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有空看看FAQ http://bbs.chinaunix.net/thread-551026-2-1.html
- 33. Why do I lose the value of global variables that are set in a loop.
- Given the following program
- #!/bin/sh
- x="this is the initial value of x"
- cat dataFile | while read line;do
- x="$line"
- done
- echo x = $x
- You may get the following for output
- x = this is the initial value of x
-
- This is because in the Bourne shell redirected control structures
- run in a subshell, so the value of x only gets changed in the
- subshell, and is lost when the loop ends.
- In other shells the same result may be seen because of the way
- pipelines are handled. In shells other than ksh (not pdksh) and
- zsh elements of a pipeline are run in subshells. In ksh and zsh,
- the last element of the pipeline is run in the current shell.
- An alternative for non-Bourne shells is to use redirection
- instead of the pipeline
- #!/bin/sh
- x="this is the initial value of x"
- while read line;do
- x="$line"
- done < dataFile
- echo x = $x
- With a Bourne shell you need to reassign file descriptors, so no
- pipline or redirection in the loop is involved.
- exec 3<&0 # save stdin
- exec < file
- while read line; do
- x=$line
- done
- exec 0<&3 # restore stdin
- Note that putting #!/bin/sh at the top of a script doesn't
- guarantee you're using the Bourne shell. Some systems link /bin/sh
- to some other shell. Check your system documentation to find out
- what shell you're really getting in this case.
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发表于 2008-05-09 10:38
