Nov 262012
 Posted by on November 26, 2012 at 12:24 pm FAQ, Linux Tagged with:  Add comments

Many of the programs that we use in linux command line produce output of some kind. This output often consists of two types. First, we have the program’s results; that is, the data the program is designed to produce. Second, we have status and error messages that tell us how the program is getting along.

If we look at a command like ls, we can see that it displays its results and its error messages on the screen. Keeping with the Unix theme of “everything is a file,” programs such as ls actually send their results to a special file called standard output (often expressed as stdout) and their status messages to another file called standard error (stderr). By default, both standard output and standard error are linked to the screen and not saved into a disk file.

In addition, many programs take input from a facility called standard input (stdin), which is, by default, attached to the keyboard. I/O redirection allows us to change where output goes and where input comes from. Normally, output goes to the screen and input comes from the keyboard, but with I/O redirection we can change that.

Redirecting Standard Output

I/O redirection allows us to redefine where standard output goes. To redirect standard output to another file instead of the screen, we use the > redirection operator followed by the name of the file. Why would we want to do this? It’s often useful to store the output of a command in a file. For example, we could tell the shell to send the output of the ls command to the file ls-output.txt instead of the screen:

[[email protected] ~]$ ls -l /usr/bin > ls-output.txt

Here, we created a long listing of the /usr/bin directory and sent the results to the file ls-output.txt. Let’s examine the redirected output of the command:

[[email protected] ~]$ ls -l ls-output.txt

-rw-rw-r– 1 me me 167878 2012-02-01 15:07 ls-output.txt

Good—a nice, large, text file. If we look at the file with less, we will see that the file ls-output.txt does indeed contain the results from our ls command:

[[email protected] ~]$ less ls-output.txt

Now, let’s repeat our redirection test but this time with a twist. We’ll change the name of the directory to one that does not exist:

[[email protected] ~]$ ls -l /bin/usr > ls-output.txt

ls: cannot access /bin/usr: No such file or directory

We received an error message. This makes sense because we specified the nonexistent directory /bin/usr, but why was the error message displayed on the screen rather than being redirected to the file ls-output.txt ? The answer is that the ls program does not send its error messages to standard output. Instead, like most well-written Unix programs, it sends its error messages to standard error. Since we redirected only standard output and not standard error, the error message was still sent to the screen. We’ll see how to redirect standard error in just a minute, but first, let’s look at what happened to our output file:

[[email protected] ~]$ ls -l ls-output.txt

-rw-rw-r– 1 me me 0 2012-02-01 15:08 ls-output.txt

The file now has zero length! This is because, when we redirect output with the > redirection operator, the destination file is always rewritten from the beginning. Since our ls command generated no results and only an error message, the redirection operation started to rewrite the file and then stopped because of the error, resulting in its truncation. In fact, if we ever need to actually truncate a file (or create a new, empty file) we can use a trick like this:

[[email protected] ~]$ > ls-output.txt

Simply using the redirection operator with no command preceding it will truncate an existing file or create a new, empty file. So, how can we append redirected output to a file instead of overwriting the file from the beginning? For that, we use the >> redirection operator, like so:

[[email protected] ~]$ ls -l /usr/bin >> ls-output.txt

Using the >> operator will result in the output being appended to the file. If the file does not already exist, it is created just as though the > operator had been used. Let’s put it to the test:

[[email protected] ~]$ ls -l /usr/bin >> ls-output.txt

[[email protected] ~]$ ls -l /usr/bin >> ls-output.txt

[[email protected] ~]$ ls -l /usr/bin >> ls-output.txt

[[email protected] ~]$ ls -l ls-output.txt

-rw-rw-r– 1 me me 503634 2012-02-01 15:45 ls-output.txt

We repeated the command three times, resulting in an output file three times as large.

Redirecting Standard Error

Redirecting standard error lacks the ease of using a dedicated redirection operator. To redirect standard error we must refer to its file descriptor. A program can produce output on any of several numbered file streams. While we have referred to the first three of these file streams as standard input, output, and error, the shell references them internally as file descriptors 0, 1, and 2, respectively. The shell provides a notation for redirecting files using the file descriptor number. Since standard error is the same as file descriptor 2, we can redirect standard error with this notation:

[[email protected] ~]$ ls -l /bin/usr 2> ls-error.txt

The file descriptor 2 is placed immediately before the redirection operator to perform the redirection of standard error to the file ls-error.txt.

Redirecting Standard Output and Standard Error to One File

There are cases in which we may wish to capture all of the output of a command to a single file. To do this, we must redirect both standard output and standard error at the same time. There are two ways to do this. First, here is the traditional way, which works with old versions of the shell:

[[email protected] ~]$ ls -l /bin/usr > ls-output.txt 2>&1

Using this method, we perform two redirections. First we redirect standard output to the file ls-output.txt, and then we redirect file descriptor 2 (standard error) to file descriptor 1 (standard output) using the notation 2>&1.

Note: Notice that the order of the redirections is significant. The redirection of standard error must always occur after redirecting standard output or it doesn’t work. In the example above, > ls-output.txt 2>&1 redirects standard error to the file ls-output.txt, but if the order is changed to 2>&1 > ls-output.txt, standard error is directed to the screen. Recent versions of bash provide a second, more streamlined method for performing this combined redirection:

[[email protected] ~]$ ls -l /bin/usr &> ls-output.txt

In this example, we use the single notation &> to redirect both standard output and standard error to the file ls-output.txt.

Disposing of Unwanted Output

Sometimes we don’t want output from a command we just want to throw it away. This applies particularly to error and status messages. The system provides a way to do this by redirecting output to a special file called /dev/null. This file is a system device called a bit bucket, which accepts input and does nothing with it. To suppress error messages from a command, we do this:

[[email protected] ~]$ ls -l /bin/usr 2> /dev/null

Redirecting Standard Input

Up to now, we haven’t encountered any commands that make use of standard input (actually we have, but we’ll reveal that surprise a little bit later), so we need to introduce one.

cat—Concatenate Files

The cat command reads one or more files and copies them to standard output like so:

cat [file…]

In most cases, you can think of cat as being analogous to the TYPE command in DOS. You can use it to display files without paging. For example,

[[email protected] ~]$ cat ls-output.txt

will display the contents of the file ls-output.txt. cat is often used to display short text files. Since cat can accept more than one file as an argument, it can also be used to join files together. Say we have downloaded a large file that has been split into multiple parts (multimedia files are often split this way on Usenet), and we want to join them back together. If the files were named

movie.mpeg.001 movie.mpeg.002 … movie.mpeg.099

we could rejoin them with this command:

[[email protected] ~]$ cat movie.mpeg.0* > movie.mpeg

Since wildcards always expand in sorted order, the arguments will be arranged in the correct order. This is all well and good, but what does this have to do with standard input? Nothing yet, but let’s try something else. What happens if we enter

cat with no arguments?

[[email protected] ~]$ cat

Nothing happens—it just sits there like it’s hung. It may seem that way, but it’s really doing exactly what it’s supposed to. If cat is not given any arguments, it reads from standard input, and since standard input is, by default, attached to the keyboard, it’s waiting for us to type something!

Try this:

[[email protected] ~]$ cat

The quick brown fox jumped over the lazy dog.

Next, type CTRL-D (i.e., hold down the CTRL key and press D) to tell cat that it has reached end-of-file (EOF) on standard input:

[[email protected] ~]$ cat

The quick brown fox jumped over the lazy dog.

The quick brown fox jumped over the lazy dog.

In the absence of filename arguments, cat copies standard input to standard output, so we see our line of text repeated. We can use this behavior to create short text files. Let’s say that we wanted to create a file called lazy_dog.txt containing the text in our example. We would do this:

[[email protected] ~]$ cat > lazy_dog.txt

The quick brown fox jumped over the lazy dog.

Enter the command followed by the text we want to place in the file. Remember to type CTRL-D at the end. Using the command line, we have implemented the world’s dumbest word processor! To see our results, we can use cat to copy the file to tandard output again:

[[email protected] ~]$ cat lazy_dog.txt

The quick brown fox jumped over the lazy dog.

Now that we know how cat accepts standard input in addition to filename arguments, let’s try redirecting standard input:

[[email protected] ~]$ cat < lazy_dog.txt

The quick brown fox jumped over the lazy dog.

Using the < redirection operator, we change the source of standard input from the keyboard to the file lazy_dog.txt. We see that the result is the same as passing a single filename argument. This is not particularly useful compared to passing a filename argument, but it serves to demonstrate using a file as a source of standard input. Other commands make better use of standard input


The ability of commands to read data from standard input and send to standard output is utilized by a shell feature called pipelines. Using the pipe operator | (vertical bar), the standard output of one command can be piped into the standard input of another.

command1 | command2.

We can use less to display, page by page, the output of any command that sends its results to standard output:

[[email protected] ~]$ ls -l /usr/bin | less

This is extremely handy! Using this technique, we can conveniently examine the output of any command that produces standard output.


Pipelines are often used to perform complex operations on data. It is possible to put several commands together into a pipeline. Frequently, the commands used this way are referred to as filters. Filters take input, change it somehow, and then output it. The first one we will try is sort. Imagine we want to make a combined list of all of the executable programs in /bin and /usr/bin, put them in sorted order, and then view the list:

[[email protected] ~]$ ls /bin /usr/bin | sort | less

Since we specified two directories (/bin and /usr/bin), the output of ls would have consisted of two sorted lists, one for each directory. By including sort in our pipeline, we changed the data to produce a single, sorted list.

uniq—Report or Omit Repeated Lines

The uniq command is often used in conjunction with sort. uniq accepts a sorted list of data from either standard input or a single filename argument (see the uniq man page for details) and, by default, removes any duplicates from the list. So, to make sure our list has no duplicates (that is, any programs of the same name that appear in both the /bin and /usr/bin directories) we will add uniq to our pipeline:

[[email protected] ~]$ ls /bin /usr/bin | sort | uniq | less

In this example, we use uniq to remove any duplicates from the output of the sort command. If we want to see the list of duplicates instead, we add the -d option to uniq like so:

[[email protected] ~]$ ls /bin /usr/bin | sort | uniq -d | less

wc—Print Line, Word, and Byte Counts

The wc (word count) command is used to display the number of lines, words, and bytes contained in files. For example:

[[email protected] ~]$ wc ls-output.txt

7902 64566 503634 ls-output.txt

In this case it prints out three numbers: lines, words, and bytes contained in ls-output.txt. Like our previous commands, if executed without command-line arguments, wc accepts standard input. The -l option limits its output to only report lines. Adding it to a pipeline is a handy way to count things. To see the number of items we have in our sorted list, we can do this:

[[email protected] ~]$ ls /bin /usr/bin | sort | uniq | wc -l


grep—Print Lines Matching a Pattern

grep is a powerful program used to find text patterns within files, like this:

grep pattern [file…]

When grep encounters a “pattern” in the file, it prints out the lines containing it. The patterns that grep can match can be very complex, but for now we will concentrate on simple text matches. We’ll cover the advanced patterns, called regular expressions

Let’s say we want to find all the files in our list of programs that have the word zip in the name. Such a search might give us an idea of which programs on our system have something to do with file compression. We would do this:

[[email protected] ~]$ ls /bin /usr/bin | sort | uniq | grep zip












There are a couple of handy options for grep: -i, which causes grep to ignore case when performing the search (normally searches are case sensitive) and -v, which tells grep to print only lines that do not match the pattern.

head/tail—Print First/Last Part of Files

Sometimes you don’t want all the output from a command. You may want only the first few lines or the last few lines. The head command prints the first 10 lines of a file, and the tail command prints the last 10 lines. By default, both commands print 10 lines of text, but this can be adjusted with the -n option:

[[email protected] ~]$ head -n 5 ls-output.txt

total 343496

-rwxr-xr-x 1 root root 31316 2011-12-05 08:58 [

-rwxr-xr-x 1 root root 8240 2011-12-09 13:39 411toppm

-rwxr-xr-x 1 root root 111276 2011-11-26 14:27 a2p

-rwxr-xr-x 1 root root 25368 2010-10-06 20:16 a52dec

[[email protected] ~]$ tail -n 5 ls-output.txt

-rwxr-xr-x 1 root root 5234 2011-06-27 10:56 znew

-rwxr-xr-x 1 root root 691 2009-09-10 04:21

-rw-r–r– 1 root root 930 2011-11-01 12:23 zonetab2pot.pyc

-rw-r–r– 1 root root 930 2011-11-01 12:23 zonetab2pot.pyo

lrwxrwxrwx 1 root root 6 2012-01-31 05:22 zsoelim -> soelim

These can be used in pipelines as well:

[[email protected] ~]$ ls /usr/bin | tail -n 5





tail has an option that allows you to view files in real time. This is useful for watching the progress of log files as they are being written. In the following example, we will look at the messages file in /var/log. Superuser privileges are required to do this on some Linux distributions, because the /var/log/messages file may contain security information.

[[email protected] ~]$ tail -f /var/log/messages

Feb 8 13:40:05 twin4 dhclient: DHCPACK from

Feb 8 13:40:05 twin4 dhclient: bound to — renewal in 1652


Feb 8 13:55:32 twin4 mountd[3953]: /var/NFSv4/musicbox exported to both and twin7.localdomain in,twin7.localdomain

Feb 8 14:07:37 twin4 dhclient: DHCPREQUEST on eth0 to port 67

Feb 8 14:07:37 twin4 dhclient: DHCPACK from

Feb 8 14:07:37 twin4 dhclient: bound to — renewal in 1771


Feb 8 14:09:56 twin4 smartd[3468]: Device: /dev/hda, SMART Prefailure

Attribute: 8 Seek_Time_Performance changed from 237 to 236

Feb 8 14:10:37 twin4 mountd[3953]: /var/NFSv4/musicbox exported to both and twin7.localdomain in,twin7.localdomain

Feb 8 14:25:07 twin4 sshd(pam_unix)[29234]: session opened for user me by


Feb 8 14:25:36 twin4 su(pam_unix)[29279]: session opened for user root by


Using the -f option, tail continues to monitor the file and when new lines are appended, they immediately appear on the display. This continues until you type CTRL-C.

tee—Read from Stdin and Output to Stdout and Files

In keeping with our plumbing analogy, Linux provides a command called tee which creates a “T” fitting on our pipe. The tee program reads standard input and copies it to both standard output (allowing the data to continue down the pipeline) and to one or more files. This is useful for capturing a pipeline’s contents at an intermediate stage of processing. Here we repeat one of our earlier examples, this time including tee to capture the entire directory listing to the file ls.txt before grep filters the pipeline’s contents:

[[email protected] ~]$ ls /usr/bin | tee ls.txt | grep zip












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