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STDIN
STDIN is used to ask for input from the keyboard. The important to keep in mind,
is that the entry ends with a newline value (\n) that normally
has to be removed from the variable. This can be accomplished
with chomp. The other feature is that any input
in Perl that uses '<>' (referred to as
the diamond operator) is automatically
copied to the Perl variable '$_'.
You can use the 'defined' command to determine if any entry
has been made. This is useful in a program loop.
while (<STDIN>) {
chomp; # this is the same as using chomp($_)
@all_lines . $_;
}
The following example is a Perl program that accepts filenames as parameters and outputs the contents of each file.
while (<>) {
print $_;
}
If the program was named 'printfiles.pl', you would enter the following to print the contents of file1 and file2.
printfiles.pl file1 file2
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STDOUT, print, printf
The command 'print' is used to write to standard output.
The print command is actually a function that takes a list of
arguements and returns a value, just like any other function.
print ((2+3), "hello");
# prints '5hello'
You can use the printf command if you are familiar with this C command.
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File handles - Open, Close
A filehandle is a variable name assigned to a disk file name. It is recommended
that filehandlers are always entered in only uppercase. Again,
be careful not to assign a name that is a protected perl name.
Examples to open a file...
open( CUSTOMER, "/usr/data/customer" );
# opens the file for reading
open( OUT, ">new_or_existing file" );
# opens for write & will create a new file if it doesn't exist
open( LOG, ">>/usr/logfile");
# this will open for write and append data
Example to close file...
close(CUSTOMER);
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Die
The die function is used to provide an error message if the open is not
successful. The variable '$!' contains the error
string. If the die statement ends with newline
'\n', it prevents the perl program name and line from printing.
open(LOG, ">>logfile") || die "Can't append to logfile, error = $!\n";
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Using File handles
Once a file is open, you can read or write to it just like using <STDIN> or <STDOUT>
The following reads the all of the lines from the file and prints each line.
open (EP, "/etc/passwd");
while (<EP>) {
chomp;
print "password line = $_\n";
}
The following
reads from IN file and writes contents to OUT
file.
open(IN, "/usr/file1") || die "can't open file, error = $!\n";
open(OUT, "/usr/newfile") || die "Can't create new file, error = $!\n";
while (<IN>) {
print OUT $_;
}
close(IN);
close(OUT);
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File Tests
Sometimes before opening a file, you may want to validate it does or doesn't
exist or check on the type of file. The syntax for performing
this is a "-" sign plus a single
test character plus the disk file name.
Example: -e "/etc/password"
would evaluate as true if the file existed. The following chart defines all of the test characters.
| Test character |
Meaning (for file or directory) |
Test character |
Meaning (for type specified) |
| -r |
is readable |
-f |
plain file |
| -R |
is readable by real user, not effective user |
-d |
directory |
| -w |
is writable |
-T |
text file |
| -W |
is writable by real user, not effective user |
-b |
binary file |
| -e |
exists |
-z |
file exists and has zero bytes |
| -s |
exists and has nonzero size |
-l |
symbolic link |
| -x |
is executable |
-s |
socket |
| -X |
is executable by real user, not effective user |
-p |
named pipe |
| -o |
is owned by user |
-c |
character special |
| -O |
is owned by real user, not effective user |
-t |
isatty() is true |
| -k |
sticky bit is set |
-M |
modifcation age in days |
| -u |
setuid |
-A |
Access age in days |
| -g |
setgid |
-C |
Inode modification in days |
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Format - Defining
The format command is a writing template to format the printed output.It consists of 3 steps.
- Define the format (usually set once)
- Loading the data into the variable portions of the format
- Invoking the format.
The define step is similiar to creating a subrouting. Note that it ends with a single period. Here is an example...
format MYFORMAT =
fieldline
value1, value2, value3
fieldline
value1, value2
Each fieldline may contain literal text or fieldholder '@'used
to insert a scarlar variable. The characters after the @ symbol
control the number of characters to print and how to justify.
| Character |
What it does |
| < |
left hand justify |
| > |
right hand justify |
| | |
center |
| #.# |
for numeric values |
| * |
Any number of characters |
format PHONES =
Phone Number List
Name: @<<<<<<<<<<<<<<<<<<<<<< Phone number: @>>>>>>>>>>>>
$name, $phone
Here is an example that allows for multiple lines
of text associated with a single line of text. Instead of the '@' symbol.
The '^' is used to signify multiline variable.
The '~~' at the beginning line suppressions
the output if there is no more text to print or repeats the
format as many times as necessary to print all data for the $comment variable.
format PEOPLE =
Name: @<<<<<<<<<<<<<<<<<<<<< Comment: ^<<<<<<<<<<<<<<<<<<<<<<<<
$name, $comment
~~ ^<<<<<<<<<<<<<<<<<<<<<<<<
$comment
To prevent a long report from continously printing, you need to create another
format routine where the name is the same as the detail section
and "_TOP" is appended. The variable
'$%' can be used to print the current
page number. The default length is 60 lines per page
and is stored in the variable '$='. The
following is an example of a top of form format and changes the default number of lines to 55.
$= = 30;
format PHONES_TOP =
Phone Number List - Page @<<
$%.
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Format - Using
The format statement is involved with a write function where
the expression for the write statement is the format name. The
following example reads a file named '/usr/source' with the following content...
Roger Petersen:630-941-9332
Scott Petersen:630-941-9338
Using the above format PHONES, the following will read the 'source' file
and create a new formatted file '/usr/phones.txt'.
open(PHONES, ">/usr/phones.txt") || die "Can't create new file, error = $!\n";
open(IN,"/usr/source") || die "Can't find file, error = $!\n";
while (<IN>) {
chomp;
($name, $phone) = split(/:/);
write (PHONES);
}
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Change directories - chdir
The function chdir() is used to change to a different directory.
The success of the change can be tested with die.
chdir("/etc") || die "Can't move to /etc, error = $!\n";
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Globbing
To list all files in a directory, the combination of putting the path name
in '< >'s and adding an '*'
does the trick. You can also use the glob function in place of the '< >'s.
@a = </etc*>;
@a = glob("/etc*");
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Removing a file
To remove a file, the function unlink is used.
unlink ("/usr/filename");
unlink <*.txt>; # remove all files ending with txt
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Renaming a file
The rename function is used to change the name of a file.
rename( $old, $new);
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Linking files
To link a new file name to an existing file name, use the link
function. To create a symbolic link, the function symlink is used.
link( $existing_file, $another_filename);
symlink( $existing_file, $another_filename);
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Change file permissions
The chmod function is used to the permissions of a file or directory.
chmod(0111, "myfile");
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Create/Delete directories
The functions mkdir or rmdir are used to create
or delete directories. The permission attributes can also be set when using the mkdir function.
mkdir("newdirectory", 0777);
rmdir("newdirectory");
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Change ownership
To change the owner and/or user group for a file, the chmod
function is used. The UID and GID must be the actual numeric values.
If you don't want to change both the UID and GID, use a '-1' for the UID or GID.
chmod(123, 2, "filename");
# UID is set to 123, GID is to 2 for the file "filename"
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