.
	      Word splitting and pathname expansion are not performed  on  the
	      words  between  the  [[  and  ]]; tilde expansion, parameter and
	      variable expansion, arithmetic expansion, command	 substitution,
	      process substitution, and quote removal are performed.

	      When  the	 == and != operators are used, the string to the right
	      of the operator is considered a pattern and matched according to
	      the  rules  described  below under Pattern Matching.  The return
	      value is 0 if the string matches or does not match the  pattern,
	      respectively,  and  1 otherwise.	Any part of the pattern may be
	      quoted to force it to be matched as a string.

	      Expressions may  be  combined  using  the	 following  operators,
	      listed in decreasing order of precedence:

	      ( expression )
		     Returns  the  value  of  expression.  This may be used to
		     override the normal precedence of operators.
	      ! expression
		     True if expression is false.
	      expression1 && expression2
		     True if both expression1 and expression2 are true.
	      expression1 || expression2
		     True if either expression1 or expression2 is true.

	      The && and || operators do not evaluate expression2 if the value
	      of  expression1  is  sufficient to determine the return value of
	      the entire conditional expression.

       for name [ in word ] ; do list ; done
	      The list of words following in is expanded, generating a list of
	      items.  The variable name is set to each element of this list in
	      turn, and list is executed each time.  If the in word  is	 omit-
	      ted,  the	 for  command  executes	 list once for each positional
	      parameter that is set (see PARAMETERS below).  The return status
	      is  the  exit  status of the last command that executes.	If the
	      expansion of the items following in results in an empty list, no
	      commands are executed, and the return status is 0.

       for (( expr1 ; expr2 ; expr3 )) ; do list ; done
	      First, the arithmetic expression expr1 is evaluated according to
	      the rules described  below  under	 ARITHMETIC  EVALUATION.   The
	      arithmetic  expression  expr2 is then evaluated repeatedly until
	      it evaluates to zero.  Each time expr2 evaluates to  a  non-zero
	      value,  list  is executed and the arithmetic expression expr3 is
	      evaluated.  If any expression is omitted, it behaves  as	if  it
	      evaluates to 1.  The return value is the exit status of the last
	      command in list that is executed, or false if any of the expres-
	      sions is invalid.

       select name [ in word ] ; do list ; done
	      The list of words following in is expanded, generating a list of
	      items.  The set of expanded words is  printed  on	 the  standard
	      error,  each  preceded  by a number.  If the in word is omitted,
	      the positional parameters are printed  (see  PARAMETERS  below).
	      The  PS3 prompt is then displayed and a line read from the stan-
	      dard input.  If the line consists of a number  corresponding  to
	      one  of  the  displayed  words, then the value of name is set to
	      that word.  If the line is empty, the words and prompt are  dis-
	      played again.  If EOF is read, the command completes.  Any other
	      value read causes name to be set to  null.   The	line  read  is
	      saved  in	 the  variable REPLY.  The list is executed after each
	      selection until a break command is executed.  The exit status of
	      select  is the exit status of the last command executed in list,
	      or zero if no commands were executed.

       case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
	      A case command first expands word, and tries to match it against
	      each pattern in turn, using the same matching rules as for path-
	      name expansion (see Pathname Expansion below).  When a match  is
	      found,  the  corresponding  list	is  executed.  After the first
	      match, no subsequent matches are attempted.  The exit status  is
	      zero if no pattern matches.  Otherwise, it is the exit status of
	      the last command executed in list.

       if list; then list; [ elif list; then list; ] ... [ else list; ] fi
	      The if list is executed.	If its exit status is zero,  the  then
	      list  is	executed.   Otherwise,	each  elif list is executed in
	      turn, and if its exit status is  zero,  the  corresponding  then
	      list is executed and the command completes.  Otherwise, the else
	      list is executed, if present.  The exit status is the exit  sta-
	      tus of the last command executed, or zero if no condition tested
	      true.

       while list; do list; done
       until list; do list; done
	      The while command continuously executes the do list as  long  as
	      the  last	 command  in list returns an exit status of zero.  The
	      until command is identical to the while command, except that the
	      test  is	negated;  the  do list is executed as long as the last
	      command in list returns a non-zero exit status.  The exit status
	      of  the  while and until commands is the exit status of the last
	      do list command executed, or zero if none was executed.

       [ function ] name () { list; }
	      This defines a function named name.  The body of the function is
	      the  list	 of  commands  between { and }.	 This list is executed
	      whenever name is specified as the name of a simple command.  The
	      exit status of a function is the exit status of the last command
	      executed in the body.  (See FUNCTIONS below.)

COMMENTS
       In a non-interactive shell, or an interactive shell in which the inter-
       active_comments	option	to  the	 shopt	builtin	 is enabled (see SHELL
       BUILTIN COMMANDS below), a word beginning with # causes that  word  and
       all  remaining  characters  on that line to be ignored.	An interactive
       shell without the interactive_comments option enabled  does  not	 allow
       comments.  The interactive_comments option is on by default in interac-
       tive shells.

QUOTING
       Quoting is used to remove the special meaning of certain characters  or
       words  to  the shell.  Quoting can be used to disable special treatment
       for special characters, to prevent reserved words from being recognized
       as such, and to prevent parameter expansion.

       Each  of	 the metacharacters listed above under DEFINITIONS has special
       meaning to the shell and must be quoted if it is to represent itself.

       When the command history expansion facilities are being used, the  his-
       tory  expansion character, usually !, must be quoted to prevent history
       expansion.

       There are  three	 quoting  mechanisms:  the  escape  character,	single
       quotes, and double quotes.

       A  non-quoted  backslash (\) is the escape character.  It preserves the
       literal value of the next character that follows, with the exception of
       <newline>.   If	a  \<newline>  pair  appears, and the backslash is not
       itself quoted, the \<newline> is treated as a line  continuation	 (that
       is, it is removed from the input stream and effectively ignored).

       Enclosing  characters  in  single quotes preserves the literal value of
       each character within the quotes.  A single quote may not occur between
       single quotes, even when preceded by a backslash.

       Enclosing  characters  in  double quotes preserves the literal value of
       all characters within the quotes, with the exception of $,  `,  and  \.
       The  characters	$  and	`  retain  their special meaning within double
       quotes.	The backslash retains its special meaning only	when  followed
       by one of the following characters: $, `, ", \, or <newline>.  A double
       quote may be quoted within double quotes by preceding it with  a	 back-
       slash.

       The  special  parameters	 *  and	 @ have special meaning when in double
       quotes (see PARAMETERS below).

       Words of the form $'string' are treated specially.  The word expands to
       string,	with  backslash-escaped characters replaced as specifed by the
       ANSI C standard.	 Backslash escape sequences, if present,  are  decoded
       as follows:
	      \a     alert (bell)
	      \b     backspace
	      \e     an escape character
	      \f     form feed
	      \n     new line
	      \r     carriage return
	      \t     horizontal tab
	      \v     vertical tab
	      \\     backslash
	      \'     single quote
	      \nnn   the  eight-bit  character	whose value is the octal value
		     nnn (one to three digits)
	      \xHH   the eight-bit character whose value  is  the  hexadecimal
		     value HH (one or two hex digits)