Why the results of these two code fragments are not the same? Post: 303030207

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Top Forums Shell Programming and Scripting Why the results of these two code fragments are not the same? Post 303030207 by Don Cragun on Thursday 7th of February 2019 05:39:18 AM

02-07-2019

Registered User

I generally avoid eval, but have you considered making your shell script more like your C++ code?

Code:

#!/bin/sh
#set -xv
i=1
while [ $i -le $# ]
do	j=$((i + 1))
	while [ $j -le $# ]
	do	if eval [ \"\${$i}\" = \"\${$j}\" ]
		then	printf '\n%s (%d,%d)\n\nExiting...\n\n' \
			    'Error: Two or more arguments are the same:' \
			    $i $j >&2
			exit 1
		fi
		j=$((j + 1))
	done
	i=$((i + 1))
done

The while loops in the above script could be simplified to for loops that are accepted by many shells (including recent versions of bash and ksh) but they are extensions to the standards and, therefore, not available in several shells. The above code just uses standard utility interfaces and shell language constructs. For shells that accept these extensions, the below code looks even more like your C++ code.

Code:

#!/bin/sh
set -xv
for ((i = 1; i <= $#; i++))
do	for ((j = i + 1; j <= $#; j++))
	do	if eval [ \"\${$i}\" = \"\${$j}\" ]
		then	printf '\n%s (%d,%d)\n\nExiting...\n\n' \
			    "Error: Two or more arguments are the same:" \
			    $i $j >&2
			exit 1
		fi
	done
done

Note that both of these script write their error messages to the standard error output instead of to standard output and both of these script print the argument numbers of the first found pair of identical argument in their diagnostics.

Don Cragun

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getusershell

getusershell(3C)					   Standard C Library Functions 					  getusershell(3C)

NAME

       getusershell, setusershell, endusershell - get legal user shells

SYNOPSIS

       #include <unistd.h>

       char *getusershell(void);

       void setusershell(void);

       void endusershell(void);

DESCRIPTION

       The  getusershell()  function  returns  a  pointer  to  a  legal  user  shell  as defined by the system manager in the file /etc/shells. If
       /etc/shells does not exist, the following locations of the standard system shells are used in its place:

       /bin/bash		  /bin/csh
       /bin/jsh 		  /bin/ksh
       /bin/pfcsh		  /bin/pfksh
       /bin/pfsh		  /bin/sh
       /bin/tcsh		  /bin/zsh
       /sbin/jsh		  /sbin/pfsh
       /sbin/sh 		  /usr/bin/bash
       /usr/bin/csh		  /usr/bin/jsh
       /usr/bin/ksh		  /usr/bin/pfcsh
       /usr/bin/pfksh		  /usr/bin/pfsh
       /usr/bin/sh		  /usr/bin/tcsh
       /usr/bin/zsh		  /usr/xpg4/bin/sh

       The getusershell() function opens the file /etc/shells, if it exists, and returns the next entry in the list of shells.

       The setusershell() function rewinds the file or the list.

       The endusershell() function closes the file, frees any memory used by getusershell() and setusershell(), and rewinds the file /etc/shells.

RETURN VALUES

       The getusershell() function returns a null pointer on EOF.

BUGS

       All information is contained in memory that may be freed with a call to endusershell(), so it must be copied if it is to be saved.

SunOS 5.10							    30 Aug 2004 						  getusershell(3C)