Input/Output Redirection

The Korn shell provides a number of operators than can be used to manipulate command input/output and files. For example, you can save command output to a file. Or instead of typing in the input to a command, it can be taken from a file. The following sections describe some of the features of Korn shell I/O redirection.

Redirecting Standard Output

The standard output of a command is by default directed to your terminal. By using the > symbol, the standard output of a command can be redirected to a file, instead of your terminal. In this example, the standard output of the echo command is put into the file p.out:

						$ echo "Hello" > p.out
						$ cat p.out
						Hello
					

If the file doesn't exist, then it is created. Otherwise, the contents are usually overwritten, usually, but not always. If you have the noclobber option set (discussed in the next section), or the file attributes (permissions and/or ownership) do not permit writing, the file will not be overwritten. If we change permission on p.out and try to direct output to it again, we get an error message:

						$ chmod 444 p.out
						$ echo "Hello again" > pwd.out
						/bin/ksh: p.out: cannot create
					

We'll see in the next section how to force overwriting of existing files using a variation of the > redirect operator.

You can also use the > command to create an empty file:

						$ > tmp
						$ ls —s tmp
						0 tmp
					

This is equivalent to touch tmp.

Standard output can be appended to a file by using the >> redirect operator. Here, the output of find is appended to deadfiles.out:

						$ echo "Dead File List" > junk.out
						$ find . —name dead.letter —print >>junk.out
						$ find . —name core —print >>junk.out
						$ cat junk.out
						Dead File List
						./mail/dead.letter
						./bin/core
					

Standard output is closed with the >&– redirect operator:

						$ echo "This is going nowhere" >&—
						$
					

This feature can be used in a Korn shell script to close the output of a group of commands instead of redirecting the output of each command individually.

The noclobber Option

To prevent redirecting output to an existing file, set the noclobber option. By default, this feature is usually disabled, but can be enabled with the set –o noclobber command:

							$ ls > file.out
							$ set —o noclobber
						

Now when we try to redirect output to file.out, an error message is returned:

							$ ls > file.out
							/bin/ksh: file.out: file already exists
						

The >| operator is used to force overwriting of a file, even if the noclobber option is enabled. Here, file.out can now be overwritten, even though the noclobber option is set:

							$ ls >| file.out
						

Redirecting Standard Input

Standard input to a command is redirected from a file using the < operator. This feature can be used to read in a mail message from a file, instead of typing it in:

						$ mail dick jane spot < mlist
					

This could also be implemented using a pipe:

						$ cat mlist | mail dick jane spot
					

In both cases, file mlist becomes the standard input to mail.

Standard input can be closed with the <&– redirect operator. Here, the standard input to the wc command is closed:

						$ cat mlist | wc —l <&—
						0
					

Not really exciting. This is useful when manipulating file descriptors with the exec command, which is discussed later in Chapter 8.

File Descriptors

File descriptors are used by processes to identify their open files. The Korn shell automatically assigns file descriptor 0 to standard input for reading, file descriptor 1 to standard output for writing, and file descriptor 2 to standard error for reading and writing. The file descriptors 3 through 9 can be used with I/O redirection operators and are opened, closed, and/or copied with the exec command, which is discussed later in Chapter 8.

Redirecting Standard Error

Most Unix commands write their error messages to standard error. As with standard output, standard error is by default directed to your terminal, but it can be redirected using the standard error file descriptor (2) and the > operator. For example, the ls command displays a message on standard error when you attempt to display information about a non-existent file:

							$ ls tmp
							tmp not found
						

Now, if you do an ls on an existent and non-existent file on the same command-line, a message about the non-existent file goes to standard error, while the output for the existent file goes to standard output:

							$ ls tmp t.out
							tmp not found
							t.out
						

By using the 2> operator, the standard error of the same command is redirected to ls.out. The standard output is still displayed directly to your terminal:

							$ ls tmp t.out 2>ls.out
							t.out
							$ cat ls.out
							tmp not found
						

There can be no space between the 2 and > symbol, otherwise the 2 is interpreted as an argument to the command.

More with File Descriptors

The >&n operator causes output to be redirected to file descriptor n. This is used when you want to direct output to a specific file descriptor. To send the output of the echo command to standard error, just add >&2 to the command-line:

							$ echo This is going to standard error >&2
							This is going to standard error
						

In the next command, the standard error and output are sent to ls.out by specifying multiple redirections on the same command line. First, >&2 causes standard output to be redirected to standard error. Then, 2>ls.out causes standard error (which is now standard output and standard error) to be sent to ls.out:

							$ ls tmp t.out 2>ls.out 1>&2
							$ cat ls.out
							tmp not found
							t.out
						

This command causes output to be redirected to both standard output and standard error:

							$ { echo "This is going to stdout" >&1 ; 
							echo "This is going to stderr" >&2 ; }
							This is going to stdout
							This is going to stderr
						

If the output of the last command was redirected to a file using the > operator, then only the standard output would be redirected. The standard error would still be displayed on your terminal.

							$ { echo "This is going to stdout" >&1 ; 
							echo "This is going to stderr" >&2 ; } >out
							This is going to stderr
							$ cat out
							This is going to stdout
						

The n>&m operator causes the output from file descriptors n and m to be merged. This operator could be used in the previous command to direct both the standard output and standard error to the same file.

							$ { echo "This is going to stdout" >&1 ; 
							echo "This is going to stderr" >&2 ; } >out 2>&1
							$ cat out
							This is going to stdout
							This is going to stderr
						

If you wanted the standard output and standard error appended to the output file, the >> operator could be used:

							$ { echo "This is going to stdout again" >&1 ; 
							echo "This is going to stderr again" >&2 ; } 
							>>out 2>&1
							$ cat out
							This is going to stdout
							This is going to stderr
							This is going to stdout again
							This is going to stderr again
						

As seen in the previous example, multiple file descriptor redirections can also be specified. To close the standard output and standard error of this ls command:

							$ ls tmp t.out >&— 2>&—
							$
						

The print and read commands have special options that allow you to redirect standard output and input with file descriptors. This is discussed in Chapter 8.

Here Documents

Here documents is a term used for redirecting multiple lines of standard input to a command. In looser terms, they allow batch input to be given to programs and are frequently used to execute interactive commands from within Korn shell scripts. The format for a here document is:

						command <<word
					

or

						command
						<<–word
					

where lines that follow are used as standard input to command until word is read. In the following example, standard input for the cat command is read until the word END is encountered:

						$ cat >> .profile <<END
						> export TERM=sun-cmd
						> export ORACLE_HOME=/apps/oracle
						> END
					

The other variation, command <<– word, deletes leading tab characters from the here document. It is often used over the first variation to produce more readable code when used within larger scripts. A good use for here documents is to automate ftp file transfers. This snippet of code could be used within a larger script to automatically ftp code to or from another server.

						$ ftp <<- END
						open aspsun
						user anonymous
						cd /usr/pub
						binary
						get ksh.tar.Z
						quit
						END
					

Here Documents and File Descriptors

Here documents can also be used with file descriptors. Instead of reading multiple lines from standard input, multiple lines are read from file descriptor n until word is read.

							command n<<word
						

or

							command n<<–word
						

Discarding Standard Output

The /dev/null file is like the system trash bin. Anything redirected to it is discarded by the system.

						$ ls >/dev/null
					

This is not the same as:

						$ ls >&—
					

which closes standard output.