Linux Tutorial #4. Redirection. Output redirection ( > )

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1 Linux Tutorial #4 Redirection Most processes initiated by Linux commands write to the standard output (that is, they write to the terminal screen), and many take their input from the standard input (that is, they read it from the keyboard). There is also the standard error, where processes write their error messages, by default, to the terminal screen. We have already seen one use of the cat command to write the contents of a file to the screen. Now type cat without specifying a file to read % cat Then type a few words on the keyboard and press the [return] key. What do you see? Finally hold the [control] key down and press [d] (written as ^D for short) to end the input. What has happened? If you run the cat command without specifying a file to read, it reads the standard input (the keyboard), and on receiving the end of file (^D) character, cat copies the user input to the standard output (the screen). In Linux, we can redirect both the input and the output of commands. The rest of this tutorial will illustrate a number of ways that redirection can be used. Output redirection ( > ) We use the > symbol to redirect the output of a command. When we put a filename after the > symbol, the output will be written to this file. Output redirection is useful for storing the output of any command in a text file for future use. For example, if you type % ls > files.txt % cat files.txt You will see that the names of all of the files in your current directory have been stored in file.txt. We can also use output redirection to capture user input from the keyboard in a file. For example, to create a file called list1.txt containing a list of fruit names, type

2 % cat > list1.txt Then type in the names of some fruit. Press [return] after each name. pear banana apple ^D {this means press [control] and [d] to stop} In this case, the cat command reads the standard input (the keyboard) and the > redirects the output, which normally goes to the screen, into a file called list1.txt. To view the contents of this file, type % cat list1.txt You should see the fruit names you typed above. Now repeat this process and create another file called list2.txt containing the following fruit: orange, plum, mango, grapefruit. To view the contents of this file, type % cat list2.txt Appending to a file ( >> ) The >> symbol can be used to append standard output to the end of a file. This can be very helpful for saving the output of several commands in one output file. For example, to add more items to the file list1.txt from above, type % cat >> list1.txt Then type in the names of more fruit peach grape orange ^D { control d } To read the contents of this modified file, type % cat list1.txt You should now have two files. One contains six fruit, the other contains four fruit. We will now use the cat command to join (concatenate) list1.txt and list2.txrt into a new file called biglist.txt. To do this, type

3 % cat list1.txt list2.txt > biglist.txt What this command is doing is reading the contents of list1.txt and list2.txt in turn, then outputting the text to the file biglist.txt. To view the contents of this file, type sort % cat biglist.txt The Linux sort command will read an input file and sort the lines in alphabetical or numerical order. This command is useful for a wide range of applications: sorting student names, sorting distances between cities, or even sorting fruit names. For example, to sort the fruit names in biglist.txt, type % sort biglist.txt You should see all of the fruit names appearing in sorted order. The sort command can also be used to sort user input from the keyboard. For example, type % sort Then type in the names of some animals. Press [Return] after each one. dog cat bird ape ^D { control d } The output will be ape bird cat dog Input redirection ( < ) We can use the < symbol to redirect the input of a command to come from a file rather than the keyboard. For example, to sort the list of fruit, type % sort < biglist.txt

4 and the sorted list will be output to the screen. Some times it is useful to combine input redirection and output redirection on one Linux command. For example, to output the sorted list of fruit names into a file called sortlist.txt, type % sort < biglist.txt > sortlist.txt Input and output redirection can be very useful for debugging a program with a collection of testing inputs. For example, %./yourprogram < input1.txt > output1.txt %./yourprogram < input2.txt > output2.txt %./yourprogram < input3.txt > output3.txt We can examine the output files using a text editor to see if the program has produced the expected output. Pipes ( ) The pipe symbol can be used to send the output of one command as the input of another command. This can be very useful for combining a series of commands without creating a lot of temporary files. Before we see how pipes work, lets see how we can combine the ls and wc commands to count the number of files in your current directory. To see a list of all of the files in your current directory, type % ls Now save this list of files in a file called files.txt by typing % ls > files.txt Now we can count the number of files in the directory by typing % wc - l < files.txt This is a bit slow and you have to remember to remove the temporary file called files.txt when you have finished. What you really want to do is connect the output of the ls command directly to the input of the wc command. This is exactly what pipes were designed for. We simply put the pipe symbol between the two commands. For example, we can get the same results as above by typing % ls wc - l

5 Summary Command command > file command >> file command < file command1 command2 sort file Meaning redirect standard output to a file append standard output to a file redirect standard input from a file pipe output of command1 to input of command2 alphabetically sort the contents of file Author: M.Stonebank@surrey.ac.uk, 9th October 2000 Edited: jgauch@uark.edu, January 2015