If statements in Linux terminal Post: 302861487

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Homework and Emergencies Homework & Coursework Questions If statements in Linux terminal Post 302861487 by sea on Wednesday 9th of October 2013 04:59:52 AM

10-09-2013

Registered User

Right, personal experience showed me that the use of [[ condition ]] is the most compatible one among shells.
But then again i only use sh and bash on diffrent linux'...

man bash says:

Code:

       ((expression))
              The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION.  If the value of the expression 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 EXPRES-
              SIONS.  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.  Conditional operators such as -f must be unquoted to be recognized as primaries.

              When used with [[, the < and > operators sort lexicographically using the current locale.

              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.
              If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters.  The return value is 0 if the string matches  (==)  or  does
              not match (!=) the pattern, and 1 otherwise.  Any part of the pattern may be quoted to force it to be matched as a string.

              An  additional binary operator, =~, is available, with the same precedence as == and !=.  When it is used, the string to the right of the operator is considered an extended regular
              expression and matched accordingly (as in regex(3)).  The return value is 0 if the string matches the pattern, and 1 otherwise.  If the regular expression is  syntactically  incor-
              rect,  the conditional expression's return value is 2.  If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters.  Any
              part of the pattern may be quoted to force it to be matched as a string.  Substrings matched by parenthesized subexpressions within the regular expression are saved  in  the  array
              variable  BASH_REMATCH.   The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expression.  The element of BASH_REMATCH with index n is
              the portion of the string matching the nth parenthesized subexpression.

              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.

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LEARN ABOUT BSD

test

TEST(1) 						      General Commands Manual							   TEST(1)

NAME

       test - condition evaluation utility

SYNOPSIS

       test expression

DESCRIPTION

       The  test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; otherwise it returns 1 (false).
       If there is no expression, test also returns 1 (false).

       All operators and flags are separate arguments to the test utility.

       The following primaries are used to construct expression:

	-b   file
	      True if file exists and is a block special file.

	-c   file
	      True if file exists and is a character special file.

	-d   file
	      True if file exists and is a directory.

	-e   file
	      True if file exists (regardless of type).

	-f   file
	      True if file exists and is a regular file.

	-g   file
	      True if file exists and its set group ID flag is set.

	-h   file
	      True if file exists and is a symbolic link.

	-n   string
	      True if the length of string is nonzero.

	-p   file
	      True if file is a named pipe

	-r   file
	      True if file exists and is readable.

	-s   file
	      True if file exists and has a size greater than zero.

	-t   [file_descriptor]
	      True if the file whose file descriptor number is file_descriptor (default 1) is open and is associated with a terminal.

	-u   file
	      True if file exists and its set user ID flag is set.

	-w   file
	      True if file exists and is writable.  True indicates only that the write flag is on.  The file is not writable on a  read-only  file
	      system even if this test indicates true.

	-x   file
	      True  if	file  exists  and is executable.  True indicates only that the execute flag is on.  If file is a directory, true indicates
	      that file can be searched.

	-z   string
	      True if the length of string is zero.

	string
	      True if string is not the null string.

	s1   =	 s2
	      True if the strings s1 and s2 are identical.

	s1   !=   s2
	      True if the strings s1 and s2 are not identical.

	n1   -eq   n2
	      True if the integers n1 and n2 are algebraically equal.

	n1   -ne   n2
	      True if the integers n1 and n2 are not algebraically equal.

	n1   -gt   n2
	      True if the integer n1 is algebraically greater than the integer n2 .

	n1   -ge   n2
	      True if the integer n1 is algebraically greater than or equal to the integer n2 .

	n1   -lt   n2
	      True if the integer n1 is algebraically less than the integer n2 .

	n1   -le   n2
	      True if the integer n1 is algebraically less than or equal to the integer n2 .

	      These primaries can be combined with the following operators:

       ! expression
	      True if expression is false.

	expression1   -a   expression2
	      True if both expression1 and expression2 are true.

	expression1   -o   expression2
	      True if either expression1 or expression2 are true.

	(expression)
	      True if expression is true.

	      The
	      -a operator has higher precedence than the -o operator.

GRAMMAR AMBIGUITY

       The test grammar is inherently ambiguous.  In order to assure a degree  of  consistency,  the  cases  described	in  the  IEEE  Std  1003.2
       ("POSIX"), section D11.2/4.62.4, standard are evaluated consistently according to the rules specified in the standards document.  All other
       cases are subject to the ambiguity in the command semantics.

RETURN VALUES

       The test utility exits with one of the following values:

       0      expression evaluated to true.

       1      expression evaluated to false or expression was missing.

       >1     An error occurred.

BUGS

       Named pipes are not implemented in 2.11BSD.

STANDARDS

       The test function is expected to be IEEE Std 1003.2 ("POSIX") compatible.

7th Edition							  March 13, 1995							   TEST(1)