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Chapter 16. I/O Redirection

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 output from a file, command, program, script, or even code block within a script (see Example 3-1 and Example 3-2) 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 16-1).

      # 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.

   : > filename
      # The > truncates file "filename" to zero length.
      # If file not present, creates zero-length file (same effect as 'touch').
      # The : serves as a dummy placeholder, producing no output.

   > filename    
      # The > truncates file "filename" to zero length.
      # If file not present, creates zero-length file (same effect as 'touch').
      # (Same result as ": >", above, but this does not work with some shells.)

      # Redirect stdout to a file.
      # Creates the file if not present, otherwise appends to it.

      # Single-line redirection commands (affect only the line they are on):
      # --------------------------------------------------------------------

      # Redirect stdout to file "filename".
      # Redirect and append stdout to file "filename".
      # Redirect stderr to file "filename".
      # Redirect and append stderr to file "filename".
      # Redirect both stdout and stderr to file "filename".

      # Redirecting stdout, one line at a time.

      echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE
      echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE
      echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE
      echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."
      # These redirection commands automatically "reset" after each line.

      # Redirecting stderr, one line at a time.

      bad_command1 2>$ERRORFILE       #  Error message sent to $ERRORFILE.
      bad_command2 2>>$ERRORFILE      #  Error message appended to $ERRORFILE.
      bad_command3                    #  Error message echoed to stderr,
                                      #+ and does not appear in $ERRORFILE.
      # These redirection commands also automatically "reset" after each line.

      # Redirects stderr to stdout.
      # Error messages get sent to same place as standard output.

      # Redirects file descriptor i to j.
      # All output of file pointed to by i gets sent to file pointed to by j.

      # Redirects, by default, file descriptor 1 (stdout) to j.
      # All stdout 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

      # Open file "filename" for reading and writing, and assign file descriptor "j" to it.
      # If "filename" does not exist, create it.
      # If file descriptor "j" is not specified, default to fd 0, stdin.
      # An application of this is writing at a specified place in a file. 
      echo 1234567890 > File    # Write string to "File".
      exec 3<> File             # Open "File" and assign fd 3 to it.
      read -n 4 <&3             # Read only 4 characters.
      echo -n . >&3             # Write a decimal point there.
      exec 3>&-                 # Close fd 3.
      cat File                  # ==> 1234.67890
      # Random access, by golly.

      # 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".

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
See Example 12-28 and Example A-15.

Multiple output streams may be redirected to one file.

ls -yz >> command.log 2>&1
#  Capture result of illegal options "yz" in file "command.log."
#  Because stderr is redirected to the file,
#+ any error messages will also be there.

#  Note, however, that the following does *not* give the same result.
ls -yz 2>&1 >> command.log
#  Outputs an error message and does not write to file.

#  If redirecting both stdout and stderr,
#+ the order of the commands makes a difference.

Closing File Descriptors


Close input file descriptor n.

0<&-, <&-

Close stdin.


Close output file descriptor n.

1>&-, >&-

Close stdout.

Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it.

# Redirecting only stderr to a pipe.

exec 3>&1                              # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&-    # Close fd 3 for 'grep' (but not 'ls').
#              ^^^^   ^^^^
exec 3>&-                              # Now close it for the remainder of the script.

# Thanks, S.C.

For a more detailed introduction to I/O redirection see Appendix E.

16.1. Using exec

An 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 16-1. Redirecting stdin using exec

# Redirecting stdin using 'exec'.

exec 6<&0          # Link file descriptor #6 with stdin.
                   # Saves 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 "Following lines read from file."
echo "-------------------------------"
echo $a1
echo $a2

echo; echo; echo

exec 0<&6 6<&-
#  Now restore stdin from fd #6, where it had been saved,
#+ and close fd #6 ( 6<&- ) to free it for other processes to use.
# <&6 6<&-    also works.

echo -n "Enter data  "
read b1  # Now "read" functions as expected, reading from normal stdin.
echo "Input read from stdin."
echo "----------------------"
echo "b1 = $b1"


exit 0

Similarly, an exec >filename command redirects stdout to a designated file. This sends all command output that would normally go to stdout to that file.

Example 16-2. Redirecting stdout using exec

# reassign-stdout.sh


exec 6>&1           # Link file descriptor #6 with stdout.
                    # Saves stdout.

exec > $LOGFILE     # stdout replaced with file "logfile.txt".

# ----------------------------------------------------------- #
# All output from commands in this block sent to file $LOGFILE.

echo -n "Logfile: "
echo "-------------------------------------"

echo "Output of \"ls -al\" command"
ls -al
echo; echo
echo "Output of \"df\" command"

# ----------------------------------------------------------- #

exec 1>&6 6>&-      # Restore stdout and close file descriptor #6.

echo "== stdout now restored to default == "
ls -al

exit 0

Example 16-3. Redirecting both stdin and stdout in the same script with exec

# upperconv.sh
# Converts a specified input file to uppercase.


if [ ! -r "$1" ]     # Is specified input file readable?
  echo "Can't read from input file!"
  echo "Usage: $0 input-file output-file"
fi                   #  Will exit with same error
                     #+ even if input file ($1) not specified (why?).

if [ -z "$2" ]
  echo "Need to specify output file."
  echo "Usage: $0 input-file output-file"
  exit $E_WRONG_ARGS

exec 4<&0
exec < $1            # Will read from input file.

exec 7>&1
exec > $2            # Will write to output file.
                     # Assumes output file writable (add check?).

# -----------------------------------------------
    cat - | tr a-z A-Z   # Uppercase conversion.
#   ^^^^^                # Reads from stdin.
#           ^^^^^^^^^^   # Writes to stdout.
# However, both stdin and stdout were redirected.
# -----------------------------------------------

exec 1>&7 7>&-       # Restore stout.
exec 0<&4 4<&-       # Restore stdin.

# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."

exit 0

I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem.

Example 16-4. Avoiding a subshell

# avoid-subshell.sh
# Suggested by Matthew Walker.



cat myfile.txt | while read line;
                 do {
                   echo $line
                   (( Lines++ ));  #  Incremented values of this variable
                                   #+ inaccessible outside loop.
                                   #  Subshell problem.

echo "Number of lines read = $Lines"     # 0
                                         # Wrong!

echo "------------------------"

exec 3<> myfile.txt
while read line <&3
do {
  echo "$line"
  (( Lines++ ));                   #  Incremented values of this variable
                                   #+ accessible outside loop.
                                   #  No subshell, no problem.
exec 3>&-

echo "Number of lines read = $Lines"     # 8


exit 0

# Lines below not seen by script.

$ cat myfile.txt

Line 1.
Line 2.
Line 3.
Line 4.
Line 5.
Line 6.
Line 7.
Line 8.



A file descriptor is simply a number that the operating system assigns to an open file to keep track of it. Consider it a simplified version of a file pointer. It is analogous to a file handle in C.


Using file descriptor 5 might cause problems. When Bash creates a child process, as with exec, the child inherits fd 5 (see Chet Ramey's archived e-mail, SUBJECT: RE: File descriptor 5 is held open). Best leave this particular fd alone.

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