The . (dot) Command |
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Normally, when you execute a shell program, a subshell is
created in which to execute it. Therefore, if you define variables
in the program, they are only good for as long as the program is
executing (when the program is done, you return to the current shell's
environment). If you wish to have the shell program executed in
the current shell (and thus make the defined variables good for
the current shell's environment), use the "dot"
command:
Make sure there is a space between the dot (.)
and the script name (otherwise the system will assume it is part
of the script name). Let's look at an example.
Create a file with the following commands:
echo $dog
dog=tired
echo $dog |
Make the script executable with the chmod command ("chmod +x dogsample",
where "dogsample"
is the name of the script). Next, define the variable dog
to be:
Run the script (by typing dogsample)
without the dot command. The results will be:
Now, check to see what the value of dog
is:
The original value for dog
appears. This is because the shell was executed in a subshell. Now,
try the dot command:
and then test the value of dog:
The value of dog
was changed because the script was run in the current shell.
The eval Command |
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The eval
command reads its arguments as input to the shell, and the resulting
commands are executed. The format is:
where arg is an argument that is
a shell command or shell program. Here is an example:
eval "grep jones $p_file | set | echo $1 $2 $4" |
eval will
execute the pipe contained in double quotes in the shell.
If you use the following:
you would receive an error message from date.
The "&"
is ignored as a special character (due to the single quotes). So,
to make the command function as expected, use eval:
and the eval
will reparse the string and thus attach the special meaning to "&".
Using Shell Expansions |
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You read about pattern matching in “File Name Generation”. Here are some examples which will simplify
some of the constructs you just learned.
When you generate lists for your for
constructs (or any other construct where you are trying to generate
filenames without needing to type in each file name), you are free
to use pattern-matching characters. For example:
for i in *.c
do
mv $i /tmp
done |
Here we generate a loop in which i
is set to each file name in the current directory that ends in ".c."
case $i in
?[dD].c ) echo $i ;;
*[!nN] ) mv $i .. ;;
* ) exit ;;
esac |
This case construct will match the value of i
on the first pattern line if it begins with any single character
(?), followed
by either "d.c"
or "D.c".
The second pattern line matches any string (including the null)
ending in any letter other than "n"
or "N". The last
expression matches anything left over.
Helpful Tips |
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Let's wrap up this section with a couple of helpful items.
To print a character that will "beep"
to alert a user, use CTRL-G
in an echo command.
To add control characters to the vi
editor, you must first type CTRL-V,
then type the control string.
To break from a "for"
or "while"
loop, use the break
statement. If you want to break out of a certain number of levels
in a nested loop construct, add break n,
where n in the number of levels of nesting.
As an example, consider:
for i
do
while true
do
...
break 2
This "break 2" gets you beyond the second "done". It breaks you out of two enclosing loops (for and while).
done
done |
To continue with the next iteration of the enclosing
loop, use the continue
statement. To continue at the next iteration of the nth enclosing
loop, use continue n.
For more items, look in the sh(1)
entry in the HP-UX Reference manual. Some
of these features will be discussed in“Example: Groupcopy”.
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