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January
19, 2004 -
Subshells & Subshell Grouping - Part
III
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Our
discussion of subshells and subshell
grouping will conclude with an example
using the tar (tape archive)
command. For this example, tar is
used on both sides of a pipe to copy a
directory tree to another disk directory
(a disk to disk copy without an
intermediary file). The general
format for this operation is:
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tar
read from source directory | tar write
to destination directory
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Since
the tar write (extract) will occur in a
different directory than the tar read, the
write operation needs to be performed
within a subshell.
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[root@hawk] # pwd
/export/home/jdoe/project1_files
[root@hawk] #
[root@hawk] # find .
.
./file1
./files2
./files3
./tmp
./tmp/file4
./tmp/file5
./tmp/file6
[root@hawk] #
[root@hawk] # mkdir /export/home/jdoe/project1_backup
[root@hawk] #
[root@hawk] # tar cvfp - . | ( cd /export/home/jdoe/project1_backup; tar
xvfp - )
a ./ 0K
a ./file1 1K
a ./files2 1K
a ./files3 1K
a ./tmp/ 0K
a ./tmp/file4 1K
a ./tmp/file5 1K
a ./tmp/file6 1K
tar: blocksize = 16
x ., 0 bytes, 0 tape blocks
x ./file1, 53 bytes, 1 tape blocks
x ./files2, 53 bytes, 1 tape blocks
x ./files3, 53 bytes, 1 tape blocks
x ./tmp, 0 bytes, 0 tape blocks
x ./tmp/file4, 52 bytes, 1 tape blocks
x ./tmp/file5, 52 bytes, 1 tape blocks
x ./tmp/file6, 52 bytes, 1 tape blocks
[root@hawk] #
[root@hawk] # cd /export/home/jdoe/project1_backup
[root@hawk] #
[root@hawk] # find .
.
./file1
./files2
./files3
./tmp
./tmp/file4
./tmp/file5
./tmp/file6
[root@hawk] #
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As you can see
from the output, the files in the source
directory tree were copied to the
destination directory.
In
review, there are both advantages and
disadvantages to using subshells, and your
ability to determine when and when not to
use them will improve as your shell
scripting skills mature. They are
effective for controlling the shell's
environment, enabling you to avoid any
undesired side effects in the current
environment. This was previously
illustrated by looking at the value of a
variable in the login shell, and then in a
subshell (view
tip). Defining a
variable within a subshell effectively
makes it a local variable (local to only
the subshell).
Another example of avoiding undesirable
changes to the login shell was
demonstrated by changing the working
directory within a subshell (view
tip). The
directory can be changed as many times as
needed without having an impact on the
login shell's working directory.
One of the major disadvantages of
utilizing subshells is the fact that more
system resources are used to run
them. It is more efficient to run
all programs in the current shell.
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