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Content-Type: text/x-zim-wiki
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Wiki-Format: zim 0.4
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Creation-Date: 2011-12-25T13:52:34+08:00
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====== The coproc keyword ======
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Created Sunday 25 December 2011
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http://wiki.bash-hackers.org/syntax/keywords/coproc
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===== Synopsis =====
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coproc [NAME] command [redirections]
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===== Description =====
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Bash 4.0 introduced the coprocesses, a feature certainly familiar to __ksh__ users.
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coproc starts __a command__ in the backgound __setting up pipes__ so that you can interact with it. Optionally, the co-process can have a name NAME.
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If NAME is given, the following command must be a__ compound command__. If no NAME ist given, the command can be a simple command or a compound command.
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===== Redirections（这里的重定向是对于coproc里的command而言的） =====
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The redirections are normal redirections that are set __after the pipe has been set up__, some examples:
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# redirecting stderr in the pipe
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$ coproc { ls thisfiledoesntexist; read ;} __2>&1 #1代表coproc的标准输出，即COPROC[1]__
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__#将在子shell里执行的{ ls thisfiledoesntexist; read ;}标准出错重定位到其标准输出(而不是当前shell的标准输出)。由于重定向是在pipe建立之后而且coproc的标准输入和标准输出是和执行该语句的当前shell相连接的，所以coproc的出错和输出将可以通过pipe访问到。__
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[2] 23084
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$ read -u ${__COPROC[0]__};printf "%s\n" "$REPLY"
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ls: cannot access thisfiledoesntexist: No such file or directory #coproc的出错可以通过其标准输出fd访问到。
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#COPROC是__当前shell__中与coproc的标准输入/输出相连的描**述符数组，COPROC[0]与coproc的标准输出相连，COPROC[1]与coproc的标准输入相连**。
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#let the output of the coprocess go to stdout
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$ __{ coproc mycoproc { awk '{print "foo" $0;fflush()}' ;} >&3 ;} 3>&1__
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{....} 分组命令代表在**当前shell中**执行其中的命令，所以3>&1，表示将{...}中使用的3描述符与__当前shell__的1描述符相连接。
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而 {...}中的>&3是在coproc与当前shell的pipe建立之后执行的，其标准输出的描述符其环境中的3相连后，实际上是与当前shell的标准输出相连。
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[2] 23092
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$ echo bar >&${mycoproc[1]}
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$ foobar
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Here we need to **save the previous file descriptor of stdout**, because by the time we want to redirect the fds of the coprocess stdout has been redirected to the pipe.
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注意，不能是：
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[geekard@geekard ~]$ { coproc mycoproc { awk '{print "foo" $0; fflush()}'; }__ 3>&1 >&3;__ } #只是重定向协程自身的fd，__与外界shell无关__。
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[geekard@geekard ~]$ echo ddd >&${mycoproc[1]}
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[geekard@geekard ~]$ jobs
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[1]+ Running coproc mycoproc { awk '{print "foo" $0; fflush()}'; } 3>&1 1>&3 &
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[geekard@geekard ~]$ __lsof -p 31330__
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COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
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bash 31330 geekard cwd DIR 8,7 4096 2490369 /home/geekard
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bash 31330 geekard rtd DIR 8,2 4096 2 /
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bash 31330 geekard txt REG 8,2 705652 15 /bin/bash
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bash 31330 geekard mem REG 8,2 47044 411422 /lib/libnss_files-2.15.so
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bash 31330 geekard mem REG 8,2 7015792 303590 /usr/lib/locale/locale-archive
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bash 31330 geekard mem REG 8,2 1949865 411409 /lib/libc-2.15.so
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bash 31330 geekard mem REG 8,2 13952 391721 /lib/libdl-2.15.so
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bash 31330 geekard mem REG 8,2 351704 264729 /usr/lib/libncursesw.so.5.9
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bash 31330 geekard mem REG 8,2 281388 391738 /lib/libreadline.so.6.2
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bash 31330 geekard mem REG 8,2 151125 411426 /lib/ld-2.15.so
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bash 31330 geekard ** 0r ** FIFO 0,8 0t0 199595 pipe
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bash 31330 geekard **1w ** FIFO 0,8 0t0 199594 pipe
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bash 31330 geekard 2u CHR 136,6 0t0 9 /dev/pts/6
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bash 31330 geekard ** 3w ** FIFO 0,8 0t0 199594 pipe
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bash 31330 geekard **10w** FIFO 0,8 0t0 199594 pipe
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bash 31330 geekard 255u CHR 136,6 0t0 9 /dev/pts/6
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[geekard@geekard ~]$ **echo** </proc/31330/fd/3 __#错误__，echo只输出命令行上的字符，__不使用重定向__。
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[geekard@geekard ~]$** cat **!$ #cat是**行缓冲**，一次读入一行，直到遇到EOF。
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cat /proc/31330/fd/3
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__fooddd__
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^C
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[geekard@geekard ~]$ echo "ddd2" >&${mycoproc[1]}
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[geekard@geekard ~]$ __read line__ <&${mycoproc[0]} #read只读取一行后返回。
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[geekard@geekard ~]$ echo $line
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fooddd2
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[geekard@geekard ~]$
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这其实是将**协程的文件描述符3**重定向到mycoproc[0]，协程的标准输出并没有改变。
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===== Pitfalls =====
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Avoid the command__ | __while read subshell
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The traditional KSH workaround to avoid the subshell when doing command | while read is to use a coprocess, unfortunately, it seems that bash's behaviour differ from KSH.
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In KSH you would do:
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ls |& #start a coprocess
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while read -p file;do echo "$file";done #read its output
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In bash:
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#DOESN'T WORK
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$ coproc ls
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[1] 23232
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$ while read __-u__ ${COPROC[0]} line;do echo "$line";done
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bash: read: line: invalid file descriptor specification
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[1]+ Done coproc COPROC ls
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By the time we start reading from the output of the coprocess, the file descriptor has been closed.
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===== Buffering =====
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In the first example, we used fflush() in the awk command, this was done on purpose, as always when you use__ pipes the I/O operations are buffered__, let's see what happens with sed:
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$ coproc sed s/^/foo/
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[1] 22981
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$ echo bar >&${COPROC[1]}
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$ read __-t 3__ -u ${COPROC[0]}; (( **$? >127** )) && echo "nothing read"
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nothing read
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Even though this example is the same as the first awk example, the read doesn't return, simply because the output is waiting in a buffer.
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===== background processes =====
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The file descriptors of the coprocesses are __available to the shell where you run coproc__, but they are __not inherited__. Here a not so meaningful illustration, suppose we want something that continuely reads the output of our coprocess and echo the result:
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#NOT WORKING
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$ coproc awk '{print "foo" $0;fflush()}'
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[2] 23100
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$ while read -u ${COPROC[0]};do echo "$REPLY";done __&__
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[3] 23104
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$ ./bash: line 243: read: 61: invalid file descriptor: Bad file
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descriptor
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it fails, because__ the descriptor is not avalaible in the subshell created by &__. evoking shell中的协程文件描述符并不会被后续的子shell继承。但是，在evoking shell中打开的其它文件描述符一般会被子shell继承。
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A possible workaround:
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#WARNING: for illustration purpose ONLY
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# this is not the way to make the coprocess print its output
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# to stdout, see the redirections above.
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$ coproc awk '{print "foo" $0;fflush()}'
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[2] 23109
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$__ exec 3<&${COPROC[0]}__
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$ while read -u 3;do echo "$REPLY";done &
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[3] 23110
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$ echo bar >&${COPROC[1]}
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$ foobar
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Here the** fd 3 is inherited**.
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===== Only one coprocess at a time =====
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The title says it all, complain to the bug-bash mailing list if you want more.
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===== Examples =====
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=== Anonymous Coprocess ===
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First let's see an example without NAME:
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$ coproc awk '{print "foo" $0;fflush()}'
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[1] 22978
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The command starts in the background, coproc returns immedately. 2 new files descriptors are now available via the __COPROC array__, We can send data to our command:
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$ echo bar >&${COPROC[1]}
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And then read its output:
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$ read -u ${COPROC[0]};printf "%s\n" "$REPLY"
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foobar
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When we don't need our command anymore, we can kill it via its pid:
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$ kill __$COPROC_PID__
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$
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[1]+ Terminated coproc COPROC awk '{print "foo" $0;fflush()}'
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=== Named Coprocess ===
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Using a named coprocess is as simple, we just need a compound command like when defining a function:
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$ coproc mycoproc __{ awk '{print "foo" $0;fflush()}' ;}__
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[1] 23058
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$ echo bar >&${mycoproc[1]}
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$ read -u ${mycoproc[0]};printf "%s\n" "$REPLY"
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foobar
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$ kill $mycoproc_PID
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$
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[1]+ Terminated coproc mycoproc { awk '{print "foo" $0;fflush()}'; }
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Redirecting the output of a script to a file and to the screen
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#!/bin/bash
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# we start tee in the background
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# redirecting its output to the stdout of the script
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{ coproc tee { tee logfile ;} __>&3 __;} __3>&1__ #invoking shell写打开文件描述符3(duplicate from its fd 1)，然后该描述符__3被子进程继承__。
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# we redirect stding and stdout of the script to our coprocess
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__exec >&${tee[1]} 2>&1__
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在使用复制文件描述符特性时，被复制到的文件描述符必须事先读打开或写打开。所以，内层的3其实继承的是外层shell打开的描述符。
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The operator
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[n]>&word
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is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. If the digits in word do not specify __a file descriptor open for output__, a redirection error occurs.
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===== Portability considerations =====
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* the coproc keyword is not specified by POSIX(R)
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* other shells might have different ways to solve the coprocess problem
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* the coproc keyword appeared in Bash version 4.0-alpha
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===== See also =====
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Anthony Thyssen's Coprocess Hints - excellent summary of everything around the topic
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http://www.ict.griffith.edu.au/anthony/info/shell/co-processes.hints
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