Linux shell programming : input/output redirection

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input/output redirection

Linux system commands take input from our terminal and send the output back to our terminal. A command normally reads its input from standard input (terminal) by default. Similarly, a command writes its output to standard output (terminal) by default.

output redirection

The output from a command can be easily redirected to a file instead of a terminal:

$ ls > files

We didn't get any output on our terminal because the output has been redirected from the default standard output device (the terminal) into the specified file:

$ ls > files
$ cat files
arr2.sh
arr.sh
...
unknown_args.sh
v.sh

Since '>' overwrites, we may want to use '>>' to append to an existing file.

input redirection

Just as the output of a command can be redirected to a file, so can the input of a command be redirected from a file. As the greater-than character '>' is used for output redirection, the less-than character '>' is used to redirect the input of a command.

The commands that normally take their input from standard input can have their input redirected from a file in this manner. For example, to count the number of lines in the file users generated above, we can execute the command as follows:

$ wc -l file_list
16 files

We can count the number of lines in the 'files' by redirecting the standard input of the wc -l command:

$ wc -l < file_list
16

Note that there is a difference in the output produced by the two forms of the wc -l command. In the first case, the number of the files in the "file_list" is displayed with the line count; in the second case, it is not.

In the first case, wc knows that it is reading its input from the 'file_list'. In the second case, it only knows that it is reading its input from standard input so it does not display 'files'.

here document

A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.

COMMAND <<InputComesFromHERE
...
...
...
InputComesFromHERE

Following is the input to the command wc -l to count total number of line:

$ wc -l << EOF
> Whose woods these are I think I know.
> His house is in the village, though;
> He will not see me stopping here
> To watch his woods fill up with snow.
> My little horse must think it queer
> To stop without a farmhouse near
> Between the woods and frozen lake
> The darkest evening of the year.
> EOF
8

Another example that writes a shell script file using "cat" and the "here document":

$ cat <<EOF > myscript.sh
#!/bin/bash
echo $PWD
EOF

This creates "myscript.sh" and it looks like this:

#!/bin/bash
echo /Users/kihyuckhong/Documents/TEMP

Or easier form by switching EOF with file name:

$ cat > myscript.sh <<EOF 
#!/bin/bash
echo $PWD
EOF

Some people like to use '-' to indicate stdin/stdout (Usage of dash (-) in place of a filename):

$ cat - > myscript.sh <<EOF 
#!/bin/bash
echo $PWD
EOF

discarding output

When we execute a program, Linux opens three file descriptors for the program:

0 standard input (stdin)
1 standard output (stdout)
2 standard error (stderr)

The redirect output symbol (>) is shorthand for 1>, which tells the shell to redirect standard output. Similarly, < is short for 0<, which redirects standard input. The symbol 2> redirects standard error.

There are cases when we need to execute a command, but we don't want the output displayed to the screen. In such cases, we can discard the output by redirecting it to the file /dev/null which is a special file that automatically discards all its output:

$ command > /dev/null

Actually, stdout goes to /dev/null (which is a dummy device that does not record any output).

To discard both output of a command and its error, we want to redirect stderr to stdout:

$ command > /dev/null 2>&1

Here 2 represents stderr and 1 represents stdout. By specifying 2>&1 the stderr also goes to the stdout (which is already redirected to /dev/null).

We can display a message on to stderr by redirecting stdin into stderr as follows: