Output Redirection

In all of the Unix commands up to this point that produced outputsuch as man and lsthe command output appears in the Terminal window. This is called the standard output device of Unix.

But the shell can also redirect the output of a command to a file instead of to the screen. This output redirection enables you to create files by writing command output to a file.

Output redirection uses the greater-than character (>) to tell the shell to place the output of a command into a file rather than listing it to the screen. If the output file already exists, it is overwritten with the new information.

Similarly, a pair of greater-than signs (>>) tells the shell to append the output of a command to the end of a file rather than erasing the file and starting from the beginning. If the output file does not already exist, the shell creates a new file with the name you specified.

This section offers some examples of output redirection.

To sort a file & output it to another file

Type sort file > output-file and press .

For example, sort sample.txt > alpha.txt would sort the lines in the file named sample.txt and write them to a file named alpha.txt.

To save a directory listing as a file

Type ls > output-file and press .

For example, ls -la > list.txt creates a file named list.txt that contains a complete directory listing in the long format (Figure 49).

Figure 49. This example shows how the `ls` command can be used to save a directory listing as a text file. The `cat` command was used in the illustration to display the contents of the new file.

To append output to an existing file

Type command >> output-file and press .

For example, ls -la Documents >> list.txt would append a directory listing for the Documents subdirectory to the list.txt file (Figure 50).

Figure 50. This example uses >> to append another directory to the one in Figure 49 and display the combined files with the `cat` command.

To create a text file with cat

Type cat > output-file (for example, cat > test.txt) and press . The cat command starts and waits for you to type text (Figure 51).

Figure 51. Using the cat command with an output file name starts `cat` and waits for text entry.

Enter the text you want to include in the file. You can press to start a new line if desired (Figure 52).

Figure 52. Enter the text you want to include in the file.

When you're finished entering text, press (Figure 53). ( is the ASCII "End Of Transmission" character.)

Figure 53. Press to save the file.

The new file is saved with the name you specified.

Tip

Normally, the cat command uses a source-file argument; that is, you normally tell cat to list a specific file to the screen. If you do not specify a source-file for cat, it takes source data from the standard input device, which is usually the keyboard, and redirects it to the output-file.

To combine files with cat

Type cat file1 ... > output-file and press .

For example, cat firstfile.txt secondfile.txt thirdfile.txt > combinedfile.txt combines the files named firstfile.txt, secondfile.txt, and thirdfile.txt, in that order, and saves them as a file named combinedfile.txt.