我有一个单个文件的目录,one.txt
。如果我运行ls | cat
,它工作正常。但是,如果我试图划分这个管道的两边,我确实看到了命令的输出以及strace,但是过程没有完成。
strace ls 2> >(stdbuf -o 0 sed 's/^/command1:/') | strace cat 2> >(stdbuf -o 0 sed 's/^/command2:/')
我得到的输出是:
command2:execve("/usr/bin/cat", ["cat"], [/* 50 vars */]) = 0
command2:brk(0) = 0x1938000
command2:mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f87e5a93000
command2:access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
<snip>
command2:open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
command2:fstat(3, {st_mode=S_IFREG|0644, st_size=106070960, ...}) = 0
command2:mmap(NULL, 106070960, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f87def8a000
command2:close(3) = 0
command2:fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
command2:fstat(0, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
command2:fadvise64(0, 0, 0, POSIX_FADV_SEQUENTIAL) = -1 ESPIPE (Illegal seek)
command2:read(0, "command1:execve(\"/usr/bin/ls\", ["..., 65536) = 4985
command1:execve("/usr/bin/ls", ["ls"], [/* 50 vars */]) = 0
command1:brk(0) = 0x1190000
command1:mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fae869c3000
command1:access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
<snip>
command1:close(3) = 0
command1:fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
command2:write(1, "command1:close(3) "..., 115) = 115
command2:read(0, "command1:mmap(NULL, 4096, PROT_R"..., 65536) = 160
command1:mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fae869c2000
one.txt
command1:write(1, "one.txt\n", 8) = 8
command2:write(1, "command1:mmap(NULL, 4096, PROT_R"..., 160) = 160
command2:read(0, "command1:close(1) "..., 65536) = 159
command1:close(1) = 0
command1:munmap(0x7fae869c2000, 4096) = 0
command1:close(2) = 0
command2:write(1, "command1:close(1) "..., 159) = 159
command2:read(0, "command1:exit_group(0) "..., 65536) = 53
command1:exit_group(0) = ?
command2:write(1, "command1:exit_group(0) "..., 53) = 53
command2:read(0, "command1:+++ exited with 0 +++\n", 65536) = 31
command1:+++ exited with 0 +++
command2:write(1, "command1:+++ exited with 0 +++\n", 31) = 31
从那时起它就会挂起。 ps显示管道中的两个命令(这里的ls和cat)正在运行。
我在RHEL7上运行Bash版本4.2.46。
我在你的strace上放了一个strace:
strace bash -c 'strace true 2> >(cat > /dev/null)'
它挂在wait4
上,表明它等在孩子身上。 ps f
证实了这一点:
24740 pts/19 Ss 0:00 /bin/bash
24752 pts/19 S+ 0:00 \_ strace true
24753 pts/19 S+ 0:00 \_ /bin/bash
24755 pts/19 S+ 0:00 \_ cat
基于此,我的工作理论是这种效果是一个僵局,因为:
strace
等待所有孩子,甚至是那些没有直接产生的孩子这表明至少有两种解决方法,这两种方法似乎都有效:
strace -D ls 2> >(nl)
{ strace ls; true; } 2> >(nl)
-D
,引用手册页,“[运行]跟踪器进程作为一个独立的孙子,而不是作为tracee的父母”。第二个强迫bash做另一个fork
运行strace
,通过添加另一个命令来做。
在这两种情况下,额外的叉子意味着过程替换不会最终成为strace
的孩子,避免了这个问题。