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| ## page was renamed from BashGuide/TheBasics/InputAndOutput #pragma section-numbers 2 [[BashGuide/TestsAndConditionals|<- Tests and Conditionals]] | [[BashGuide/CompoundCommands|Compound Commands ->]] ---- = Input And Output = <<TableOfContents>> This basic principle of computer science applies just as well to applications started through [[BASH]]. [[BASH]] makes it fairly easy to play around with the input and output of commands, which gives us great flexibility and incredible opportunities for automation. -------- <<Anchor(File_Descriptors)>> == File Descriptors == Input and output from and to processes occurs via so-called ''File Descriptors'' (in short: FDs). FDs are kind of like pointers to sources of data. When something reads from or writes to that FD, the data is being read from or written to the FD's data source. FDs can point to regular files, but they can also point to other data sources, like sockets, pipes, or devices. By default, every new process starts with three FDs. They are referred to by the names ''Standard Input'', ''Standard Output'' and ''Standard Error''. In short, they are respectively called `stdin`, `stdout` and `stderr`. In an interactive shell, or in a script running on a terminal, the ''Standard Input'' is how bash sees the characters you type on your keyboard. The ''Standard Output'' is where the program sends most of its normal information so that the user can see it, and the ''Standard Error'' is where the program sends its error messages. Be aware that GUI applications work in the same way; but the actual GUI doesn't work via these FDs. GUI applications can still read and write from and to the standard FDs, but they usually don't. Usually, they do all the user interaction via that GUI; making it hard for [[BASH]] to control. As a result, we'll stick to simple terminal applications. Those, we can easily feed data on their "Standard Input", and read data from on their "Standard Output" and "Standard Error". Let's make these definitions a little more concrete. Here's a demonstration of how "Standard Input" and "Standard Output" work: {{{ $ read -p "What is your name? " name; echo "Good day, $name. Would you like some tea?" What is your name? lhunath Good day, lhunath. Would you like some tea? }}} `read` is a command that reads information from `stdin` and stores it in a variable. We specified `name` to be that variable. Once `read` has read a line of information from `stdin`, it finishes and lets `echo` display a message. `echo` uses `stdout` to send its output to. `stdin` is connected to your terminal's input device, which is probably your keyboard. `stdout` is connected to your terminal's output device, which we assume is your computer monitor. As a result, you can type in your name and are then greeted with a friendly message on your monitor, offering you a cup of tea. So what is `stderr`? Let's demonstrate: {{{ $ rm secrets rm: cannot remove `secrets': No such file or directory }}} Unless you have a file called `secrets` in your current directory, that `rm` command will fail and show an error message explaining what went wrong. Error messages like these are by convention displayed on `stderr`. `stderr` is also connected to your terminal's output device, just like `stdout`. As a result, error messages display on your monitor just like the messages on `stdout`. However, this separation makes it easy to keep errors separated from the application's normal messages. Some people like to use wrappers to make all the output on `stderr` red, so that they can see the error messages more clearly. This is not generally advisable, but it is a simple example of the many options this separation provides us with. Another common practice is writing `stderr` to a special log file. -------- . '''Good Practice: <<BR>> Remember that when you create scripts, you should send your custom error messages to the `stderr` FD. This is a convention and it is very convenient when applications follow the convention. As such, so should you! You're about to learn redirection soon, but let me show you quickly how it's done:''' {{{ echo "Uh oh. Something went really bad.." >&2 }}} ---- . ''File Descriptor'': A numeric index referring to one of a process's open files. Each command has at least three basic descriptors: FD 0 is `stdin`, FD 1 is `stdout` and FD 2 is `stderr`. -------- <<Anchor(Redirection)>> == Redirection == The most basic form of input/output manipulation in [[BASH]] is ''Redirection''. ''Redirection'' is used to change the data source or destination of an application's FDs. That way, you can send the application's output to a file instead of the terminal, or have the application read from a file instead of from the keyboard. Redirection, too, comes in different shapes. There's ''File Redirection'', ''File Descriptor manipulation'', ''Heredocs'' and ''Herestrings''. -------- . '''In The Manual: [[http://www.gnu.org/software/bash/manual/bashref.html#SEC37|Redirections]]''' ---- . ''Redirection'': This is the practice of changing a certain FD to read its input from or send its output to elsewhere. -------- <<Anchor(File_Redirection)>> === File Redirection === ''File Redirection'' is the most basic form of redirection. I'll start with this so you can grasp the concept of redirection well. {{{ $ echo "The story of William Tell. > > It was a cold december night. Too cold to write." > story $ cat story The story of William Tell. It was a cold december night. Too cold to write. }}} As a result, the `echo` command will not send its output to the terminal; rather, the `> story` redirection '''changes the destination of the `stdout` FD''' so that it now points to a file called `story`. Be aware that before the `echo` command is executed, [[BASH]] first checks to see whether that file `story` actually exists. If it doesn't, it is created as an empty file, so that the FD can be pointed to it. This behaviour can be toggled with ''Shell Options'' (see later). It should be noted that this redirection is in effect only for the single `echo` command it was applied to. Other commands executed after that will continue sending their output to the script's `stdout` location. We then use the application `cat` to print out the contents of that file. `cat` is an application that reads the contents of all the files you pass it as arguments. It then outputs each file one after another on `stdout`. In essence, it con'''cat'''enates the contents of all the files you pass it as arguments. '''Warning:''' Far too many code examples and shell tutorials on the Internet tell you to use `cat` whenever you need to read the contents of a file. '''This is highly ill-advised!''' `cat` only serves well to concatenate contents of multiple files together, or as a quick tool on the shell prompt to see what's inside a file. You should '''NOT''' use `cat` to read from files in your scripts. There will almost always be far better ways to do this. Please keep this warning in mind. Useless use of `cat` will merely result in an extra process to create, and often results in poorer read speed because `cat` cannot determine the context of what it's reading and the purpose for that data. When we use `cat` without passing any kind of arguments, it obviously doesn't know what files to read. In this case, `cat` will just read from `stdin` instead of from a file (much like `read`). Since `stdin` is normally not a regular file, starting `cat` without any arguments will seem to do nothing: {{{ $ cat }}} It doesn't even give you back your shell prompt! What's going on? `cat` is still reading from `stdin`, which is your keyboard. Anything you type now will be sent to `cat`. As soon as you hit the ''Enter'' key, `cat` will do what it normally does: it will display what it reads on `stdout`, just the same way as when it displayed our story on `stdout`: {{{ $ cat test? test? }}} Why does it say `test?` twice now? Well, as you type, your terminal shows you all the characters that you send to `stdin` before sending them there. That results in the first `test?` that you see. As soon as you hit ''Enter'', `cat` has read a line from `stdin`, and shows it on `stdout`, which is also your terminal; hence, resulting in the second line: `test?`. You can press ''Ctrl+D'' to send your terminal the ''End of File'' character. That'll cause `cat` to think `stdin` has closed. It will stop reading from it and return you to your prompt. Let's use file redirection to attach a file to `stdin`, so that `stdin` is no longer reading from our keyboard, but instead, now reads from the file: {{{ $ cat < story The story of William Tell. It was a cold december night. Too cold to write. }}} The result of this is exactly the same as the result from our previous `cat story`; except this time, the way it works is a little different. In our first example, `cat` opened an FD to the file `story` and read its contents through that FD. In the second example, `cat` simply reads from `stdin`, just like it did when it was reading from our keyboard. However, this time, the `< story` operation has '''modified''' `stdin` so that its data source is the file `story` rather than our keyboard. Redirection operators can be preceded by a number. That number denotes the FD that it changes. Let's summarize with some examples: * '''`command > file`''': Send the `stdout` of command to `file`. * '''`command 1> file`''': Send the `stdout` of command to `file`. Since stdout was open on FD 1, that's the number we precede the redirection operator with. Note, the effect of this is identical to the previous example, because FD 1 is the default FD for the '''>''' redirection operator. * '''`command < file`''': Use the contents of `file` when `command` reads from `stdin`. * '''`command 0< file`''': Use the contents of `file` when `command` reads from `stdin`, exactly as in the previous example, since FD 0 (stdin) is the default for the '''<''' redirection operator. The number for the `stderr` FD is 2. So, let's try sending the output of `stderr` to a file: {{{ $ for homedir in /home/* > do rm "$homedir/secret" > done 2> errors }}} In this example, we're looping over each directory (or file) in `/home`. We then try to delete the file `secret` in each of them. Some `homedir`s may not have a secret, or we may not have permission to remove it. As a result, the `rm` operation will fail and send an error message on `stderr`. You may have noticed that our redirection operator isn't on `rm`, but it's on that `done` thing. Why is that? Well, this way, the redirection applies to all output to `stderr` made inside the whole loop. Let's see what the result of our loop was: {{{ $ cat errors rm: cannot remove `/home/axxo/secret': No such file or directory rm: cannot remove `/home/lhunath/secret': No such file or directory }}} Two error messages in our error log file. Two people that didn't have a `secret` file in their home directory. If you're writing a script, and you expect that running a certain command may fail on occasion, but don't want the script's user to be bothered by the possible error messages that command may produce, you can silence an FD. Silencing it is as easy as normal ''File Redirection''. We're just going to send all output to that FD into the system's black hole: {{{ $ for homedir in /home/* > do rm "$homedir/secret" > done 2> /dev/null }}} The file `/dev/null` is '''always''' empty, no matter what you write or read from it. As such, when we write our error messages to it, they just disappear. The `/dev/null` file remains as empty as ever before. That's because it's not a normal file; it's a ''virtual'' device. Some people call `/dev/null` the ''bit bucket''. There is one last thing you should learn about ''File Redirection''. It's interesting that you can make error log files like this to keep your error messages; but as I mentioned before, [[BASH]] makes sure that the file exists before trying to redirect to it. [[BASH]] '''also makes sure the file is empty''' before redirecting to it. As a result, each time we run our loop to delete secret files, our log file will be truncated empty before we fill it up again with new error messages. What if we'd like to keep a record of any error messages generated by our loop? What if we don't want that file to be truncated each time we start our loop? The solution is achieved by doubling the redirection operator. `>` becomes `>>`. `>>` will not empty a file, it will just append new data to the end of it! {{{ $ for homedir in /home/* > do rm "$homedir/secret" > done 2>> errors }}} Hooray! -------- . '''Good Practice: <<BR>> It's a good idea to use redirection whenever an application needs file data and is built to read data from `stdin`. A lot of bad examples on the Internet tell you to pipe (see later) the output of `cat` into processes; but this is nothing more than a very ''bad'' idea. <<BR>> When designing an application that could be fed data from a variety of different sources, it is often best simply to have your application read from `stdin`; that way, the user can use redirection to feed it whatever data she wishes. An application that reads standard input in a generalized way is called a ''filter''.''' ---- . '''In The Manual: [[http://www.gnu.org/software/bash/manual/bashref.html#SEC38|Redirecting Input]], [[http://www.gnu.org/software/bash/manual/bashref.html#SEC39|Redirecting Output]], [[http://www.gnu.org/software/bash/manual/bashref.html#SEC40|Appending Redirected Output]], [[http://www.gnu.org/software/bash/manual/bashref.html#SEC41|Redirecting Standard Output and Standard Error]]''' -------- <<Anchor(File_Descriptor_Manipulation)>> === File Descriptor Manipulation === Now that you know how to manipulate process input and output by sending it to and reading it from files, let's make it a little more interesting still. It's possible to change the source and desination of FDs to point to or from files, as you know. It's also possible to copy one FD to another. Let's prepare a simple testbed: {{{ $ echo "I am a proud sentence." > file }}} We've made a file called `file`, and written a proud sentence into it. It's time I introduce a new application to you. Its name is `grep`, and it's incredibly powerful. `grep` is that one thing that you need more than anything else in your household. It basically takes a ''search string'' as its first argument and one or more files as extra arguments. Just like `cat`, `grep` also uses `stdin` if you don't specify any files as extra arguments. `grep` reads the files (or `stdin` if none were provided) and searches for the ''search string'' you gave it. Most versions of `grep` even support a `-r` switch, which makes it take directories as well as files as extra arguments, and then searches all the files and directories in those directories that you gave it. Here's an example of how `grep` can work: {{{ $ ls house/ drawer closet dustbin sofa $ grep -r socks house/ house/sofa:socks }}} In this silly example we have a directory called `house` with several pieces of furniture in it as files. If we're looking for our `socks` in each of those files, we send `grep` to search the directory `house/`. `grep` will search everything in there, open each file and look through its contents. In our example, `grep` finds `socks` in the file `house/sofa`; presumably tucked away under a pillow. You want a more realistic example? Sure: {{{ $ grep "$HOSTNAME" /etc/* /etc/hosts:127.0.0.1 localhost Lyndir }}} Here we instruct `grep` to search for whatever `$HOSTNAME` expands to in whatever files `/etc/*` expands to. It finds my hostname, which is `Lyndir`, in the file `/etc/hosts`, and shows me the line in that file that contains the ''search string''. OK, now that you understand `grep`, let's continue with our ''File Descriptor Manipulation''. Remember that we created a file called `file`, and wrote a proud sentence to it? Let's use `grep` to find where that proud sentence is now: {{{ $ grep proud * file:I am a proud sentence. }}} Good! `grep` found our sentence in `file`. It writes the result of its operation to `stdout` which is shown on our terminal. Now let's see if we can make `grep` send an error message, too: {{{ $ grep proud file 'not a file' file:I am a proud sentence. grep: not a file: No such file or directory }}} This time, we instruct `grep` to search for the string `proud` in the files '`file`' and '`not a file`'. `file` exists, and the sentence is in there, so `grep` happily writes the result to `stdout`. It moves on to the next file to scan, which is '`not a file`'. `grep` can't open this file to read its content, because it doesn't exist. As a result, `grep` emits an error message on `stderr` which is still connected to our terminal. Now, how would you go about silencing this `grep` statement completely? We'd like to send all the output that appears on the terminal to a file instead; let's call it `proud.log`: {{{ $ grep proud file 'not a file' > proud.log 2> proud.log }}} Does that look about right? We first use `>` to send `stdout` to `proud.log`, and then use `2>` to send `stderr` to `proud.log` as well. Almost, but not quite. If you run this command (at least on some computers), and then look in `proud.log`, you'll see there's only an error message, not the output from `stdout`. We've created a very bad condition here. We've created two FDs that both point to the same file, independently of each other. The results of this are not well-defined. Depending on how the operating system handles FDs, some information written via one FD may clobber information written through the other FD. {{{ $ echo "I am a very proud sentence with a lot of words in it, all for you." > file2 $ grep proud file2 'not a file' > proud.log 2> proud.log $ cat proud.log grep: not a file: No such file or directory of words in it, all for you. }}} What happened here? `grep` opened `file2` first, found what we told it to look for, and then wrote our very proud sentence to `stdout` (FD 1). FD 1 pointed to `proud.log`, so the information was written to that file. However, we also had another FD (FD 2) pointed to this same file, and specifically, pointed to the ''beginning'' of this file. When `grep` tried to open '`not a file`' to read it, it couldn't. Then, it wrote an error message to `stderr` (FD 2), which was pointing to the beginning of `proud.log`. As a result, the second write operation overwrote information from the first one! We need to prevent having two FDs working on the same destination or source. We can do this by duplicating FDs: {{{ $ grep proud file 'not a file' > proud.log 2>&1 }}} In order to understand these, you need to remember: always read file redirections from left to right. This is the order in which [[BASH]] processes them. First, `stdout` is changed so that it points to our `proud.log`. Then, we use the `>&` syntax to duplicate FD 1 and put this duplicate in FD 2. A duplicate FD works differently from having two independent FDs pointing to the same place. Write operations that go through either one of them are exactly the same. There won't be a mix-up with one FD pointing to the start of the file while the other has already moved on. Be careful not to confuse the order: {{{ $ grep proud file 'not a file' 2>&1 > proud.log }}} This will duplicate `stderr` to where `stdout` points (which is the terminal), and then `stdout` will be redirected to `proud.log`. As a result, `stdout`'s messages will be logged, but the error messages will still go to the terminal. ''Oops''. '''Note: <<BR>> For convenience, [[BASH]] also makes yet another form of redirection available to you. The `&>` redirection operator is actually just a shorter version of what we did here; redirecting both `stdout` and `stderr` to a file :''' {{{ $ grep proud file 'not a file' &> proud.log }}} This is the same as `> proud.log 2>&1`, but not portable to BourneShell. It is not recommended practice, but you should recognize it if you see it used in someone else's scripts. '''''TODO: Moving FDs and Opening FDs RW.''''' -------- . '''In The Manual: [[http://www.gnu.org/software/bash/manual/bashref.html#SEC44|Duplicating File Descriptors]], [[http://www.gnu.org/software/bash/manual/bashref.html#SEC45|Moving File Descriptors]], [[http://www.gnu.org/software/bash/manual/bashref.html#SEC46|Opening File Descriptors for Reading and Writing]]''' ---- . '''In the FAQ: <<BR>> [[BashFAQ/014|How can I redirect the output of multiple commands at once?]] . [[BashFAQ/032|How can I redirect the output of 'time' to a variable or file?]] . [[BashFAQ/040|How do I use dialog to get input from the user?]] . [[BashFAQ/047|How can I redirect stderr to a pipe?]] . [[BashFAQ/055|Tell me all about 2>&1 -- what's the difference between 2>&1 >foo and >foo 2>&1, and when do I use which?]]''' -------- <<Anchor(Heredocs_And_Herestrings)>> === Heredocs And Herestrings === Files aren't all that. They're boring, really. Strings are so much more interesting. They're not permanent like files on a hard disk, but they're easy to work with, easy to make and easy to manipulate. ''Heredocs'' and ''Herestrings'' allow you to perform ''Redirection'' as you would with files, just by using strings instead. Let's try it out! {{{ $ grep proud <<END > I am a proud sentence. > END I am a proud sentence. }}} This is a ''Heredoc'' (or ''Here Document''). ''Heredoc''s aren't really useful unless you're trying to embed long strings of several lines inside your scripts, which is bad practice. You should keep your logic (your code) and your input (your data) separated, preferably in different files. The way ''Heredoc''s work, is by adding the `<<STRING` redirection to a command. That'll instruct that command's `stdin` that it has to start reading from the script (or the command line, if you're not in a script). The input of the ''Heredoc'' stops as soon as you repeat whatever string you added to the end of the `<<`. In the example above, I used the string `END`, but it can really be any single word. Beware that all text following the ''Heredoc'' operator is sent to the command's `stdin` almost literally (`$` and backticks have to be escaped with `\` or will be expanded). That also means any spaces you use for indenting your script. The terminator string (in our case `END`) must be in the beginning of the line. {{{ echo "Let's test abc:" if [[ abc = a* ]]; then cat <<END abc seems to start with an a! END fi }}} Will result in: {{{ Let's test abc: abc seems to start with an a! }}} You can avoid this by temporarily removing the indentation for the lines of your ''Heredoc''s. However, that distorts your pretty and consistent indentation. There is an alternative. If you use `<<-END` instead of `<<END` as your ''Heredoc'' operator, [[BASH]] removes any `tab` characters in the beginning of each line of your ''Heredoc'' content before sending it to the command. That way you can still use tabs to indent your ''Heredoc'' content with the rest of your code. Those tabs will not be sent to the command that receives your ''Heredoc''. This also means you can use tabs to indent your terminator string. Finally, note that if you quote the word that you're using to delimit your ''Heredoc'', [[BASH]] won't perform any substitutions on the contents. Try this example with and without the quote characters, to see the difference: {{{ $ cat <<'XYZ' > My home directory is $HOME > XYZ My home directory is $HOME }}} Now let's check out the very similar but more compact ''Herestring''s: {{{ $ grep proud <<<"I am a proud sentence" I am a proud sentence. }}} This time, `stdin` reads its information straight from the string you put after the `<<<` operator. This is very convenient to send data that's in variables into processes: {{{ $ grep proud <<<"$USER sits proudly on his throne in $HOSTNAME." lhunath sits proudly on his throne in Lyndir. }}} ''Herestring''s are shorter, less intrusive and overall more convenient than their bulky ''Heredoc'' counterpart. However, they are not portable to the Bourne shell. Later on, you will learn about pipes and how they can be used to send the output of a command into another command's `stdin`. Many people use pipes to send the output of a variable as `stdin` into a command. However, for this purpose, ''Herestring''s should be preferred. They do not create a subshell and are lighter both to the shell and to the style of your shell script: {{{ $ echo 'Wrap this silly sentence.' | fmt -t -w 20 Wrap this silly sentence. $ fmt -t -w 20 <<< 'Wrap this silly sentence.' Wrap this silly sentence. }}} -------- . '''Good Practice: <<BR>> Long heredocs are usually a bad idea because scripts should contain logic, not data. If you have a large document that your script needs, you should ship it in a separate file along with your script. Herestrings, however, come in handy quite often, especially for sending variable content (rather than files) to filters like `grep` or `sed`.''' ---- . '''In The Manual: [[http://www.gnu.org/software/bash/manual/bashref.html#SEC42|Here Documents]], [[http://www.gnu.org/software/bash/manual/bashref.html#SEC43|Here Strings]]''' -------- <<Anchor(Pipes)>> == Pipes == Now that you can effortlessly manipulate ''File Descriptors'' to direct certain types of output to certain files, it's time you learn some more ingenious tricks available through I/O redirection. You can use ''File Redirection'' to write output to files or read input from files. But what if you want to connect the output of one application directly to the input of another? That way, you could build a sort of chain to process output. If you already know about `FIFO`s, you could use something like this to that end: {{{ $ ls $ mkfifo myfifo; ls myfifo $ grep bea myfifo & [1] 32635 $ echo "rat > cow > deer > bear > snake" > myfifo bear }}} We use the `mkfifo` command to create a new file in the current directory named '`myfifo`'. This is no ordinary file, however, but a `FIFO` (which stands for ''First In, First Out''). `FIFO`s are special files that serve data on a `First In, First Out`-basis. When you read from a `FIFO`, you will only receive data as soon as another process writes to it. As such, a `FIFO` never really contains any data. So long as no process writes to it, any read operation on the `FIFO` will '''block''' as it waits for data to become available. The same works for writes to the `FIFO` -- they will block until another process reads from the `FIFO`. In our example, the `FIFO` called `myfifo` is read from by `grep`. `grep` waits for data to become available on the `FIFO`. That's why we append the grep command with the `&` operator, which puts it in the background. That way, we can continue typing and executing commands while `grep` runs and waits for data. Our `echo` statement feeds data to the `FIFO`. As soon as this data becomes available, the running `grep` command reads it in and processes it. The result is displayed. We have successfully sent data from the `echo` command to the `grep` command. But these temporary files are a real annoyance. You may not have write permissions. You need to remember to clean up any temporary files you create. You need to make sure that data is going in and out, or the `FIFO` might just end up blocking for no reason. For these reasons, another feature is made available: ''Pipes''. A pipe basically just connects the `stdout` of one process to the `stdin` of another, effectively ''piping'' the data from one process into another. The entire set of commands that are piped together is called a ''pipeline''. Let's try our above example again, but using pipes: {{{ $ echo "rat > cow > deer > bear > snake" | grep bea bear }}} The pipe is created using the `|` operator between two commands that are connected with the pipe. The former command's `stdout` is connected to the latter command's `stdin`. As a result, `grep` can read `echo`'s output and display the result of its operation, which is `bear`. Pipes are widely used as a means of post-processing application output. `FIFO`s are, in fact, also referred to as `named pipes`. They accomplish the same results as the pipe operator, but through a filename. '''Note: <<BR>> The pipe operator creates a subshell environment to run each process in. This is important to know because any variables that you modify or initialize inside the second command will appear unmodified outside of it. Let's illustrate:''' {{{ $ message=Test $ echo "Salut, le monde!" | { read message; echo "The message is: $message"; } The message is: Salut, le monde! $ echo "The message is: $message" The message is: Test }}} Once the pipeline ends, so do the subshells that were created for it. Along with those subshells, any modifications made in them are lost. So be careful! -------- . '''Good Practice: <<BR>> Pipes are a very attractive means of post-processing application output. You should, however, be careful not to over-use pipes. If you end up making a pipeline that consists of three or more applications, it is time to ask yourself whether you're doing things a smart way. You might be able to use more application features of one of the post-processing applications you've used earlier in the pipe. Each new command in a pipeline causes a new subshell and a new application to be loaded. It also makes it very hard to follow the logic in your script!''' ---- . '''In The Manual: [[http://www.gnu.org/software/bash/manual/bashref.html#SEC17|Pipelines]]''' ---- . '''In the FAQ: <<BR>> [[BashFAQ/024|I set variables in a loop. Why do they suddenly disappear after the loop terminates? Or, why can't I pipe data to read?]] . [[BashFAQ/027|How can two processes communicate using named pipes (fifos)?]] . [[BashFAQ/047|How can I redirect stderr to a pipe?]] . [[BashFAQ/001|How can I read a file line-by-line?]] . [[BashFAQ/055|Tell me all about 2>&1 -- what's the difference between 2>&1 >foo and >foo 2>&1, and when do I use which?]]''' -------- <<Anchor(Miscellaneous_Operators)>> == Miscellaneous Operators == Aside from the standard I/O operators, bash also provides a few more advanced operators that make life on the shell that much nicer. === Process Substitution === A cousin of the pipe is the process substitution operator (`<()`, `>()`). It's a convenient way to use named pipes without having to create temporary files. Whenever you think you need a temporary file to do something, process substitution might be a better way to handle things. What it does, is basically run the command inside the brackets. With the `<()` operator, the command's output is put in a sort of temporary file that's created by bash. The operator itself in your command is replaced by the filename of that file. After your whole command finishes, the file is cleaned up. Here's how we can put that into action: Imagine a situation where you want to see the difference between the output of two commands. Ordinarily, you'd have to put the two outputs in two files and `diff` those: {{{ $ head -n 1 .dictionary > file1 $ tail -n 1 .dictionary > file2 $ diff -y file1 file2 Aachen | zymurgy $ rm file1 file2 }}} Using the ''Process Substitution'' operator, we can do all that with a one-liner and no need for manual cleanup: {{{ $ diff -y <(head -n 1 .dictionary) <(tail -n 1 .dictionary) Aachen | zymurgy }}} The `<(..)` part is replaced by the temporary file created by bash, so `diff` actually sees something like this: {{{ $ diff -y /dev/fd/63 /dev/fd/62 }}} Here we see how bash runs diff when we use process substitution. It runs our `head` and `tail` commands, redirects their respective outputs to the files `/dev/fd/63` and `/dev/fd/62`. Then it runs the `diff` command, passing those filenames where originally we had put the respective filename's process substitution operator. The actual implementation of the temporary files depends from system to system. In fact, you can see what the above would actually look like to `diff` on your box by putting an `echo` in front of our command: {{{ $ echo diff -y <(head -n 1 .dictionary) <(tail -n 1 .dictionary) diff -y /dev/fd/63 /dev/fd/62 }}} The `>(..)` operator is much like the `<(..)` operator, but instead of redirecting the command's output to a file, we redirect the file to the command's input. It's used for cases where you're running a command that writes to a file, but you want it to write to another command instead: {{{ $ tar -cf >(ssh host tar x) . }}} === Here Strings === Much like the earlier mentioned ''Here Documents'' are the bash ''Here Strings''. A ''Here String'' is created by using the operator `<<<`. It is almost the same as a ''Here Document'', but instead of redirecting multiple lines of text to a command's `stdin`, we're redirecting one argument to it. Note that the argument may still consist of anything a string can hold; which means it can still be multiple lines long. It's a great replacement for `echo "something" | somecommand`, because it doesn't incur the overhead of the subshell, and it's slightly neater in code. Moreover, very often, the `echo` solution is not an option, because it runs `somecommand` in a subshell. That means you can't set any bash variables in there! Let's illustrate: {{{ $ echo "well, well, what have we here?" | tr "wh" "v'" vell, vell, v'at 'ave ve 'ere? $ tr "wh" "v'" <<< "well, well, what have we here?" vell, vell, v'at 'ave ve 'ere? $ echo "$USER" | read name # 'name' is set in a subshell! Can't access it in the rest of the script. $ read name <<< "$USER" # 'name' is set in the main shell, now it's accessible to the rest of the script. }}} Combining the `<<<` operator with the `$''` operator is often very handy. The latter is like `echo -e`, but more reliable. It expands escape codes, such as `\n`, `\a`, etc. We can use it for a shorter replacement of heredocs: {{{ $ grep '[A-C]' <<< $'Olivia\nPeter\nAlfred\nCornelia' Alfred Cornelia }}} ---- [[BashGuide/TestsAndConditionals|<- Tests and Conditionals]] | [[BashGuide/CompoundCommands|Compound Commands ->]] |
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