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Full Discussion: Bash scripting, question about a variable
Top Forums Shell Programming and Scripting Bash scripting, question about a variable Post 302414949 by cfajohnson on Wednesday 21st of April 2010 11:23:31 AM
Old 04-21-2010
Quote:
Originally Posted by bakunin
True. You can guarantee that "$1" will never contain a whitespace, then?

It doesn't matter whether $1 contains white space or not. That is not literal whitespace. $1 could be $' \t\n' or all spaces and it wouldn't matter.

Literal whitespace means an actual space on the command line, not contained in a variable.

(I should also include other characters special to the shell, such as a semi-colon.)
Quote:
Quoting variables out of habit, even in cases where it is not necessarily required, is just staying out of troubles, doing otherwise is asking for them.

Doing it out of habit, instead of out of knowledge, is not good practice.

Doing it where it is never necessary is not good practice.
Quote:
Of course there is a point and i named it: precaution. That in this specific case it would not be necessary i did say explicitly said in the very next sentence, explaining on a counterexample what i meant.

We might disagree philosophically about what constitutes robust programming practices: i believe that many aspects of any often-employed activity (like, for instance, programming for a programmer) is driven by habits as much as conscient effort. My experience is that when i act habitually in a safe way i am generally better off then when i habitually employ a potential hazard - i will probably forget to take the necessary precaution in the one case where it will matter.

To come back from the general to the problem at hand, variable expansion: all i "risk" are two keystrokes and 2 bytes on a disk - compared to the avoided potential problems in cases like the one i cited this is not too much, i suppose.

There is no potential problem in not quoting the assignment of one variable (or a concatenation of variables or strings not containing special characters) to a variable.

OK, I'll agree that quoting the assignment doesn't hurt, but it is not necessary, and certainly not more robust.
 

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LEARN ABOUT FREEBSD

ddb

DDB(8)							    BSD System Manager's Manual 						    DDB(8)

NAME

     ddb -- configure DDB kernel debugger properties

SYNOPSIS

     ddb capture [-M -core] [-N -system] print
     ddb capture [-M -core] [-N -system] status
     ddb script scriptname
     ddb script scriptname=script
     ddb scripts
     ddb unscript scriptname
     ddb pathname

DESCRIPTION

     The ddb utility configures certain aspects of the ddb(4) kernel debugger from user space that are not configured at compile-time or easily
     via sysctl(8) MIB entries.

     To ease configuration, commands can be put in a file which is processed using ddb as shown in the last synopsis line.  An absolute pathname
     must be used.  The file will be read line by line and applied as arguments to the ddb utility.  Whitespace at the beginning of lines will be
     ignored as will lines where the first non-whitespace character is '#'.

OUTPUT CAPTURE

     The ddb utility can be used to extract the contents of the ddb(4) output capture buffer of the current live kernel, or from the crash dump of
     a kernel on disk.	The following debugger commands are available from the command line:

     capture [-M core] [-N system] print
	     Print the current contents of the ddb(4) output capture buffer.

     capture [-M core] [-N system] status
	     Print the current status of the ddb(4) output capture buffer.

SCRIPTING

     The ddb utility can be used to configure aspects of ddb(4) scripting from user space; scripting support is described in more detail in
     ddb(4).  Each of the debugger commands is available from the command line:

     script scriptname
	     Print the script named scriptname.

     script scriptname=script
	     Define a script named scriptname.	As many scripts contain characters interpreted in special ways by the shell, it is advisable to
	     enclose script in quotes.

     scripts
	     List currently defined scripts.

     unscript scriptname
	     Delete the script named scriptname.

EXIT STATUS

     The ddb utility exits 0 on success, and >0 if an error occurs.

EXAMPLES

     The following example defines a script that will execute when the kernel debugger is entered as a result of a break signal:

	   ddb script kdb.enter.break="show pcpu; bt"

     The following example will delete the script:

	   ddb unscript kdb.enter.break

     For further examples, see the ddb(4) and textdump(4) manual pages.

SEE ALSO

     ddb(4), textdump(4), sysctl(8)

HISTORY

     The ddb utility first appeared in FreeBSD 7.1.

AUTHORS

     Robert N M Watson

BUGS

     Ideally, ddb would not exist, as all pertinent aspects of ddb(4) could be configured directly via sysctl(8).

BSD
								 December 24, 2008							       BSD
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