Redirection (Unix)
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Redirection is a function common to most Unix shells which allow standard streams to be redirected to user-specified locations.
Programmatically, it is done with the dup2(2) system call, or its less-flexible but higher-level stdio analogues, freopen(3) and popen(3).
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[edit] Redirecting standard input and standard output
Redirection is usually implemented by placing certain characters between commands. In this context, the characters are often referred to as hoinkies in order to "avoid confusion with other bracket-type operators" (Bryant and O'Hallaron 2003). Typically, the syntax of these characters is as follows:
command1 > file1
executes command1, placing the output in file1.
command1 < file1
executes command1, using file1 as the source of input (as opposed to the keyboard).
command1 < infile > outfile
combines the two capabilities: command1 reads from infile and writes to outfile
[edit] Piping
Programs can be run together such that one program reads the output from another with no need for an explicit intermediate file:
command1 | command2
executes command1, using its output as the input for command2 (commonly called piping, since the "|" character is known as a "pipe").
A good example for command piping is combining echo with another command to achieve something interactive in a non-interactive shell, e.g.
echo -e "user\npass" | ftp localhost
. This would run the ftp client and enter user, press return, then enter pass.
[edit] Redirecting to and from the standard file handles
In Unix shells derived from the original Bourne shell, the first two actions can be further modified by placing a number (the file descriptor) immediately before the character; this will affect which stream is used for the redirection. The Unix standard I/O streams are:
| Handle | Name | Description |
|---|---|---|
| 0 | stdin | Standard input |
| 1 | stdout | Standard output |
| 2 | stderr | Standard error |
For example:
command1 2> file1
executes command1, directing the standard error stream to file1 (useful since standard error outputs to the terminal by default and is unaffected by redirection unless so specified).
In shells derived from csh (the C shell), the syntax instead appends the & character to the redirect characters, thus achieving a similar result.
Another useful capability is to redirect one standard file handle to another. The most popular variation is to merge standard error into standard output so error messages can be processed together with (or alternately to) the usual output. Example:
find / -name .profile > results 2>&1
will try to find files/directories named .profile in the root directory. Executed without redirection, it will output hits to stdout and errors (e.g. for lack of privilege to read certain directories) to stderr. If we redirect standard output to file results, the error messages will continue to spam the console. To see both hits and error messages in file results, we merge stderr (handle 2) into stdout (handle 1) using 2>&1 .
It's possible use 2>&1 before ">" but it doesn't work. Infact, when the interpreter reads 2>&1, it doesn't know yet where standard output is redirected and then standard error isn't merged.
If the merged output is to be piped into another program, the file merge sequence 2>&1 must precede the pipe symbol, thus:
find / -name .profile 2>&1 | less
A simplified form of the command:
command > file 2>&1
is:
command &>file
or:
command >&file
[edit] Chained pipelines
The redirection and piping tokens can be chained together to create complex commands. For example:
ls | grep '.sh' | sort > shlist
lists the contents of the current directory, where this output is filtered to only contain lines which contain .sh, sort this resultant output alphabetically, and place the final output in shlist.
This type of construction is used very commonly in Unix shell scripts.
[edit] Reference
- Bryant, Randal E.; David O'Hallaron (2003). Computer Systems: A Programmer's Perspective. Pearson Education, Inc., 794. ISBN 0-13-034074-X.
