debugging.html

Shall高级编程

><B
><A
NAME="TRAPREF1"
></A
>Trapping signals</B
></P
><DL
><DT
><B
CLASS="COMMAND"
>trap</B
></DT
><DD
><P
>Specifies an action on receipt of a signal; also
	        useful for debugging.

	      <DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
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WIDTH="25"
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SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="SIGNALD"
></A
>A
		<I
CLASS="FIRSTTERM"
>signal</I
> is simply a message
		sent to a process, either by the kernel or another
		process, telling it to take some specified action
		(usually to terminate).  For example, hitting a
		<B
CLASS="KEYCAP"
>Control</B
>-<B
CLASS="KEYCAP"
>C</B
>,
		sends a user interrupt, an INT signal, to a running
		program.</P
></TD
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>

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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;trap '' 2
   2&nbsp;# Ignore interrupt 2 (Control-C), with no action specified. 
   3&nbsp;
   4&nbsp;trap 'echo "Control-C disabled."' 2
   5&nbsp;# Message when Control-C pressed.</PRE
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>
	      </P
></DD
></DL
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX76"
></A
><P
><B
>Example 29-5. Trapping at exit</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Hunting variables with a trap.
   3&nbsp;
   4&nbsp;trap 'echo Variable Listing --- a = $a  b = $b' EXIT
   5&nbsp;#  EXIT is the name of the signal generated upon exit from a script.
   6&nbsp;#
   7&nbsp;#  The command specified by the "trap" doesn't execute until
   8&nbsp;#+ the appropriate signal is sent.
   9&nbsp;
  10&nbsp;echo "This prints before the \"trap\" --"
  11&nbsp;echo "even though the script sees the \"trap\" first."
  12&nbsp;echo
  13&nbsp;
  14&nbsp;a=39
  15&nbsp;
  16&nbsp;b=36
  17&nbsp;
  18&nbsp;exit 0
  19&nbsp;#  Note that commenting out the 'exit' command makes no difference,
  20&nbsp;#+ since the script exits in any case after running out of commands.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ONLINE"
></A
><P
><B
>Example 29-6. Cleaning up after <B
CLASS="KEYCAP"
>Control-C</B
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# logon.sh: A quick 'n dirty script to check whether you are on-line yet.
   3&nbsp;
   4&nbsp;umask 177  # Make sure temp files are not world readable.
   5&nbsp;
   6&nbsp;
   7&nbsp;TRUE=1
   8&nbsp;LOGFILE=/var/log/messages
   9&nbsp;#  Note that $LOGFILE must be readable
  10&nbsp;#+ (as root, chmod 644 /var/log/messages).
  11&nbsp;TEMPFILE=temp.$$
  12&nbsp;#  Create a "unique" temp file name, using process id of the script.
  13&nbsp;#     Using 'mktemp' is an alternative.
  14&nbsp;#     For example:
  15&nbsp;#     TEMPFILE=`mktemp temp.XXXXXX`
  16&nbsp;KEYWORD=address
  17&nbsp;#  At logon, the line "remote IP address xxx.xxx.xxx.xxx"
  18&nbsp;#                      appended to /var/log/messages.
  19&nbsp;ONLINE=22
  20&nbsp;USER_INTERRUPT=13
  21&nbsp;CHECK_LINES=100
  22&nbsp;#  How many lines in log file to check.
  23&nbsp;
  24&nbsp;trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT
  25&nbsp;#  Cleans up the temp file if script interrupted by control-c.
  26&nbsp;
  27&nbsp;echo
  28&nbsp;
  29&nbsp;while [ $TRUE ]  #Endless loop.
  30&nbsp;do
  31&nbsp;  tail -n $CHECK_LINES $LOGFILE&#62; $TEMPFILE
  32&nbsp;  #  Saves last 100 lines of system log file as temp file.
  33&nbsp;  #  Necessary, since newer kernels generate many log messages at log on.
  34&nbsp;  search=`grep $KEYWORD $TEMPFILE`
  35&nbsp;  #  Checks for presence of the "IP address" phrase,
  36&nbsp;  #+ indicating a successful logon.
  37&nbsp;
  38&nbsp;  if [ ! -z "$search" ] #  Quotes necessary because of possible spaces.
  39&nbsp;  then
  40&nbsp;     echo "On-line"
  41&nbsp;     rm -f $TEMPFILE    #  Clean up temp file.
  42&nbsp;     exit $ONLINE
  43&nbsp;  else
  44&nbsp;     echo -n "."        #  The -n option to echo suppresses newline,
  45&nbsp;                        #+ so you get continuous rows of dots.
  46&nbsp;  fi
  47&nbsp;
  48&nbsp;  sleep 1  
  49&nbsp;done  
  50&nbsp;
  51&nbsp;
  52&nbsp;#  Note: if you change the KEYWORD variable to "Exit",
  53&nbsp;#+ this script can be used while on-line
  54&nbsp;#+ to check for an unexpected logoff.
  55&nbsp;
  56&nbsp;# Exercise: Change the script, per the above note,
  57&nbsp;#           and prettify it.
  58&nbsp;
  59&nbsp;exit 0
  60&nbsp;
  61&nbsp;
  62&nbsp;# Nick Drage suggests an alternate method:
  63&nbsp;
  64&nbsp;while true
  65&nbsp;  do ifconfig ppp0 | grep UP 1&#62; /dev/null &#38;&#38; echo "connected" &#38;&#38; exit 0
  66&nbsp;  echo -n "."   # Prints dots (.....) until connected.
  67&nbsp;  sleep 2
  68&nbsp;done
  69&nbsp;
  70&nbsp;# Problem: Hitting Control-C to terminate this process may be insufficient.
  71&nbsp;#+         (Dots may keep on echoing.)
  72&nbsp;# Exercise: Fix this.
  73&nbsp;
  74&nbsp;
  75&nbsp;
  76&nbsp;# Stephane Chazelas has yet another alternative:
  77&nbsp;
  78&nbsp;CHECK_INTERVAL=1
  79&nbsp;
  80&nbsp;while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD"
  81&nbsp;do echo -n .
  82&nbsp;   sleep $CHECK_INTERVAL
  83&nbsp;done
  84&nbsp;echo "On-line"
  85&nbsp;
  86&nbsp;# Exercise: Discuss the relative strengths and weaknesses
  87&nbsp;#           of each of these various approaches.</PRE
></TD
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></TABLE
><HR></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="OPTION"
>DEBUG</TT
> argument to
	<B
CLASS="COMMAND"
>trap</B
> causes a specified action to execute
	after every command in a script. This permits tracing variables,
	for example.

      <DIV
CLASS="EXAMPLE"
><HR><A
NAME="VARTRACE"
></A
><P
><B
>Example 29-7. Tracing a variable</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;trap 'echo "VARIABLE-TRACE&#62; \$variable = \"$variable\""' DEBUG
   4&nbsp;# Echoes the value of $variable after every command.
   5&nbsp;
   6&nbsp;variable=29
   7&nbsp;
   8&nbsp;echo "Just initialized \"\$variable\" to $variable."
   9&nbsp;
  10&nbsp;let "variable *= 3"
  11&nbsp;echo "Just multiplied \"\$variable\" by 3."
  12&nbsp;
  13&nbsp;exit $?
  14&nbsp;
  15&nbsp;#  The "trap 'command1 . . . command2 . . .' DEBUG" construct is
  16&nbsp;#+ more appropriate in the context of a complex script,
  17&nbsp;#+ where placing multiple "echo $variable" statements might be
  18&nbsp;#+ clumsy and time-consuming.
  19&nbsp;
  20&nbsp;# Thanks, Stephane Chazelas for the pointer.
  21&nbsp;
  22&nbsp;
  23&nbsp;Output of script:
  24&nbsp;
  25&nbsp;VARIABLE-TRACE&#62; $variable = ""
  26&nbsp;VARIABLE-TRACE&#62; $variable = "29"
  27&nbsp;Just initialized "$variable" to 29.
  28&nbsp;VARIABLE-TRACE&#62; $variable = "29"
  29&nbsp;VARIABLE-TRACE&#62; $variable = "87"
  30&nbsp;Just multiplied "$variable" by 3.
  31&nbsp;VARIABLE-TRACE&#62; $variable = "87"</PRE
></TD
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><HR></DIV
>

      </P
></TD
></TR
></TABLE
></DIV
><P
>Of course, the <B
CLASS="COMMAND"
>trap</B
> command has other uses
        aside from debugging.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="MULTIPLEPROC"
></A
><P
><B
>Example 29-8. Running multiple processes (on an SMP box)</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# parent.sh
   3&nbsp;# Running multiple processes on an SMP box.
   4&nbsp;# Author: Tedman Eng
   5&nbsp;
   6&nbsp;#  This is the first of two scripts,
   7&nbsp;#+ both of which must be present in the current working directory.
   8&nbsp;
   9&nbsp;
  10&nbsp;
  11&nbsp;
  12&nbsp;LIMIT=$1         # Total number of process to start
  13&nbsp;NUMPROC=4        # Number of concurrent threads (forks?)
  14&nbsp;PROCID=1         # Starting Process ID
  15&nbsp;echo "My PID is $$"
  16&nbsp;
  17&nbsp;function start_thread() {
  18&nbsp;        if [ $PROCID -le $LIMIT ] ; then
  19&nbsp;                ./child.sh $PROCID&#38;
  20&nbsp;                let "PROCID++"
  21&nbsp;        else
  22&nbsp;           echo "Limit reached."
  23&nbsp;           wait
  24&nbsp;           exit
  25&nbsp;        fi
  26&nbsp;}
  27&nbsp;
  28&nbsp;while [ "$NUMPROC" -gt 0 ]; do
  29&nbsp;        start_thread;
  30&nbsp;        let "NUMPROC--"
  31&nbsp;done
  32&nbsp;
  33&nbsp;
  34&nbsp;while true
  35&nbsp;do
  36&nbsp;
  37&nbsp;trap "start_thread" SIGRTMIN
  38&nbsp;
  39&nbsp;done
  40&nbsp;
  41&nbsp;exit 0
  42&nbsp;
  43&nbsp;
  44&nbsp;
  45&nbsp;# ======== Second script follows ========
  46&nbsp;
  47&nbsp;
  48&nbsp;#!/bin/bash
  49&nbsp;# child.sh
  50&nbsp;# Running multiple processes on an SMP box.
  51&nbsp;# This script is called by parent.sh.
  52&nbsp;# Author: Tedman Eng
  53&nbsp;
  54&nbsp;temp=$RANDOM
  55&nbsp;index=$1
  56&nbsp;shift
  57&nbsp;let "temp %= 5"
  58&nbsp;let "temp += 4"
  59&nbsp;echo "Starting $index  Time:$temp" "$@"
  60&nbsp;sleep ${temp}
  61&nbsp;echo "Ending $index"
  62&nbsp;kill -s SIGRTMIN $PPID
  63&nbsp;
  64&nbsp;exit 0
  65&nbsp;
  66&nbsp;
  67&nbsp;# ======================= SCRIPT AUTHOR'S NOTES ======================= #
  68&nbsp;#  It's not completely bug free.
  69&nbsp;#  I ran it with limit = 500 and after the first few hundred iterations,
  70&nbsp;#+ one of the concurrent threads disappeared!
  71&nbsp;#  Not sure if this is collisions from trap signals or something else.
  72&nbsp;#  Once the trap is received, there's a brief moment while executing the
  73&nbsp;#+ trap handler but before the next trap is set.  During this time, it may
  74&nbsp;#+ be possible to miss a trap signal, thus miss spawning a child process.
  75&nbsp;
  76&nbsp;#  No doubt someone may spot the bug and will be writing 
  77&nbsp;#+ . . . in the future.
  78&nbsp;
  79&nbsp;
  80&nbsp;
  81&nbsp;# ===================================================================== #
  82&nbsp;
  83&nbsp;
  84&nbsp;
  85&nbsp;# ----------------------------------------------------------------------#
  86&nbsp;
  87&nbsp;
  88&nbsp;
  89&nbsp;#################################################################
  90&nbsp;# The following is the original script written by Vernia Damiano.
  91&nbsp;# Unfortunately, it doesn't work properly.
  92&nbsp;#################################################################
  93&nbsp;
  94&nbsp;#!/bin/bash
  95&nbsp;
  96&nbsp;#  Must call script with at least one integer parameter
  97&nbsp;#+ (number of concurrent processes).
  98&nbsp;#  All other parameters are passed through to the processes started.
  99&nbsp;
 100&nbsp;
 101&nbsp;INDICE=8        # Total number of process to start
 102&nbsp;TEMPO=5         # Maximum sleep time per process
 103&nbsp;E_BADARGS=65    # No arg(s) passed to script.
 104&nbsp;
 105&nbsp;if [ $# -eq 0 ] # Check for at least one argument passed to script.
 106&nbsp;then
 107&nbsp;  echo "Usage: `basename $0` number_of_processes [passed params]"
 108&nbsp;  exit $E_BADARGS
 109&nbsp;fi
 110&nbsp;
 111&nbsp;NUMPROC=$1              # Number of concurrent process
 112&nbsp;shift
 113&nbsp;PARAMETRI=( "$@" )      # Parameters of each process
 114&nbsp;
 115&nbsp;function avvia() {
 116&nbsp;         local temp
 117&nbsp;         local index
 118&nbsp;         temp=$RANDOM
 119&nbsp;         index=$1
 120&nbsp;         shift
 121&nbsp;         let "temp %= $TEMPO"
 122&nbsp;         let "temp += 1"
 123&nbsp;         echo "Starting $index Time:$temp" "$@"
 124&nbsp;         sleep ${temp}
 125&nbsp;         echo "Ending $index"
 126&nbsp;         kill -s SIGRTMIN $$
 127&nbsp;}
 128&nbsp;
 129&nbsp;function parti() {
 130&nbsp;         if [ $INDICE -gt 0 ] ; then
 131&nbsp;              avvia $INDICE "${PARAMETRI[@]}" &#38;
 132&nbsp;                let "INDICE--"
 133&nbsp;         else
 134&nbsp;                trap : SIGRTMIN
 135&nbsp;         fi
 136&nbsp;}
 137&nbsp;
 138&nbsp;trap parti SIGRTMIN
 139&nbsp;
 140&nbsp;while [ "$NUMPROC" -gt 0 ]; do
 141&nbsp;         parti;
 142&nbsp;         let "NUMPROC--"
 143&nbsp;done
 144&nbsp;
 145&nbsp;wait
 146&nbsp;trap - SIGRTMIN
 147&nbsp;
 148&nbsp;exit $?
 149&nbsp;
 150&nbsp;: &#60;&#60;SCRIPT_AUTHOR_COMMENTS
 151&nbsp;I had the need to run a program, with specified options, on a number of
 152&nbsp;different files, using a SMP machine. So I thought [I'd] keep running
 153&nbsp;a specified number of processes and start a new one each time . . . one
 154&nbsp;of these terminates.
 155&nbsp;
 156&nbsp;The "wait" instruction does not help, since it waits for a given process
 157&nbsp;or *all* process started in background. So I wrote [this] bash script
 158&nbsp;that can do the job, using the "trap" instruction.
 159&nbsp;  --Vernia Damiano
 160&nbsp;SCRIPT_AUTHOR_COMMENTS</PRE
></TD
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><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><TT
CLASS="USERINPUT"
><B
>trap '' SIGNAL</B
></TT
> (two adjacent
	apostrophes) disables SIGNAL for the remainder of the
	script. <TT
CLASS="USERINPUT"
><B
>trap SIGNAL</B
></TT
> restores
	the functioning of SIGNAL once more. This is useful to
	protect a critical portion of a script from an undesirable
	interrupt.</P
></TD
></TR
></TABLE
></DIV
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;	trap '' 2  # Signal 2 is Control-C, now disabled.
   2&nbsp;	command
   3&nbsp;	command
   4&nbsp;	command
   5&nbsp;	trap 2     # Reenables Control-C
   6&nbsp;	</PRE
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><A
NAME="AEN17884"
></A
><P
><A
HREF="bashver3.html#BASH3REF"
>Version 3</A
> of Bash adds the
	following special variables for use by the debugger.

       <OL
TYPE="1"
><LI
><P
>$BASH_ARGC</P
></LI
><LI
><P
>$BASH_ARGV</P
></LI
><LI
><P
>$BASH_COMMAND</P
></LI
><LI
><P
>$BASH_EXECUTION_STRING</P
></LI
><LI
><P
>$BASH_LINENO</P
></LI
><LI
><P
>$BASH_SOURCE</P
></LI
><LI
><P
><A
HREF="variables2.html#BASHSUBSHELLREF"
>$BASH_SUBSHELL</A
></P
></LI
></OL
></P
></DIV
></TD
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></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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><A
NAME="FTN.AEN17836"
HREF="debugging.html#AEN17836"
>[1]</A
></TD
><TD
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VALIGN="TOP"
WIDTH="95%"
><P
>By convention, <TT
CLASS="REPLACEABLE"
><I
>signal
		0</I
></TT
> is assigned to <A
HREF="exit-status.html#EXITCOMMANDREF"
>exit</A
>.  </P
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