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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. [5] The Unix standard I/O streams are: [6]
Process substitution can also be used to capture output that would normally go to a file, and redirect it to the input of a process. The Bash syntax for writing to a process is >(command). Here is an example using the tee, wc and gzip commands that counts the lines in a file with wc -l and compresses it with gzip in one pass:
In the calling process the lowest numbered unused file descriptor will be used for the new file descriptor number. When using the dup2() system call it performs the same task as dup() with the exception of using the file descriptor number specified in the newfd variable of the call, in that newfd is adjusted to refer to the oldfd file description.
This is because the redirection form opens the file as the stdin file descriptor which command can fully access, while the cat form simply provides the data as a stream of bytes. Another common case where cat is unnecessary is where a command defaults to operating on stdin, but will read from a file, if the filename is given as an argument.
A file descriptor open when an exec call is made remains open in the new process image, unless was fcntl ed with FD_CLOEXEC or opened with O_CLOEXEC (the latter was introduced in POSIX.1-2001). This aspect is used to specify the standard streams (stdin, stdout and stderr) of the new program.
The output can be stored to another file rather than printed using redirection. As an example, the two following files list the known fathers and the mothers of some people. Both files have been sorted on the join field — this is a requirement of the program. george jim kumar gunaware albert martha george sophie
Unix directories do not contain files. Instead, they contain the names of files paired with references to so-called inodes, which in turn contain both the file and its metadata (owner, permissions, time of last access, etc., but no name). Multiple names in the file system may refer to the same file, a feature termed a hard link. [1]
In computing, a here document (here-document, here-text, heredoc, hereis, here-string or here-script) is a file literal or input stream literal: it is a section of a source code file that is treated as if it were a separate file.