Shell Programming and Scripting

mysqldump -u ${MYSQL_USERNAME} --password=${MYSQL_PASSWORD} -C -Q -e --create-options ${i} | gzip -c > ${i}.sql.gz
	
	if [ $? -ne 0 ]
	then
		echo "mysqldump of ${i} failed."
		EXIT_CODE=1
	fi

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13. How do I get the exit code of cmd1 in cmd1|cmd2

    First, note that cmd1 exit code could be non-zero and still don't
    mean an error. This happens for instance in

    cmd | head -1

    you might observe a 141 (or 269 with ksh93) exit status of cmd1,
    but it's because cmd was interrupted by a SIGPIPE signal when
    "head -1" terminated after having read one line.

    To know the exit status of the elements of a pipeline
    cmd1 | cmd2 | cmd3

    a. with zsh:

       The exit codes are provided in the pipestatus special array.
       cmd1 exit code is in $pipestatus[1], cmd3 exit code in
       $pipestatus[3], so that $? is always the same as
       $pipestatus[-1].

    b. with bash:

       The exit codes are provided in the PIPESTATUS special array.
       cmd1 exit code is in ${PIPESTATUS[0]}, cmd3 exit code in
       ${PIPESTATUS[2]}, so that $? is always the same as
       ${PIPESTATUS: -1}.

    c. with any other Bourne like shells

       You need to use a trick to pass the exit codes to the main
       shell.  You can do it using a pipe(2). Instead of running
       "cmd1", you run "cmd1; echo $?" and make sure $? makes it way
       to the shell.

       exec 3>&1
       eval `
         # now, inside the `...`, fd4 goes to the pipe
         # whose other end is read and passed to eval;
         # fd1 is the normal standard output preserved
         # the line before with exec 3>&1
         exec 4>&1 >&3 3>&-
         {
           cmd1 4>&-; echo "ec1=$?;" >&4
         } | {
           cmd2 4>&-; echo "ec2=$?;" >&4
         } | cmd3
         echo "ec3=$?;" >&4
       `

    d. with a POSIX shell

       You can use this function to make it easier:

       run() {
         j=1
         while eval "\${pipestatus_$j+:} false"; do
           unset pipestatus_$j
           j=$(($j+1))
         done
         j=1 com= k=1 l=
         for a; do
           if [ "x$a" = 'x|' ]; then
             com="$com { $l "'3>&-
                         echo "pipestatus_'$j'=$?" >&3
                       } 4>&- |'
             j=$(($j+1)) l=
           else
             l="$l \"\$$k\""
           fi
           k=$(($k+1))
         done
         com="$com $l"' 3>&- >&4 4>&-
                    echo "pipestatus_'$j'=$?"'
         exec 4>&1
         eval "$(exec 3>&1; eval "$com")"
         exec 4>&-
         j=1
         while eval "\${pipestatus_$j+:} false"; do
           eval "[ \$pipestatus_$j -eq 0 ]" || return 1
           j=$(($j+1))
         done
         return 0
       }

       use it as:

       run cmd1 \| cmd2 \| cmd3
       exit codes are in $pipestatus_1, $pipestatus_2, $pipestatus_3

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