#!/bin/shx="this is the initial value of x"cat /tmp/tmp | while read line;do        x="$line"        echo $xdoneecho x = $x

1,a

[qiu.li@l-tdata1.tkt.cn6 ~]$ ./tmp.sh 1,ax = 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 redirectioninstead of the pipeline

#!/bin/shx="this is the initial value of x"while read line;do        x="$line"        echo $xdone < /tmp/tmpecho x = $x

[qiu.li@l-tdata1.tkt.cn6 ~]$ ./tmp.sh 1,ax = 1,a

#!/bin/shx="this is the initial value of x"exec 3<&0         # save stdinexec < /tmp/tmpwhile read line; do        x=$line        echo $xdoneexec 0<&3        # restore stdinecho x = $x

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.