# I/O redirection

This chapter describes more about the powerful UNIX mechanism of redirecting input, output and errors. Topics include:

- Standard input, output and errors

- Redirection operators

- How to use output of one command as input for another

- How to put output of a command in a file for later referrence

- How to append output of multiple commands to a file

- Input redirection

- Handling standard error messages

- Combining redirection of input, output and error streams

- Output filters

## Simple redirections

### What are standard input and standard output?

Most Linux commands read input, such as a file or another attribute for the command, and write output. By default, input is being given with the keyboard, and output is displayed on your screen. Your keyboard is your standard input (stdin) device, and the screen or a particular terminal window is the standard output (stdout) device.

input default input
output default output
input standard input
stdin
output standard output
stdout

However, since Linux is a flexible system, these default settings don't necessarily have to be applied. The standard output, for example, on a heavily monitored server in a large environment may be a printer.

### The redirection operators

#### Output redirection with > and |

Sometimes you will want to put output of a command in a file, or you may want to issue another command on the output of one command. This is known as redirecting output. Redirection is done using either the > (greater-than symbol), or using the | (pipe) operator which sends the standard output of one command to another command as standard input.

output redirection with >
greater-than
pipe
output redirection with |

As we saw before, the `cat` command concatenates files and puts them all together to the standard output. By redirecting this output to a file, this file name will be created - or overwritten if it already exists, so take care.

I/O redirection saving output in a file

    nancy:~> cat test1
    some words

    nancy:~> cat test2
    some other words

    nancy:~> cat test1 test2 > test3

    nancy:~> cat test3
    some words
    some other words

Don't overwrite!

Be careful not to overwrite existing (important) files when redirecting output. Many shells, including Bash, have a built-in feature to protect you from that risk: `noclobber`. See the Info pages for more information. In Bash, you would want to add the `set `-o` noclobber` command to your `.bashrc` configuration file in order to prevent accidental overwriting of files.

Bash noclobber
noclobber
.bashrc
I/O redirection prevent overwrite

Redirecting nothing to an existing file is equal to emptying the file:

I/O redirection emptying files

    nancy:~> ls -l list
    -rw-rw-r--    1 nancy   nancy     117 Apr  2 18:09 list

    nancy:~> > list

    nancy:~> ls -l list
    -rw-rw-r--    1 nancy   nancy       0 Apr  4 12:01 list

This process is called truncating.

truncating

The same redirection to an nonexistent file will create a new empty file with the given name:

    nancy:~> ls -l newlist
    ls: newlist: No such file or directory

    nancy:~> > newlist

    nancy:~> ls -l newlist
    -rw-rw-r--  1 nancy   nancy	    0 Apr  4 12:05 newlist

gives some more examples on the use of this sort of redirection.

Some examples using piping of commands:

piping examples

To find a word within some text, display all lines matching pattern1, and exclude lines also matching pattern2 from being displayed:

`grep pattern1 `file` | grep `-v` pattern2`

To display output of a directory listing one page at a time:

`ls `-la` | less`

To find a file in a directory:

`ls `-l` | grep part_of_file_name`

#### Input redirection

In another case, you may want a file to be the input for a command that normally wouldn't accept a file as an option. This redirecting of input is done using the < (less-than symbol) operator.

input redirection
less-than

Below is an example of sending a file to somebody, using input redirection.

    andy:~> mail mike@somewhere.org < to_do

If the user mike exists on the system, you don't need to type the full address. If you want to reach somebody on the Internet, enter the fully qualified address as an argument to `mail`.

This reads a bit more difficult than the beginner's `cat `file` | mail someone`, but it is of course a much more elegant way of using the available tools.

#### Combining redirections

The following example combines input and output redirection. The file `text.txt` is first checked for spelling mistakes, and the output is redirected to an error log file:

I/O redirection combining

`spell < `text.txt` > `error.log``

The following command lists all commands that you can issue to examine another file when using `less`:

    mike:~> less --help | grep -i examine
      :e [file]      Examine a new file.
      :n          *  Examine the (N-th) next file from the command line.
      :p          *  Examine the (N-th) previous file from the command line.
      :x          *  Examine the first (or N-th) file from the command line.

The `-i` option is used for case-insensitive searches - remember that UNIX systems are very case-sensitive.

If you want to save output of this command for future reference, redirect the output to a file:

    mike:~> less --help | grep -i examine > examine-files-in-less

    mike:~> cat examine-files-in-less
      :e [file]      Examine a new file.
      :n          *  Examine the (N-th) next file from the command line.
      :p          *  Examine the (N-th) previous file from the command line.
      :x          *  Examine the first (or N-th) file from the command line.

Output of one command can be piped into another command virtually as many times as you want, just as long as these commands would normally read input from standard input and write output to the standard output. Sometimes they don't, but then there may be special options that instruct these commands to behave according to the standard definitions; so read the documentation (man and Info pages) of the commands you use if you should encounter errors.

Again, make sure you don't use names of existing files that you still need. Redirecting output to existing files will replace the content of those files.

#### The >> operator

Instead of overwriting file data, you can also append text to an existing file using two subsequent greater-than signs:

I/O redirection append output

Example:

    mike:~> cat wishlist
    more money
    less work

    mike:~> date >> wishlist

    mike:~> cat wishlist
    more money
    less work
    Thu Feb 28 20:23:07 CET 2002

The `date` command would normally put the last line on the screen; now it is appended to the file `wishlist`.

## Advanced redirection features

### Use of file descriptors

There are three types of I/O, which each have their own identifier, called a file descriptor:

file descriptors overview

- standard input: 0

- standard output: 1

- standard error: 2

  standard error

In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is >, the redirection refers to the standard output (file descriptor 1).

Some practical examples will make this more clear:

file descriptors examples
`ls > `dirlist` 2>&1`

will direct both standard output and standard error to the file `dirlist`, while the command

`ls 2>&1 > `dirlist``

will only direct standard output to `dirlist`. This can be a useful option for programmers.

Things are getting quite complicated here, don't confuse the use of the ampersand here with the use of it in , where the ampersand is used to run a process in the background. Here, it merely serves as an indication that the number that follows is not a file name, but rather a location that the data stream is pointed to. Also note that the bigger-than sign should not be separated by spaces from the number of the file descriptor. If it would be separated, we would be pointing the output to a file again. The example below demonstrates this:

    [nancy@asus /var/tmp]$ ls 2> tmp

    [nancy@asus /var/tmp]$ ls -l tmp
    -rw-rw-r--  1 nancy nancy 0 Sept  7 12:58 tmp

    [nancy@asus /var/tmp]$ ls 2 > tmp
    ls: 2: No such file or directory

The first command that nancy executes is correct (eventhough no errors are generated and thus the file to which standard error is redirected is empty). The second command expects that `2` is a file name, which does not exist in this case, so an error is displayed.

All these features are explained in detail in the Bash Info pages.

### Examples

#### Analyzing errors

If your process generates a lot of errors, this is a way to thoroughly examine them:

file descriptors examples
`command 2>&1 | less`

This is often used when creating new software using the `make` command, such as in:

    andy:~/newsoft> make all 2>&1 | less
    --output ommitted--

#### Separating standard output from standard error

Constructs like these are often used by programmers, so that output is displayed in one terminal window, and errors in another. Find out which pseudo terminal you are using issuing the `tty` command first:

I/O redirection splitting stdout and stderr

    andy:~/newsoft> make all 2> /dev/pts/7

#### Writing to output and files simultaneously

You can use the `tee` command to copy input to standard output and one or more output files in one move. Using the `-a` option to `tee` results in appending input to the file(s). This command is useful if you want to both see and save output. The `>` and `>>` operators do not allow to perform both actions simultaneously.

tee
I/O redirection write to stdout and file

This tool is usually called on through a pipe (`|`), as demonstrated in the example below:

    mireille ~/test> date | tee file1 file2
    Thu Jun 10 11:10:34 CEST 2004

    mireille ~/test> cat file1
    Thu Jun 10 11:10:34 CEST 2004

    mireille ~/test> cat file2
    Thu Jun 10 11:10:34 CEST 2004

    mireille ~/test> uptime | tee -a file2
     11:10:51 up 21 days, 21:21, 57 users,  load average: 0.04, 0.16, 0.26

    mireille ~/test> cat file2
    Thu Jun 10 11:10:34 CEST 2004
     11:10:51 up 21 days, 21:21, 57 users,  load average: 0.04, 0.16, 0.26

## Filters

When a program performs operations on input and writes the result to the standard output, it is called a filter. One of the most common uses of filters is to restructure output. We'll discuss a couple of the most important filters below.

### More about grep

As we saw in , `grep` scans the output line per line, searching for matching patterns. All lines containing the pattern will be printed to standard output. This behavior can be reversed using the `-v` option.

grep I/O redirection

Some examples: suppose we want to know which files in a certain directory have been modified in February:

filtering grep

    jenny:~> ls -la | grep Feb

The `grep` command, like most commands, is case sensitive. Use the `-i` option to make no difference between upper and lower case. A lot of GNU extensions are available as well, such as `--colour`, which is helpful to highlight searchterms in long lines, and `--after-context`, which prints the number of lines after the last matching line. You can issue a recursive `grep` that searches all subdirectories of encountered directories using the `-r` option. As usual, options can be combined.

Regular expressions can be used to further detail the exact character matches you want to select out of all the input lines. The best way to start with regular expressions is indeed to read the `grep` documentation. An excellent chapter is included in the `grep` Info page. Since it would lead us too far discussing the ins and outs of regular expressions, it is strongly advised to start here if you want to know more about them.

regular expressions getting started

Play around a bit with `grep`, it will be worth the trouble putting some time in this most basic but very powerful filtering command. The exercises at the end of this chapter will help you to get started, see .

### Filtering output

The command `sort` arranges lines in alphabetical order by default:

sort
filtering sort

    thomas:~> cat people-I-like | sort
    Auntie Emmy
    Boyfriend
    Dad
    Grandma
    Mum
    My boss

But there are many more things `sort` can do. Looking at the file size, for instance. With this command, directory content is sorted smallest files first, biggest files last:

files size
sort file size
`ls `-la` | sort `-nk` 5`

Old sort syntax

You might obtain the same result with `ls `-la` | sort `+4n``, but this is an old form which does not comply with the current standards.

The `sort` command is also used in combination with the `uniq` program (or `sort `-u``) to sort output and filter out double entries:

sort unique

    thomas:~> cat itemlist
    1
    4
    2
    5
    34
    567
    432
    567
    34
    555

    thomas:~> sort itemlist | uniq
    1
    2
    34
    4
    432
    5
    555
    567

## Summary

In this chapter we learned how commands can be linked to each other, and how input from one command can be used as output for another command.

Input/output redirection is a common task on UNIX and Linux machines. This powerful mechanism allows flexible use of the building blocks UNIX is made of.

The most commonly used redirections are `>` and `|`. Refer to for an overview of redirection commands and other shell constructs.

New commands in chapter 5: I/O redirection

Command | Meaning
--- | ---
`date` | Display time and date information.
`set` | Configure shell options.
`sort` | Sort lines of text.
`uniq` | Remove duplicate lines from a sorted file.

## Exercises

These exercises give more examples on how to combine commands. The main goal is to try and use the Enter key as little as possible.

All exercises are done using a normal user ID, so as to generate some errors. While you're at it, don't forget to read those man pages!

- Use the `cut` command on the output of a long directory listing in order to display only the file permissions. Then pipe this output to `sort` and `uniq` to filter out any double lines. Then use the `wc` to count the different permission types in this directory.

- Put the output of `date` in a file. Append the output of `ls` to this file. Send this file to your local mailbox (don't specify anything @domain, just the user name will do). When using Bash, you will see a new mail notice upon success.

- List the devices in `/dev` which are currently used by your UID. Pipe through `less` to view them properly.

- Issue the following commands as a non-privileged user. Determine standard input, output and error for each command.

  - `cat `nonexistentfile``

  - `file `/sbin/ifconfig``

  - `grep root `/etc/passwd /etc/nofiles` > `grepresults``

  - `/etc/init.d/sshd start > `/var/tmp/output``

  - `/etc/init.d/crond start > `/var/tmp/output` 2>&1`

  Now check your results by issuing the commands again, now redirecting standardoutput to the file `/var/tmp/output` and standard error to the file `/var/tmp/error`.

- How many processes are you currently running?

- How many invisible files are in your home directory?

- Use `locate` to find documentation about the kernel.

- Find out which file contains the following entry:

      root:x:0:0:root:/root:/bin/bash

  And this one:

      system:		root

- See what happens upon issuing this command:

  `> time; date >> time; cat < time`

- What command would you use to check which script in `/etc/init.d` starts a given process?