There are always three default "files"
open, stdin (the keyboard),
stdout (the screen), and
stderr (error messages output to the
screen). These, and any other open files, can be redirected.
Redirection simply means capturing the output of a file, command,
program, script, or even code block within a script (see Example 3-2 and Example 3-3) and sending it as
input to another file, command, program, or script.
Each open file gets assigned a file descriptor.
[1]
The file descriptors for stdin,
stdout, and stderr are
0, 1, and 2, respectively. For opening additional files, there
remain descriptors 3 to 9. It is sometimes useful to assign one of
these additional file descriptors to stdin,
stdout, or stderr
as a temporary duplicate link.
[2]
This simplifies restoration to normal after complex redirection
and reshuffling (see Example 3-72).
>
# Redirect stdout to a file.
# Creates the file if not present, otherwise overwrites it.
ls -lR > dir-tree.list
# Creates a file containing a listing of the directory tree.
>>
# Redirect stdout to a file.
# Creates the file if not present, otherwise appends to it.
2> &1
# Redirects stderr to stdout.
# Has the effect of making visible error messages that might otherwise not be seen.
i> &j
# Redirects file descriptor i to j
# All output of file pointed to by i gets sent to file pointed to by j
<
# Accept input from a file.
# Companion command to ">", and often used in combination with it.
grep search-word <filename
|
# Pipe.
# General purpose process and command chaining tool.
# Similar to ">", but more general in effect.
# Useful for chaining commands, scripts, files, and programs together.
cat *.txt | sort | uniq > result-file
# Sorts the output of all the .txt files and deletes duplicate lines,
# finally saves results to "result-file". |
Note: Multiple instances of input and output redirection
and/or pipes can be combined in a single command
line.
command < input-file > output-file
command1 | command2 | command3 > output-file |
- n<&-
close input file descriptor
n
- <&-
close stdin
- n>&-
close output file descriptor n
- >&-
close stdout
The exec <filename command redirects
stdin to a file. From that point on, all
stdin comes from that file, rather than its
normal source (usually keyboard input). This provides a method of
reading a file line by line and possibly parsing each line of input
using sed and/or awk.
Example 3-72. Redirecting stdin using
exec
#!/bin/bash
# Redirecting stdin using 'exec'.
exec 6<&0 # Link file descriptor #6 with stdin.
exec < data-file # stdin replaced by file "data-file"
read a1 # Reads first line of file "data-file".
read a2 # Reads second line of file "data-file."
echo
echo "Following lines read from file."
echo "-------------------------------"
echo $a1
echo $a2
echo; echo; echo
exec 0<&6 # Now restore stdin from fd #6, where it had been saved.
echo -n "Enter data "
read b1 # Now "read" functions as expected, reading from normal stdin.
echo "Input read from stdin."
echo "----------------------"
echo "b1 = $b1"
echo
exit |
Blocks of code, such as while,
until, and for
loops, even if/then test blocks can also
incorporate redirection of stdin. The
< operator at the the end of the code block
accomplishes this.
Example 3-73. Redirected while loop
#!/bin/bash
if [ -z $1 ]
then
Filename=names.data # Default, if no filename specified.
else
Filename="$1"
fi
while [ "$name" != Smith ] # Why is variable $name in quotes?
do
read name # Reads from $Filename, rather than stdin.
echo $name
done <$Filename # Redirects stdin to file $Filename.
exit 0 |
Example 3-74. Redirected until loop
#!/bin/bash
# Same as previous example, but with "until" loop.
if [ -z $1 ]
then
Filename=names.data # Default, if no filename specified.
else
Filename="$1"
fi
# while [ "$name" != Smith ]
until [ "$name" = Smith ] # Change != to =.
do
read name # Reads from $Filename, rather than stdin.
echo $name
done <$Filename # Redirects stdin to file $Filename.
# Same results as with "while" loop in previous example.
exit 0 |
Example 3-75. Redirected for loop
#!/bin/bash
if [ -z $1 ]
then
Filename=names.data # Default, if no filename specified.
else
Filename="$1"
fi
line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target file.
# Very contrived and kludgy, nevertheless shows that
# it's possible to redirect stdin within a "for" loop...
# if you're clever enough.
for name in `seq $line_count` # Recall that "seq" prints sequence of numbers.
# while [ "$name" != Smith ] -- more complicated than a "while" loop --
do
read name # Reads from $Filename, rather than stdin.
echo $name
if [ "$name" = Smith ] # Need all this extra baggage here.
then
break
fi
done <$Filename # Redirects stdin to file $Filename.
exit 0 |
Example 3-76. Redirected if/then test
#!/bin/bash
if [ -z $1 ]
then
Filename=names.data # Default, if no filename specified.
else
Filename="$1"
fi
TRUE=1
if [ $TRUE ]
then
read name
echo $name
fi <$Filename
# Reads only first line of file.
# An if/then test has no way of iterating unless embedded in a loop.
exit 0 |
Clever use of I/O redirection permits parsing and stitching
together snippets of files and command output. One possible
application of this might be generating report and log
files.
Note: Here documents are a special case of
I/O redirection. See Section 3.24.
