center tables in a markdown file
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| Descriptoion | struct passwd member | POSIX.1 | FreeBSD 8.0 | Linux 32.0 | Mac OS X 10.6.8 | Solaris |
|:-----------------------------|:---------------------|:---------:|:-----------:|:----------:|:---------------:|:---------:|
| user name | char *pw_name | $\bullet$ | $\bullet$ | $\bullet$ | $\bullet$ | $\bullet$ |
_Figure 6.1 Fields in `/etc/passwd` file_
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