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Here you notice that we use a new special variable called $1. This variable contains the first argument given to a program. Say we run<br>
smartzip articles.zip<br>
then $1 will contain the string articles.zip<br>
<br>
The select statement is a bash specific extension and is very good for interactive use. The user can select a choice from a list of different values:<br>
select var in ... ; do<br>
break<br>
done<br>
.... now $var can be used ....<br>
<br>
Here is an example:<br>
#!/bin/sh<br>
echo "What is your favourite OS?"<br>
select var in "linux" "Gnu Hurd" "Free BSD" "Other"; do<br>
break<br>
done<br>
echo "You have selected $var"<br>
<br>
Here is what the script does:<br>
What is your favourite OS?<br>
1) linux<br>
2) Gnu Hurd<br>
3) Free BSD<br>
4) Other<br>
#? 1<br>
You have selected linux<br>
<br>
In the shell you have the following loop statements available:<br>
while ...; do<br>
....<br>
done<br>
<br>
The while-loop will run while the expression that we test for is true. The keyword "break" can be used to leave the loop at any point in time. With the keyword "continue" the loop continues with the next iteration and skips the rest of the loop body.<br>
<br>
The for-loop takes a list of strings (strings separated by space) and assigns them to a variable:<br>
for var in ....; do<br>
....<br>
done<br>
<br>
The following will e.g. print the letters A to C on the screen:<br>
#!/bin/sh<br>
for var in A B C ; do<br>
echo "var is $var"<br>
done<br>
<br>
A more useful example script, called showrpm, prints a summary of the content of a number of RPM-packages:<br>
#!/bin/sh<br>
# list a content summary of a number of RPM packages<br>
# USAGE: showrpm rpmfile1 rpmfile2 ...<br>
# EXAMPLE: showrpm /cdrom/RedHat/RPMS/*.rpm<br>
for rpmpackage in $*; do<br>
if [ -r "$rpmpackage" ];then<br>
echo "=============== $rpmpackage =============="<br>
rpm -qi -p $rpmpackage<br>
else<br>
echo "ERROR: cannot read file $rpmpackage"<br>
fi<br>
done<br>
<br>
Above you can see the next special variable, $* which contains all the command line arguments. If you run<br>
showrpm openssh.rpm w3m.rpm webgrep.rpm<br>
then $* contains the 3 strings openssh.rpm, w3m.rpm and webgrep.rpm.<br>
<br>
The GNU bash knows until-loops as well but generally while and for loops are sufficient.<br>
<br>
Quoting<br>
Before passing any arguments to a program the shell tries to expand wildcards and variables. To expand means that the wildcard (e.g. *) is replaced by the appropriate file names or that a variable is replaced by its value. To change this behaviour you can use quotes: Let's say we have a number of files in the current directory. Two of them are jpg-files, mail.jpg and tux.jpg.<br>
#!/bin/sh<br>
echo *.jpg<br>
<br>
This will print "mail.jpg tux.jpg".<br>
Quotes (single and double) will prevent this wildcard expansion:<br>
#!/bin/sh<br>
echo "*.jpg"<br>
echo '*.jpg'<br>
<br>
This will print "*.jpg" twice.<br>
Single quotes are most strict. They prevent even variable expansion. Double quotes prevent wildcard expansion but allow variable expansion:<br>
#!/bin/sh<br>
echo $SHELL<br>
echo "$SHELL"<br>
echo '$SHELL'<br>
<br>
This will print:<br>
/bin/bash<br>
/bin/bash<br>
$SHELL<br>
<br>
Finally there is the possibility to take the special meaning of any single character away by preceeding it with a backslash:<br>
echo *.jpg<br>
echo $SHELL<br>
<br>
This will print:<br>
*.jpg<br>
$SHELL<br>
<br>
Here documents<br>
Here documents are a nice way to send several lines of text to a command. It is quite useful to write a help text in a script without having to put echo in front of each line. A "Here document" starts with << followed by some string that must also appear at the end of the here document. Here is an example script, called ren, that renames multiple files and uses a here document for its help text:<br>
#!/bin/sh<br>
# we have less than 3 arguments. Print the help text:<br>
if [ $# -lt 3 ] ; then<br>
cat <<HELP<br>
ren -- renames a number of files using sed regular expressions<br>
<br>
USAGE: ren 'regexp' 'replacement' files...<br>
<br>
EXAMPLE: rename all *.HTM files in *.html:<br>
ren 'HTM$' 'html' *.HTM<br>
<br>
HELP<br>
exit 0<br>
fi<br>
OLD="$1"<br>
NEW="$2"<br>
# The shift command removes one argument from the list of<br>
# command line arguments.<br>
shift<br>
shift<br>
# $* contains now all the files:<br>
for file in $*; do<br>
if [ -f "$file" ] ; then<br>
newfile=`echo "$file" | sed "s/${OLD}/${NEW}/g"`<br>
if [ -f "$newfile" ]; then<br>
echo "ERROR: $newfile exists already"<br>
else<br>
echo "renaming $file to $newfile ..."<br>
mv "$file" "$newfile"<br>
fi<br>
fi<br>
done<br>
<br>
This is the most complex script so far. Let's discuss it a little bit. The first if-statement tests if we have provided at least 3 command line parameters. (The special variable $# contains the number of arguments.) If not, the help text is sent to the command cat which in turn sends it to the screen. After printing the help text we exit the program. If there are 3 or more arguments we assign the first argument to the variable OLD and the second to the variable NEW. Next we shift the command line parameters twice to get the third argument into the first position of $*. With $* we enter the for loop. Each of the arguments in $* is now assigned one by one to the variable $file. Here we first test that the file really exists and then we construct the new file name by using find and replace with sed. The backticks are used to assign the result to the variable newfile. Now we have all we need: The old file name and the new one. This is then used with the command mv to rename the files.<br>
<br>
Functions<br>
As soon as you have a more complex program you will find that you use the same code in several places and also find it helpful to give it some structure. A function looks like this:<br>
functionname()<br>
{<br>
# inside the body $1 is the first argument given to the function<br>
# $2 the second ...<br>
body<br>
}<br>
<br>
You need to "declare" functions at the beginning of the script before you use them.<br>
<br>
Here is a script called xtitlebar which you can use to change the name of a terminal window. If you have several of them open it is easier to find them. The script sends an escape sequence which is interpreted by the terminal and causes it to change the name in the titlebar. The script uses a function called help. As you can see the function is defined once and then used twice:<br>
#!/bin/sh<br>
# vim: set sw=4 ts=4 et:<br>
<br>
help()<br>
{<br>
cat <<HELP<br>
xtitlebar -- change the name of an xterm, gnome-terminal or kde konsole<br>
<br>
USAGE: xtitlebar [-h] "string_for_titelbar"<br>
<br>
OPTIONS: -h help text<br>
<br>
EXAMPLE: xtitlebar "cvs"<br>
<br>
HELP<br>
exit 0<br>
}<br>
<br>
# in case of error or if -h is given we call the function help:<br>
[ -z "$1" ] && help<br>
[ "$1" = "-h" ] && help<br>
<br>
# send the escape sequence to change the xterm titelbar:<br>
echo -e "</FONT><br>
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