File Descriptors

The next few redirectors in Table 7-1 depend on the notion of a file descriptor. Like the device files used with <>, this is a low-level UNIX I/O concept that is of interest only to systems programmers—and then only occasionally. You can get by with a few basic facts about them; for the whole story, look at the entries for read( ), write( ), fcntl( ), and others in Section 2 of the UNIX manual. You might wish to refer to UNIX Power Tools by Shelley Powers, Jerry Peek, Tim O'Reilly, and Mike Loukides (O'Reilly).

File descriptors are integers starting at 0 that refer to particular streams of data associated with a process. When a process starts, it usually has three file descriptors open. These correspond to the three standards: standard input (file descriptor 0), standard output (1), and standard error (2). If a process opens additional files for input or output, they are assigned to the next available file descriptors, starting with 3.

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By far the most common use of file descriptors with bash is in saving standard error in a file. For example, if you want to save the error messages from a long job in a file so that they don't scroll off the screen, append 2> file to your command. If you also want to save standard output, append > file1 2> file2.

This leads to another programming task.

Task 7-2

You want to start a long job in the background (so that your terminal is freed up) and save both standard output and standard error in a single log file. Write a script that does this.

We'll call this script start. The code is very terse:

"$@" > logfile 2>&1 &

This line executes whatever command and parameters follow start. (The command cannot contain pipes or output redirectors.) It sends the command's standard output to logfile.

Then, the redirector 2>&1 says, "send standard error (file descriptor 2) to the same place as standard output (file descriptor 1)." Since standard output is redirected to logfile, standard error will go there too. The final & puts the job in the background so that you get your shell prompt back.

As a small variation on this theme, we can send both standard output and standard error into a pipe instead of a file: command 2>&1 | ... does this. (Make sure you understand why.) Here is a script that sends both standard output and standard error to the logfile (as above) and to the terminal:

"$@" 2>&1 | tee logfile &

The command tee takes its standard input and copies it to standard output and the file given as argument.

These scripts have one shortcoming: you must remain logged in until the job completes. Although you can always type jobs (see Chapter 1) to check on progress, you can't leave your terminal until the job finishes, unless you want to risk a breach of security.[1] We'll see how to solve this problem in the next chapter.

The other file-descriptor-oriented redirectors (e.g., <& n) are usually used for reading input from (or writing output to) more than one file at the same time. We'll see an example later in this chapter. Otherwise, they're mainly meant for systems programmers, as are <&- (force standard input to close) and >&- (force standard output to close).

Before we leave this topic, we should just note that 1> is the same as >, and 0< is the same as <. If you understand this, then you probably know all you need to know about file descriptors.

 

[1] Don't put it past people to come up to your unattended terminal and cause mischief!