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 4-1 and Example 4-2) 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).
1 > 2 # Redirect stdout to a file. 3 # Creates the file if not present, otherwise overwrites it. 4 5 ls -lR > dir-tree.list 6 # Creates a file containing a listing of the directory tree. 7 8 : > filename 9 # The > truncates file "filename" to zero length. 10 # The : serves as a dummy placeholder, producing no output. 11 12 >> 13 # Redirect stdout to a file. 14 # Creates the file if not present, otherwise appends to it. 15 16 2>&1 17 # Redirects stderr to stdout. 18 # Error messages get sent to same place as standard output. 19 20 i>&j 21 # Redirects file descriptor i to j. 22 # All output of file pointed to by i gets sent to file pointed to by j. 23 24 >&j 25 # Redirects, by default, file descriptor 1 (stdout) to j. 26 # All stdout gets sent to file pointed to by j. 27 28 0< 29 < 30 # Accept input from a file. 31 # Companion command to ">", and often used in combination with it. 32 # 33 # grep search-word <filename 34 35 36 [j]<>filename 37 # Open file "filename" for reading and writing, and assign file descriptor "j" to it. 38 # If "filename" does not exist, create it. 39 # If file descriptor "j" is not specified, default to fd 0, stdin. 40 # 41 # An application of this is writing at a specified place in a file. 42 echo 1234567890 > File # Write string to "File". 43 exec 3<> File # Open "File" and assign fd 3 to it. 44 read -n 4 <&3 # Read only 4 characters. 45 echo -n . >&3 # Write a decimal point there. 46 exec 3>&- # Close fd 3. 47 cat File # ==> 1234.67890 48 # Random access, by golly. 49 50 51 52 | 53 # Pipe. 54 # General purpose process and command chaining tool. 55 # Similar to ">", but more general in effect. 56 # Useful for chaining commands, scripts, files, and programs together. 57 cat *.txt | sort | uniq > result-file 58 # Sorts the output of all the .txt files and deletes duplicate lines, 59 # finally saves results to "result-file". |
Multiple instances of input and output redirection and/or pipes can be combined in a single command line.
1 command < input-file > output-file 2 3 command1 | command2 | command3 > output-file |
Multiple output streams may be redirected to one file.
1 ls -yz >> command.log 2>&1 2 # Capture result of illegal options "yz" to "ls" in file "command.log". 3 # Because stderr redirected to the file, any error messages will also be there. |
Close input file descriptor n.
Close stdin.
Close output file descriptor n.
Close stdout.
Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it.
1 # Redirecting only stderr to a pipe. 2 3 exec 3>&1 # Save current "value" of stdout. 4 ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'ls' and 'grep'. 5 exec 3>&- # Now close it for the remainder of the script. 6 7 # Thanks, S.C. |
For a more detailed introduction to I/O redirection see Appendix D.
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 16-1. Redirecting stdin using exec
1 #!/bin/bash 2 # Redirecting stdin using 'exec'. 3 4 5 exec 6<&0 # Link file descriptor #6 with stdin. 6 7 exec < data-file # stdin replaced by file "data-file" 8 9 read a1 # Reads first line of file "data-file". 10 read a2 # Reads second line of file "data-file." 11 12 echo 13 echo "Following lines read from file." 14 echo "-------------------------------" 15 echo $a1 16 echo $a2 17 18 echo; echo; echo 19 20 exec 0<&6 6<&- 21 # Now restore stdin from fd #6, where it had been saved, 22 # and close fd #6 ( 6<&- ) to free it for other processes to use. 23 # <&6 6<&- also works. 24 25 echo -n "Enter data " 26 read b1 # Now "read" functions as expected, reading from normal stdin. 27 echo "Input read from stdin." 28 echo "----------------------" 29 echo "b1 = $b1" 30 31 echo 32 33 exit 0 |
| [1] | 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. |
| [2] | 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. |