I/O redirection

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

One of the powers of the Unix command line is the use of input/output redirection and pipes.

This chapter explains redirection of input, output and error streams.

stdin, stdout, and stderr

The bash shell has three basic streams; it takes input from stdin (stream 0), it sends output to stdout (stream 1) and it sends error messages to stderr (stream 2) .

The drawing below has a graphical interpretation of these three streams.

The keyboard often serves as stdin, whereas stdout and stderr both go to the display. This can be confusing to new Linux users because there is no obvious way to recognize stdout from stderr. Experienced users know that separating output from errors can be very useful.

The next sections will explain how to redirect these streams.

output redirection

> stdout

stdout can be redirected to a file with a greater than sign. While scanning the line, the shell will see the > sign and will clear the file.

The > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1).

[student@linux ~]$ echo It is cold today!
It is cold today!
[student@linux ~]$ echo It is cold today! > winter.txt
[student@linux ~]$ cat winter.txt 
It is cold today!
[student@linux ~]$

Note that the bash shell effectively removes the redirection from the command line before argument 0 is executed. This means that in the case of this command:

echo hello > greetings.txt

the shell only counts two arguments (echo = argument 0, hello = argument 1). The redirection is removed before the argument counting takes place.

output file is erased

While scanning the line, the shell will see the > sign and will clear the file! Since this happens before resolving argument 0, this means that even when the command fails, the file will have been cleared!

[student@linux ~]$ cat winter.txt 
It is cold today!
[student@linux ~]$ zcho It is cold today! > winter.txt
-bash: zcho: command not found
[student@linux ~]$ cat winter.txt 
[student@linux ~]$

noclobber

Erasing a file while using > can be prevented by setting the noclobber option.

[student@linux ~]$ cat winter.txt 
It is cold today!
[student@linux ~]$ set -o noclobber
[student@linux ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[student@linux ~]$ set +o noclobber
[student@linux ~]$

overruling noclobber

The noclobber can be overruled with >|.

[student@linux ~]$ set -o noclobber
[student@linux ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[student@linux ~]$ echo It is very cold today! >| winter.txt
[student@linux ~]$ cat winter.txt 
It is very cold today!
[student@linux ~]$

>> append

Use >> to append output to a file.

[student@linux ~]$ echo It is cold today! > winter.txt
[student@linux ~]$ cat winter.txt 
It is cold today!
[student@linux ~]$ echo Where is the summer ? >> winter.txt
[student@linux ~]$ cat winter.txt 
It is cold today!
Where is the summer ?
[student@linux ~]$

error redirection

2> stderr

Redirecting stderr is done with 2>. This can be very useful to prevent error messages from cluttering your screen.

The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing 1> is the same as >.

[student@linux ~]$ find / > allfiles.txt 2> /dev/null
[student@linux ~]$

2>&1

To redirect both stdout and stderr to the same file, use 2>&1.

[student@linux ~]$ find / > allfiles_and_errors.txt 2>&1
[student@linux ~]$

Note that the order of redirections is significant. For example, the command

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error made a copy of the standard output before the standard output was redirected to dirlist.

output redirection and pipes

By default you cannot grep inside stderr when using pipes on the command line, because only stdout is passed.

student@linux:~$ rm file42 file33 file1201 | grep file42
rm: cannot remove ‘file42’: No such file or directory
rm: cannot remove ‘file33’: No such file or directory
rm: cannot remove ‘file1201’: No such file or directory

With 2>&1 you can force stderr to go to stdout. This enables the next command in the pipe to act on both streams.

student@linux:~$ rm file42 file33 file1201 2>&1 | grep file42
rm: cannot remove ‘file42’: No such file or directory

You cannot use both 1>&2 and 2>&1 to switch stdout and stderr.

student@linux:~$ rm file42 file33 file1201 2>&1 1>&2 | grep file42
rm: cannot remove ‘file42’: No such file or directory
student@linux:~$ echo file42 2>&1 1>&2 | sed 's/file42/FILE42/' 
FILE42

You need a third stream to switch stdout and stderr after a pipe symbol.

student@linux:~$ echo file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' 
file42
student@linux:~$ rm file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' 
rm: cannot remove ‘FILE42’: No such file or directory

joining stdout and stderr

The &> construction will put both stdout and stderr in one stream (to a file).

student@linux:~$ rm file42 &> out_and_err
student@linux:~$ cat out_and_err 
rm: cannot remove ‘file42’: No such file or directory
student@linux:~$ echo file42 &> out_and_err
student@linux:~$ cat out_and_err 
file42
student@linux:~$

input redirection

< stdin

Redirecting stdin is done with < (short for 0<).

[student@linux ~]$ cat < text.txt
one
two
[student@linux ~]$ tr 'onetw' 'ONEZZ' < text.txt
ONE
ZZO
[student@linux ~]$

<< here document

The here document (sometimes called here-is-document) is a way to append input until a certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or can be called with Ctrl-D.

[student@linux ~]$ cat <<EOF > text.txt
> one
> two
> EOF
[student@linux ~]$ cat text.txt 
one
two
[student@linux ~]$ cat <<brol > text.txt
> brel
> brol
[student@linux ~]$ cat text.txt 
brel
[student@linux ~]$

<<< here string

The here string can be used to directly pass strings to a command. The result is the same as using echo string | command (but you have one less process running).

student@linux~$ base64 <<< linux-training.be
bGludXgtdHJhaW5pbmcuYmUK
student@linux~$ base64 -d <<< bGludXgtdHJhaW5pbmcuYmUK
linux-training.be

See rfc 3548 for more information about base64.

confusing redirection

The shell will scan the whole line before applying redirection. The following command line is very readable and is correct.

cat winter.txt > snow.txt 2> errors.txt

But this one is also correct, but less readable.

2> errors.txt cat winter.txt > snow.txt

Even this will be understood perfectly by the shell.

< winter.txt > snow.txt 2> errors.txt cat

quick file clear

So what is the quickest way to clear a file ?

>foo

And what is the quickest way to clear a file when the noclobber option is set ?

>|bar

practice: input/output redirection

1. Activate the noclobber shell option.

2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file.

3. When listing all shell options, which character represents the noclobber option ?

4. Deactivate the noclobber option.

5. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line.

6. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input.

solution: input/output redirection