A couple of BASH scripting questions

[oshunluvr]

These should be easy;

#1) What is the function of -f in a script? Example: [ -f $AVAILABLE ] || exit 0

#2) Does the case statement below pick the first answer in the list and then exit?

	case $governors in
		*interactive*)
			GOVERNOR="interactive"
			break
			;;
		*ondemand*)
			GOVERNOR="ondemand"
			case $(uname -m) in
				ppc64*)
					SAMPLING=100
				;;
			esac
			break
			;;
		*powersave*)
			GOVERNOR="powersave"
			break
			;;
		*)
			exit 0
			;;
	esac

This is a snippet from /etc/init.d/ondemand. i believe what this does it cycle through the list of available governors until it finds one then uses it. On my system, "interactive" isn't available so "ondemand" get used. "powersave" is also available but the above statement uses "ondemand" because it's ahead of "powersave". If I rearranged the order of the stanzas (putting powersave above ondemand) then it would set powersave rather than ondemand. Is this right?

[andystmartin]

These should be easy;

#1) What is the function of -f in a script? Example: [ -f $AVAILABLE ] || exit 0

#2) Does the case statement below pick the first answer in the list and then exit?

	case $governors in
		*interactive*)
			GOVERNOR="interactive"
			break
			;;
		*ondemand*)
			GOVERNOR="ondemand"
			case $(uname -m) in
				ppc64*)
					SAMPLING=100
				;;
			esac
			break
			;;
		*powersave*)
			GOVERNOR="powersave"
			break
			;;
		*)
			exit 0
			;;
	esac

This is a snippet from /etc/init.d/ondemand. i believe what this does it cycle through the list of available governors until it finds one then uses it. On my system, "interactive" isn't available so "ondemand" get used. "powersave" is also available but the above statement uses "ondemand" because it's ahead of "powersave". If I rearranged the order of the stanzas (putting powersave above ondemand) then it would set powersave rather than ondemand. Is this right?

[ -f "$FILE" ] || exit 0

tests for the existence of a file contained in the variable. If it does not exist, the script exits. "[" is a synonym for /usr/bin/test. If you look in /usr/bin there is an executable named "[" in that directory (older versions of UNIX used to put it in /bin). "man test" will describe all the options.

The bourne shell's case statement checks for a match between what is contained in $governors and the regular expressions preceded by ")". It goes from top to bottom. If there is a match, it executes the block of code up to the double-semicolon (;;), then exits the case statement. *) matches anything. If none of the previous regular expressions were matched, that block is executed.

[elijathegold]

1: -f in a test returns true if the file exists and is a regular file. In your example, if the file name held in the AVAILABLE variable and it isn't a directory, block device etc. exists then true else false.

2: Yes, from the posted snippet, that would appear to do what you want. It is worth backing up the file before changing it though.

[oshunluvr]

Great, thanks guys. I had a hard time searching for "-f" - it's too short to google

[Snowhog]

Great, thanks guys. I had a hard time searching for "-f" - it's too short to google

Maybe, but searching on how is "-f" used in a bash script found this: Bash Guide for Beginners, Chapter 7. Conditional Statements, 7.1. Introduction to if

[sithlord48]

just remember -f will only work with files. use -d to do the same with a directory.

[quaksoul1]

Great, thanks guys. I had a hard time searching for "-f" - it's too short to google

Normally case would not "cycle". But governors contains longer string:

conservative ondemand userspace powersave performance

So indeed wildcard will match any of the patterns in that string, in order. Easy way apparently to give preference, although traditionally, case wouldn't be used for that. In procedural or object-oriented language, you might need to do list lookup for that, and need more structures.

[jlittle]

. Easy way apparently to give preference, although traditionally, case wouldn't be used for that.

In "traditional" shell scripts case statements would be used for their relatively predictable pattern matching; doing it in an if statement was limited and treacherous, partly owing to quoting issues. I think the inner case statement is an example.
I'll spare you my rant about scripts clinging to 30 year old ways.

Regards, John Little