[TriLUG] bash cmd line processing question

[Andrew Perrin]

On Thu, 14 Feb 2002, M. Mueller (bhu5nji) wrote:

> why does the following command work:
> 
> $kill -SIGHUP `cat /var/run/syslogd.pid`

Backticks (``) mean "execute the command in the backticks and pass the
result back." So in this case, kill gets two arguments: -SIGHUP and
the STDOUT stream from the command:
cat /var/run/syslogd.pid
which is, presumably, the PID of the syslogd process; it is whatever the
contents of /var/run/syslogd.pid are.

> 
> why does the following command *not* work:
> 
> $kill -SIGHUP 'cat /var/run/syslogd.pid'
> 

Regular quotes ('') mean "treat the contents as a single argument instead
of breaking them up in normal shell parsing manner." Again, kill gets two
argument: -SIGHUP and the string:
cat /var/run/syslogd.pid
which is, with some certainty, not a PID (since PID's are numeric and "cat
/var/run/syslogd.pid" is not).

Hope this helps.

----------------------------------------------------------------------
Andrew J Perrin - andrew_perrin at unc.edu - http://www.unc.edu/~aperrin
 Assistant Professor of Sociology, U of North Carolina, Chapel Hill
      269 Hamilton Hall, CB#3210, Chapel Hill, NC 27599-3210 USA