ops102_4_2.pdf

Input/Output Redirection and Pipes OPS102 Week 4 Class 2 Tiayyba Riaz/John Sellens January 29, 2024 Seneca Polytechnic Outline The Unix Philosophy Input/Output Redirection Connecting Commands with Pipes OPS102 W4C2 - Input/Output Redirection and Pipes 1/10 The Unix Philosophy...

Input/Output Redirection and Pipes OPS102 Week 4 Class 2 Tiayyba Riaz/John Sellens January 29, 2024 Seneca Polytechnic Outline The Unix Philosophy Input/Output Redirection Connecting Commands with Pipes OPS102 W4C2 - Input/Output Redirection and Pipes 1/10 The Unix Philosophy The Unix Philosophy The early developers of Unix established a “philosophy”, as a result of their brilliance and experience. In his book “A Quarter-Century of UNIX” (1994), Peter H. Salus summarized the Unix Philosophy as Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface. Many of the tools and commands we have seen so far reflect these ideas. Many Linux/Unix commands act as “filters” – they read input, process or modify it, and send it along as output. https://en.wikipedia.org/wiki/Unix_philosophy OPS102 W4C2 - Input/Output Redirection and Pipes 2/10 Input/Output Redirection Input/Output Redirection We give commands to the shell via the terminal and the shell executes them. But commands often need input and produce output. Usually input comes from files, and output goes to the terminal. Commands can also give errors if something goes wrong, these error messages are also, normally, printed on screen. Commands can also get their input from other commands or the terminal. Output and error messages can be sent (“redirected”) to files (or other commands) instead of displaying them on the terminal. OPS102 W4C2 - Input/Output Redirection and Pipes 3/10 Standard Input, Output, and Error Normally, every program or command is invoked with three open file descriptors, which often default to being attached to the terminal: 0 – the standard input 1 – the standard output 2 – the standard error output C programmers will recognize these as stdin, stdout, and stderr from "stdio.h". The shell provides convenient ways to attach these file descriptors to files, devices, or other commands. OPS102 W4C2 - Input/Output Redirection and Pipes 4/10 Input/Output Redirection and the Shell The shell uses special characters and syntax on command lines to implement I/O redirection. Use less than < before a filename to read input from that file. Use greater than > before a filename to write output to that file. I remember > as it looks like an arrow pointing into a file. If the file already exists, it is emptied before writing, otherwise it is created. Use two greater thans >> before a filename to append (add to the end of) output to that file. e.g. "tr '[A-Z]' '[a-z]' lowercase.txt" I/O redirection can be used with any command, though it’s less useful with some commands e.g. "date" doesn’t read input. OPS102 W4C2 - Input/Output Redirection and Pipes 5/10 Redirecting Error Output Remember that there is also the standard error output on file descriptor 2. "gcc -o myprog myprog.c 2>gccerrors.txt" "gcc -o myprog myprog.c 2>>allerrors.txt" "./myprog >myoutput.txt 2>&1" The "2>&1" means send file descriptor 2 output (errors) wherever file descriptor 1 output is currently going. OPS102 W4C2 - Input/Output Redirection and Pipes 6/10 Input/Output and "/dev/null" Remember that the "/dev" directory contains special device files. "/dev/null" is a place you can read nothing from, or write anything to. It’s also called the “bitbucket”, or the “black hole”. "/dev/null" is handy if you don’t want to give any input to a command, or you want to ignore any output (or errors). e.g. "grep Linux * 2>/dev/null" looks for “Linux” in all matching files, but throws away any error messages (say, if you don’t have read permission on some files). OPS102 W4C2 - Input/Output Redirection and Pipes 7/10 Connecting Commands with Pipes Inter Process Communication: Pipes What if we want the put the output of a command to work as the input of another command? Commands can send their standard output directly to the standard input of other commands. Two or more simple commands can be combined to form a more powerful command sequence. No intermediate files need to be created. This is the magic of pipes! OPS102 W4C2 - Input/Output Redirection and Pipes 8/10 How To Use Pipes We make a connection (pipe) between two commands by using the "|" pipe operator between commands. Many commands can be piped together especially Linux filter commands such as "sort", "cut", "more", "less", etc. The shell uses the pipe(2) system call to connect the necessary file descriptors before running commands. As usual, each command processes its input and generates its output. Commands must be chained in a specific order, depending on what you wish to accomplish (obviously). Example pipe use: "ls -al | sort -nk 5" Also works on Windows command prompt similarly to Linux: "dir | sort" https://en.wikipedia.org/wiki/Pipeline_(Unix) OPS102 W4C2 - Input/Output Redirection and Pipes 9/10 Advantages of Using Pipes The most obvious advantage is convenience for the user. It’s a simple quick syntax, and very powerful and useful. It also means that intermediate output does not have to be written to and read from temporary files. Disk input/output is one of the slowest parts of computing. With multi-processor machines, more than one command can actually be running at the same time. Parallel, rather than serial, processing usually reduces the time taken to complete a task. OPS102 W4C2 - Input/Output Redirection and Pipes 10/10