Bash disaster prevention - Revision history

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2009-10-02T19:07:35Z

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← Older revision		Revision as of 19:07, 2 October 2009
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	==A horror story==		==A horror story==
	You listen to music and have built up a quite extensive library of digital music in your computer under the directory /home/alex/music. Everyday, you copy a folder from under music folder (e.g. [[wikipedia:beatles\|beatles]]/[[wikipedia:Yellow_Submarine_%28album%29\|yellowsubmarine]]) to your [[wikipedia:MP3 player\|MP3 player]], which appears as a usb drive (say /mnt/usbstick). You write a small script called loadmusic.bash and save it in /home/alex/,<nowiki>		You listen to music and have built up a quite extensive library of digital music in your computer under the directory /home/alex/music. Everyday, you copy a folder from under music folder (e.g. [[wikipedia:beatles\|beatles]]/[[wikipedia:Yellow_Submarine_%28album%29\|yellowsubmarine]]) to your [[wikipedia:MP3 player\|MP3 player]], which appears as a usb drive (say /mnt/usbstick). You write a small script called loadmusic.bash and save it in /home/alex/,<nowiki>
	</nowiki><pre><nowiki>		</nowiki>
			<pre><nowiki>
	#!/bin/bash		#!/bin/bash
	cd /mnt/usbstick/todaysmusic # Go to the appropriate folder		cd /mnt/usbstick/todaysmusic # Go to the appropriate folder
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	and call it as		and call it as
	<nowiki>		<nowiki>
	</nowiki><pre><nowiki>		</nowiki>
			<pre><nowiki>
	$ loadmusic.bash yellowsubmarine		$ loadmusic.bash yellowsubmarine
	</nowiki></pre>		</nowiki></pre>
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	On one day, you simply forget to insert the usb key, before running the command:		On one day, you simply forget to insert the usb key, before running the command:
	<nowiki>		<nowiki>
	</nowiki><pre><nowiki>		</nowiki>
			<pre><nowiki>
	loadmusic.bash : line 2: cd: /mnt/usbstick/todaysmusic: No such file or directory		loadmusic.bash : line 2: cd: /mnt/usbstick/todaysmusic: No such file or directory
	Done!		Done!
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	Whenever there is the potential for pitfalls like the above, use some form of general error handler to at least tell the script to exit without trying the rest of the commands.		Whenever there is the potential for pitfalls like the above, use some form of general error handler to at least tell the script to exit without trying the rest of the commands.
	<nowiki>		<nowiki>
	</nowiki><pre><nowiki>		</nowiki>
			<pre><nowiki>
	#!/bin/bash		#!/bin/bash
	## End of error handling		## End of error handling
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	, so there's no danger of bash trying to be smart.		, so there's no danger of bash trying to be smart.
	<nowiki>		<nowiki>
	</nowiki><pre><nowiki>		</nowiki>
			<pre><nowiki>
	loadmusic.bash: line 7: cd: /mnt/usbstick/todaysmusic: No such file or directory		loadmusic.bash: line 7: cd: /mnt/usbstick/todaysmusic: No such file or directory
	Error		Error
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	Look at the following code snippet: <nowiki>		Look at the following code snippet: <nowiki>
	</nowiki><pre><nowiki>#!/bin/bash		</nowiki>
			<pre><nowiki>#!/bin/bash
	date=`date +%Y%m%d`		date=`date +%Y%m%d`
	file="cpc_rfe_v2.0_sa_dly.bin.${date}.gz"		file="cpc_rfe_v2.0_sa_dly.bin.${date}.gz"

Root at 13:48, 2 June 2006

2006-06-02T13:48:04Z

← Older revision		Revision as of 13:48, 2 June 2006
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	Like any other versatile tool, consequences of misusing of UNIX scripting can be disasterous. ~~It often helps~~ to ~~save the day~~		Like any other versatile tool, consequences of misusing of UNIX scripting can be disasterous. Careful consideration of possible pitfalls in scripting is a very much necessary art. However, it is equally important to have some 'safety-value' in case something goes terribly wrong.

	==A horror story==		==A horror story==
	You listen to music and have built up a quite extensive library of digital music in your computer under the directory /home/alex/music. Everyday, you copy a folder from under music folder (e.g. [[wikipedia:beatles\|beatles]]/[[wikipedia:Yellow_Submarine_%28album%29\|yellowsubmarine]]) to your [[wikipedia:MP3 player\|MP3 player]], which appears as a usb drive (say /mnt/usbstick). You write a small script called loadmusic.bash and save it in /home/alex/,<nowiki>		You listen to music and have built up a quite extensive library of digital music in your computer under the directory /home/alex/music. Everyday, you copy a folder from under music folder (e.g. [[wikipedia:beatles\|beatles]]/[[wikipedia:Yellow_Submarine_%28album%29\|yellowsubmarine]]) to your [[wikipedia:MP3 player\|MP3 player]], which appears as a usb drive (say /mnt/usbstick). You write a small script called loadmusic.bash and save it in /home/alex/,<nowiki>

Root: /* More useful error handling */

2006-06-02T13:42:01Z

More useful error handling

← Older revision		Revision as of 13:42, 2 June 2006
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	echo "Now processing $file"		echo "Now processing $file"
	./process.bash $file		./process.bash $file
	~~PRE FORMATTED~~
	</nowiki></pre>		</nowiki></pre>

Root: /* More useful error handling */

2006-06-02T13:41:42Z

More useful error handling

← Older revision		Revision as of 13:41, 2 June 2006
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	{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}		{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}

	Every well behaved Unix command has a exit status expressed by an integer value between 0 and 255. 0 indicates success. Other codes are less universal. In many systems, 127 indicates "Command not found" error. 126 - "permission denied" and 130 - "Process ~~inturrupted~~" (e.g. Ctrl+C).		Every well behaved Unix command has a exit status expressed by an integer value between 0 and 255. 0 indicates success. Other codes are less universal. In many systems, 127 indicates "Command not found" error. 126 - "permission denied" and 130 - "Process interrupted" (e.g. Ctrl+C).

	Look at the following code snippet: <nowiki>		Look at the following code snippet: <nowiki>
	</nowiki>#!/bin/bash		</nowiki><pre><nowiki>#!/bin/bash
	date=`date +%Y%m%d`		date=`date +%Y%m%d`
	file="cpc_rfe_v2.0_sa_dly.bin.${date}.gz"		file="cpc_rfe_v2.0_sa_dly.bin.${date}.gz"
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	wget $site/$file -nv		wget $site/$file -nv
	if [ $? -ne 0 ]; then		if [ $? -ne 0 ]; then
	echo "Some error in retrieving $file."		echo "Some error in retrieving $file."
	exit 1		exit 1
	fi		fi
	echo "Now processing $file"		echo "Now processing $file"
	./process.bash $file		./process.bash $file
	~~<pre><nowiki>~~
	PRE FORMATTED		PRE FORMATTED
	</nowiki></pre>		</nowiki></pre>

	It tries to download a file from a ftp site. Then the code check the exit status of wget, which should be zero only if everything went all right. Otherwise, it will be non-zero and the script print a meaningful error message and exit.		It tries to download a file from a ftp site. Then the code check the exit status of wget, which should be zero only if everything went all right. Otherwise, it will be non-zero and the script print a meaningful error message and exit.

	[[Category:Computing]]		[[Category:Computing]]

Root: /* More useful error handling */

2006-06-02T13:41:12Z

More useful error handling

← Older revision		Revision as of 13:41, 2 June 2006
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	==More useful error handling==		==More useful error handling==
	{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}		{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}

			Every well behaved Unix command has a exit status expressed by an integer value between 0 and 255. 0 indicates success. Other codes are less universal. In many systems, 127 indicates "Command not found" error. 126 - "permission denied" and 130 - "Process inturrupted" (e.g. Ctrl+C).

			Look at the following code snippet: <nowiki>
			</nowiki>#!/bin/bash
			date=`date +%Y%m%d`
			file="cpc_rfe_v2.0_sa_dly.bin.${date}.gz"
			site="ftp://ftp.cpc.ncep.noaa.gov/fews/S.Asia/data"
			wget $site/$file -nv
			if [ $? -ne 0 ]; then
			echo "Some error in retrieving $file."
			exit 1
			fi
			echo "Now processing $file"
			./process.bash $file
			<pre><nowiki>
			PRE FORMATTED
			</nowiki></pre>

			It tries to download a file from a ftp site. Then the code check the exit status of wget, which should be zero only if everything went all right. Otherwise, it will be non-zero and the script print a meaningful error message and exit.

			[[Category:Computing]]

Root: /* Error handling */

2006-05-12T01:18:22Z

Error handling

← Older revision		Revision as of 01:18, 12 May 2006
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	<nowiki>		<nowiki>
	</nowiki><pre><nowiki>		</nowiki><pre><nowiki>
	: line 7: cd: /mnt/usbstick/todaysmusic: No such file or directory		loadmusic.bash: line 7: cd: /mnt/usbstick/todaysmusic: No such file or directory
	Error		Error
	</nowiki></pre>		</nowiki></pre>

Root: /* Error handling */

2006-05-12T01:17:54Z

Error handling

← Older revision		Revision as of 01:17, 12 May 2006
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	<tt>exit</tt>		<tt>exit</tt>
	, so there's no danger of bash trying to be smart.		, so there's no danger of bash trying to be smart.
			<nowiki>
			</nowiki><pre><nowiki>
			: line 7: cd: /mnt/usbstick/todaysmusic: No such file or directory
			Error
			</nowiki></pre>

	==More useful error handling==		==More useful error handling==
	{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}		{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}

Root: /* More useful error handling */

2006-05-12T01:13:45Z

More useful error handling

← Older revision		Revision as of 01:13, 12 May 2006
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	==More useful error handling==		==More useful error handling==
	{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:~~return value~~\|return value]] of each individual command can be checked. }}		{{hbox\|trap type of error handling is useful for preventing disasters. For more graceful error handling, the [[wikipedia:Return_statement\|return value]] of each individual command can be checked. }}

Root: /* Error handling */

2006-05-12T01:12:05Z

Error handling

← Older revision		Revision as of 01:12, 12 May 2006
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	==Error handling==		==Error handling==
	{{wbox\|Never, ever use dangerous commands like rm -rf (or rm by itself for that matter) or mv in a script that doesn't have a proper error handler.}}		{{wbox\|Never, ever use dangerous commands like <tt>rm -rf</tt> (or <tt>rm</tt> by itself, for that matter,) or mv in a script that doesn't have a proper error handler.}}

	Whenever there is the potential for pitfalls like the above, use some form of general error handler to at least tell the script to exit without trying the rest of the commands.		Whenever there is the potential for pitfalls like the above, use some form of general error handler to at least tell the script to exit without trying the rest of the commands.

Root: /* Error handling */

2006-05-12T01:11:29Z

Error handling

← Older revision		Revision as of 01:11, 12 May 2006
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	which will execute the function		which will execute the function
	<tt>handle()</tt>		<tt>handle()</tt>
	as soon as it encounters and error. And inside		as soon as it encounters and error. And inside <tt>handle()</tt>
	<tt>handle()</tt>
	, we have		, we have
	<tt>exit</tt>		<tt>exit</tt>

Bash disaster prevention - Revision history

Root: Text replace - '' to ' '

Root at 13:48, 2 June 2006

Root: /* More useful error handling */

Root: /* More useful error handling */

Root: /* More useful error handling */

Root: /* Error handling */

Root: /* Error handling */

Root: /* More useful error handling */

Root: /* Error handling */

Root: /* Error handling */

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