While executing shell scripts, you often do NOT want error messages cluttering up the normal program output.These errors usually do not help the person searching for a particular file. For instance, while searching for files, one typically gets permission denied errors.Why Error Redirection?Įrror re-direction is one of the very popular features of Unix/Linux.įrequent UNIX users will reckon that many commands give you massive amounts of errors. Error redirection is routing the errors to a file other than the screen. Fileīy default, error stream is displayed on the screen. As explained before a file descriptor, is associated with each of these files. These files are always present whenever a program is run. Whenever you execute a program/command at the terminal, 3 files are always open, viz., standard input, standard output, standard error. If the output is sent to File Descriptor of the printer, the program output would have been printed. When a program is executed the output is sent to File Descriptor of the screen, and you see program output on your monitor. Every File has an associated number called File Descriptor (FD). Regular file, Directories, and even Devices are files. Let’s look at some advance re-direction techniques which make use of File Descriptors. This would attach the file with the email, and it would be sent to the recipient. But if you want to attach a File to email you can use the input re-direction operator in the following format. You can type the contents of the email using the standard device keyboard. The ‘<‘ symbol is used for input(STDIN) redirectionĮxample: The mail program in Linux can help you send emails from the Terminal. If the sound configurations in your PC are correct, this command will play the file music.mp3 Input redirection The cat command reads the file music.mp3 and sends the output to /dev/audio which is the audio device. You can redirect standard output, to not just files, but also devices! $ cat music.mp3 > /dev/audio If you do not want a file to be overwritten but want to add more content to an existing file, then you should use ‘>‘ operator. If there is an existing file with the same name, the redirected command will delete the contents of that file and then it may be overwritten.” Note: Use the correct file name while redirecting command output to a file. Here the output of command ls -al is re-directed to file “listings” instead of your screen.
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