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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>Debugging</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="UP"TITLE="Advanced Topics"HREF="part5.html"><LINKREL="PREVIOUS"TITLE="Of Zeros and Nulls"HREF="zeros.html"><LINKREL="NEXT"TITLE="Options"HREF="options.html"><METAHTTP-EQUIV="Content-Style-Type"CONTENT="text/css"><LINKREL="stylesheet"HREF="common/kde-common.css"TYPE="text/css"><METAHTTP-EQUIV="Content-Type"CONTENT="text/html; charset=iso-8859-1"><METAHTTP-EQUIV="Content-Language"CONTENT="en"><LINKREL="stylesheet"HREF="common/kde-localised.css"TYPE="text/css"TITLE="KDE-English"><LINKREL="stylesheet"HREF="common/kde-default.css"TYPE="text/css"TITLE="KDE-Default"></HEAD><BODYCLASS="CHAPTER"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#AA0000"VLINK="#AA0055"ALINK="#AA0000"STYLE="font-family: sans-serif;"><DIVCLASS="NAVHEADER"><TABLESUMMARY="Header navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="zeros.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom"></TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="options.html"ACCESSKEY="N">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="CHAPTER"><H1><ANAME="DEBUGGING"></A>Chapter 29. Debugging</H1><TABLEBORDER="0"WIDTH="100%"CELLSPACING="0"CELLPADDING="0"CLASS="EPIGRAPH"><TR><TDWIDTH="45%"> </TD><TDWIDTH="45%"ALIGN="LEFT"VALIGN="TOP"><I><P><I>Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.</I></P><P><I>--Brian Kernighan</I></P></I></TD></TR></TABLE><P>The Bash shell contains no built-in debugger, and only bare-bones debugging-specific commands and constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non-functional script.</P><DIVCLASS="EXAMPLE"><HR><ANAME="EX74"></A><P><B>Example 29-1. A buggy script</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 # ex74.sh 3 4 # This is a buggy script. 5 # Where, oh where is the error? 6 7 a=37 8 9 if [$a -gt 27 ] 10 then 11 echo $a 12 fi 13 14 exit 0</PRE></TD></TR></TABLE><HR></DIV><P>Output from script: <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="SCREEN"> <TTCLASS="COMPUTEROUTPUT">./ex74.sh: [37: command not found</TT></PRE></TD></TR></TABLE> What's wrong with the above script? Hint: after the <ICLASS="FIRSTTERM">if</I>.</P><DIVCLASS="EXAMPLE"><HR><ANAME="MISSINGKEYWORD"></A><P><B>Example 29-2. Missing <AHREF="internal.html#KEYWORDREF">keyword</A></B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 # missing-keyword.sh: What error message will this generate? 3 4 for a in 1 2 3 5 do 6 echo "$a" 7 # done # Required keyword 'done' commented out in line 7. 8 9 exit 0 </PRE></TD></TR></TABLE><HR></DIV><P>Output from script: <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="SCREEN"> <TTCLASS="COMPUTEROUTPUT">missing-keyword.sh: line 10: syntax error: unexpected end of file</TT> </PRE></TD></TR></TABLE> Note that the error message does <SPANCLASS="emphasis"><ICLASS="EMPHASIS">not</I></SPAN> necessarily reference the line in which the error occurs, but the line where the Bash interpreter finally becomes aware of the error. </P><P>Error messages may disregard comment lines in a script when reporting the line number of a syntax error.</P><P>What if the script executes, but does not work as expected? This is the all too familiar logic error.</P><DIVCLASS="EXAMPLE"><HR><ANAME="EX75"></A><P><B>Example 29-3. <ICLASS="FIRSTTERM">test24</I>: another buggy script</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 # This script is supposed to delete all filenames in current directory 4 #+ containing embedded spaces. 5 # It doesn't work. 6 # Why not? 7 8 9 badname=`ls | grep ' '` 10 11 # Try this: 12 # echo "$badname" 13 14 rm "$badname" 15 16 exit 0</PRE></TD></TR></TABLE><HR></DIV><P>Try to find out what's wrong with <AHREF="debugging.html#EX75">Example 29-3</A> by uncommenting the <TTCLASS="USERINPUT"><B>echo "$badname"</B></TT> line. Echo statements are useful for seeing whether what you expect is actually what you get.</P><P>In this particular case, <TTCLASS="USERINPUT"><B>rm "$badname"</B></TT> will not give the desired results because <TTCLASS="VARNAME">$badname</TT> should not be quoted. Placing it in quotes ensures that <BCLASS="COMMAND">rm</B> has only one argument (it will match only one filename). A partial fix is to remove to quotes from <TTCLASS="VARNAME">$badname</TT> and to reset <TTCLASS="VARNAME">$IFS</TT> to contain only a newline, <TTCLASS="USERINPUT"><B>IFS=$'\n'</B></TT>. However, there are simpler ways of going about it. <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 # Correct methods of deleting filenames containing spaces. 2 rm *\ * 3 rm *" "* 4 rm *' '* 5 # Thank you. S.C.</PRE></TD></TR></TABLE> </P><P>Summarizing the symptoms of a buggy script, <OLTYPE="1"><LI><P>It bombs with a <SPANCLASS="QUOTE">"<SPANCLASS="ERRORNAME">syntax error</SPAN>"</SPAN> message, or</P></LI><LI><P>It runs, but does not work as expected (<SPANCLASS="ERRORNAME">logic error</SPAN>).</P></LI><LI><P>It runs, works as expected, but has nasty side effects (<SPANCLASS="ERRORNAME">logic bomb</SPAN>).</P></LI></OL> </P><P><ANAME="DEBUGTOOLS"></A></P><P>Tools for debugging non-working scripts include <OLTYPE="1"><LI><P><AHREF="internal.html#ECHOREF">echo</A> statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on.</P><DIVCLASS="TIP"><TABLECLASS="TIP"WIDTH="90%"BORDER="0"><TR><TDWIDTH="25"ALIGN="CENTER"VALIGN="TOP"><IMGSRC="common/tip.png"HSPACE="5"ALT="Tip"></TD><TDALIGN="LEFT"VALIGN="TOP"><P>Even better is an <BCLASS="COMMAND">echo</B> that echoes only when <ICLASS="FIRSTTERM">debug</I> is on. <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 ### debecho (debug-echo), by Stefano Falsetto ### 2 ### Will echo passed parameters only if DEBUG is set to a value. ### 3 debecho () { 4 if [ ! -z "$DEBUG" ]; then 5 echo "$1" >&2 6 # ^^^ to stderr 7 fi 8 } 9 10 DEBUG=on 11 Whatever=whatnot 12 debecho $Whatever # whatnot 13 14 DEBUG= 15 Whatever=notwhat 16 debecho $Whatever # (Will not echo.)</PRE></TD></TR></TABLE> </P></TD></TR></TABLE></DIV></LI><LI><P>using the <AHREF="extmisc.html#TEEREF">tee</A> filter to check processes or data flows at critical points.</P></LI><LI><P>setting option flags <TTCLASS="OPTION">-n -v -x</TT></P><P><TTCLASS="USERINPUT"><B>sh -n scriptname</B></TT> checks for syntax errors without actually running the script. This is the equivalent of inserting <TTCLASS="USERINPUT"><B>set -n</B></TT> or <TTCLASS="USERINPUT"><B>set -o noexec</B></TT> into the script. Note that certain types of syntax errors can slip past this check.</P><P><TTCLASS="USERINPUT"><B>sh -v scriptname</B></TT> echoes each command before executing it. This is the equivalent of inserting <TTCLASS="USERINPUT"><B>set -v</B></TT> or <TTCLASS="USERINPUT"><B>set -o verbose</B></TT> in the script.</P><P>The <TTCLASS="OPTION">-n</TT> and <TTCLASS="OPTION">-v</TT> flags work well together. <TTCLASS="USERINPUT"><B>sh -nv scriptname</B></TT> gives a verbose syntax check.</P><P><TTCLASS="USERINPUT"><B>sh -x scriptname</B></TT> echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting <TTCLASS="USERINPUT"><B>set -x</B></TT> or <TTCLASS="USERINPUT"><B>set -o xtrace</B></TT> in the script.</P><P><ANAME="UNDVARERR"></A></P><P>Inserting <TTCLASS="USERINPUT"><B>set -u</B></TT> or <TTCLASS="USERINPUT"><B>set -o nounset</B></TT> in the script runs it, but gives an <SPANCLASS="ERRORNAME">unbound variable</SPAN> error message at each attempt to use an undeclared variable.</P></LI><LI><P>Using an <SPANCLASS="QUOTE">"assert"</SPAN> function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.)</P><DIVCLASS="EXAMPLE"><HR><ANAME="ASSERT"></A><P><B>Example 29-4. Testing a condition with an <ICLASS="FIRSTTERM">assert</I></B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 # assert.sh 3 4 ####################################################################### 5 assert () # If condition false, 6 { #+ exit from script 7 #+ with appropriate error message. 8 E_PARAM_ERR=98 9 E_ASSERT_FAILED=99 10 11 12 if [ -z "$2" ] # Not enough parameters passed 13 then #+ to assert() function. 14 return $E_PARAM_ERR # No damage done. 15 fi 16 17 lineno=$2 18 19 if [ ! $1 ] 20 then 21 echo "Assertion failed: \"$1\"" 22 echo "File \"$0\", line $lineno" # Give name of file and line number. 23 exit $E_ASSERT_FAILED 24 # else 25 # return 26 # and continue executing the script. 27 fi 28 } # Insert a similar assert() function into a script you need to debug. 29 ####################################################################### 30 31 32 a=5 33 b=4 34 condition="$a -lt $b" # Error message and exit from script. 35 # Try setting "condition" to something else 36 #+ and see what happens. 37 38 assert "$condition" $LINENO 39 # The remainder of the script executes only if the "assert" does not fail. 40 41 42 # Some commands. 43 # Some more commands . . . 44 echo "This statement echoes only if the \"assert\" does not fail." 45 # . . . 46 # More commands . . . 47 48 exit $?</PRE></TD></TR></TABLE><HR></DIV></LI><LI><P>Using the <AHREF="variables2.html#LINENOREF">$LINENO</A> variable and the <AHREF="internal.html#CALLERREF">caller</A> builtin.</P></LI><LI><P><ANAME="DEBUGTRAP"></A></P><P>trapping at exit.</P><P>The <BCLASS="COMMAND">exit</B> command in a script triggers a signal <SPANCLASS="RETURNVALUE">0</SPAN>, terminating the process, that is, the script itself. <ANAME="AEN17836"HREF="#FTN.AEN17836">[1]</A> It is often useful to trap the <BCLASS="COMMAND">exit</B>, forcing a <SPANCLASS="QUOTE">"printout"</SPAN> of variables, for example. The <BCLASS="COMMAND">trap</B> must be the first command in the script.</P></LI></OL> </P><DIVCLASS="VARIABLELIST"><P⌨️ 快捷键说明
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