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How do I read user input into a variable in Bash | Shell Scripting Tutorial for Beginners
How do I read user input into a variable in Bash | Shell Scripting Tutorial for Beginners
How do I read user input into a variable in Bash | Shell Scripting Tutorial for Beginners
IBM Documentation [1]
Understanding standard input, standard output, and standard error. Once a command begins running, it has access to three files:
– If you invoke a shell command, REXX program, or C program from the ISPF shell, standard output cannot be directed to your terminal screen. You can specify a z/OS UNIX file or use the default, a temporary file.
– If you invoke a shell command from the shell or from a C program or from a REXX program invoked from TSO READY, standard error is directed to your terminal screen by default.. – If you invoke a shell command, REXX program, or C program from
Understanding stdin, stderr and stdout in Linux [2]
There is a decent chance that if you have used Linux operating systems then you might have encountered the three famous data streams known as stdin, stderr and stdout. All these are different in their functions and have their own uses but one thing common between all three of them is that they are data streams that bash creates.
In terms of computing, a data stream is something that gives us the ability to transfer data from a source to an outflow and vice versa. The source and the outflow are the two end points of the data stream
Now we know a little about the data streams, let’s know more about the three famous data streams.. stdin − It stands for standard input, and is used for taking text as an input.
Bash Tutorial => STDIN, STDOUT and STDERR explained [3]
Commands have one input (STDIN) and two kinds of outputs, standard output (STDOUT) and standard error (STDERR).. Standard input is used to provide input to a program.
root@server~# ls anotherfile ls: cannot access ‘anotherfile’: No such file or directory. Standard error is (as the name implies) used for error messages
STDIN, STDOUT and STDERR are the three standard streams. They are identified to the shell by a number rather than a name:
stdin, stdout, stderr [4]
Linux is built being able to run instructions from the command line using switches to create the output.. The question of course is how do we make use of that?
Under normal circumstances every Linux program has three streams opened when it starts; one for input; one for output; and one for printing diagnostic or error messages. These are typically attached to the user’s terminal (see man tty(4)) but might instead refer to files or other devices, depending on what the parent process chose to set up.
Generally standard input, referred to as “stdin”, comes from the keyboard.. When you type stuff, you’re typing it on stdin (a standard input terminal)
LinuxComptia101&102 Flashcards [5]
|A shell metacharacter used to pipe Standard Output from one command to the Standard Input of another command.|. |A shell metacharacter used to obtain Standard Input from a file.|
|A command used to create special variables that are shortcuts to longer command strings.|. |A variable that is altered by loop constructs to ensure that commands are not executed indefinitely.|
Common decision constructs include if, case, &&, and ||.|. |A command used to display or echo output to the terminal screen
IBM Documentation [6]
xargs – Construct an argument list and run a command. xargs [-I placeholder] [-i [placeholder]] [-L number] [-l [number]] [-n number] [ -ptx] [-E [eofstr]] [-e [eofstr]] [-s size] [command [argument …]]
This template looks like a normal command, except that it lacks some arguments. xargs adds arguments from standard input (the standard input) to complete the command, then runs the resulting command
In these cases, an incorrect double-byte string would be detected during command-line parsing.. The maximum length of a constructed command is LINE_MAX bytes.
UNIX Shell Programming QuickStart [7]
of the script and lets the kernel know what shell will be interpreting. by the full pathname to the shell, and can be followed by options to
Wildcards must be escaped with either a backslash or. Local variables are in scope for the current shell.
Local variables can also be defined with the built-in declare. set for the currently running shell and any process spawned from that
Learning the bash Shell, 3rd Edition [Book] [8]
If you have become familiar with the customization techniques we presented in the previous chapter, you have probably run into various modifications to your environment that you want to make but can’t—yet. bash has some of the most advanced programming capabilities of any command interpreter of its type
In fact, bash can be used as a complete environment for writing software prototypes.. Some aspects of bash programming are really extensions of the customization techniques we have already seen, while others resemble traditional programming language features
Experience with a conventional programming language like Pascal or C is helpful (though not strictly necessary) for subsequent chapters. Throughout the rest of the book, we will encounter occasional programming problems, called tasks, whose solutions make use of the concepts we cover.
Catching user input [9]
The read built-in command is the counterpart of the echo and printf commands. One line is read from the standard input, or from the file descriptor supplied as an argument to the -u option
If there are fewer words read from the input stream than there are names, the remaining names are assigned empty values.. The characters in the value of the IFS variable are used to split the input line into words or tokens; see Section 3.4.8
If no names are supplied, the line read is assigned to the variable REPLY.. The return code of the read command is zero, unless an end-of-file character is encountered, if read times out or if an invalid file descriptor is supplied as the argument to the -u option.
Special Characters [10]
What makes a character special? If it has a meaning beyond its literal meaning, a meta-meaning, then we refer to it as a special character. Along with commands and keywords, special characters are building blocks of Bash scripts.
Comments may also occur following the end of a command.. Comments may also follow whitespace at the beginning of a line.
/g’ -e ‘s/_/_ /g’` ) # Excerpted from life.sh script. A command may not follow a comment on the same line
Read values into a shell variable from a pipe [11]
The questions is how to catch output from a command to save in variable(s) for use later in a script. I might repeat some earlier answers but I try to line up all the answers I can think up to compare and comment, so bear with me.
In contrast other shells define all piped commands as subshells including the last.. But having to copy with other shells, bash f.ex., another construct must be used.
To catch more values this construct is useful and works in bash and ksh:. There is a variant which I have noticed work in bash but not in ksh:
Unpack arguments into a command line flags using shell script [12]
I am trying to create a shell script that will unpack multiple arguments and put them in a one-line with multiple flags. # How script will be run ./script “database” “collection1 collection2 collection3”
${collections} variable which takes space-separated arguments into an array or something and calls each collection together with the
Shell Command Language [13]
This chapter contains the definition of the Shell Command Language.. This chapter describes the syntax of that command language as it is used by the sh utility and the system() and popen() functions defined in the System Interfaces volume of POSIX.1-2017.
The specific details are included in the cited sections of this chapter.. The shell reads its input from a file (see sh), from the -c option or from the system() and popen() functions defined in the System Interfaces volume of POSIX.1-2017
The shell breaks the input into tokens: words and operators; see Token Recognition.. The shell parses the input into simple commands (see Simple Commands) and compound commands (see Compound Commands).
How to redirect shell command output [14]
Shell scripts provide a very powerful feature: the ability to redirect the output from commands and scripts and send it to files, devices, or even as input to other commands or scripts.. Commands and scripts in a shell can generate two basic types of outputs:
– STDERR: The error output from a command/script (file descriptor 2). By default, STDOUT and STDERR are sent to your terminal’s screen.
A file descriptor is a unique identifier for a file or other I/O resource.. There are multiple ways to redirect output from shell scripts and commands.
Shell Scripting for Beginners – How to Write Bash Scripts in Linux [15]
Shell scripting is an important part of process automation in Linux. Scripting helps you write a sequence of commands in a file and then execute them.
You can perform daily tasks efficiently and even schedule them for automatic execution.. You can also set certain scripts to execute on startup such as showing a particular message on launching a new session or setting certain environment variables.
– What is a bash script and how do you identify it?. – How to create your first bash script and execute it.
Bash Reference Manual [16]
Next: Introduction, Previous: (dir), Up: (dir) [Contents][Index]. This text is a brief description of the features that are present in the Bash shell (version 5.2, 19 September 2022)
This is Edition 5.2, last updated 19 September 2022,. Bash contains features that appear in other popular shells, and some features that only appear in Bash
The following menu breaks the features up into categories, noting which features were inspired by other shells and which are specific to Bash.. This manual is meant as a brief introduction to features found in Bash
NSWI177 [17]
Labs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14.. In this lab we will complete our tour of shell scripting: you will learn about conditions and loops in the shell, and about variables
Especially the for loops over list of files are often used directly on the command-line without enclosing them in a full-fledged script.. Before diving into shell programming, we need to introduce you to ShellCheck
These issues are not syntax errors neither logical errors. The issues raised by ShellCheck are patterns that are well-known to cause unexpected behavior, degrade performance or may be even hiding some nasty surprises.
Piping in Unix or Linux [18]
A pipe is a form of redirection (transfer of standard output to some other destination) that is used in Linux and other Unix-like operating systems to send the output of one command/program/process to another command/program/process for further processing. The Unix/Linux systems allow the stdout of a command to be connected to the stdin of another command
The pipe is used to combine two or more commands, and in this, the output of one command acts as input to another command, and this command’s output may act as input to the next command, and so on. It can also be visualized as a temporary connection between two or more commands/ programs/ processes
This direct connection between commands/ programs/ processes allows them to operate simultaneously and permits data to be transferred between them continuously rather than having to pass it through temporary text files or through the display screen.. Pipes are unidirectional i.e., data flows from left to right through the pipeline.
Variables, Reading Input, and Mathematics [19]
We’ve seen variables a few times in our journey so far. In this chapter we’ll look at variables in a bit more detail
Variables are places where the system, the shell, or shell users like ourselves can store data.. We’ve already seen variables a few times in this book
When we want to use a variable in the shell, we use the. If you run this command you will see something like this:
Bourne Shell Programming [20]
Many standard utilities (rdist, make, cron, etc.) allow you to specify a command to run at a certain time. Usually, this command is simply passed to the Bourne shell, which means that you can execute whole scripts, should you choose to do so.
If you want to modify the boot-time behavior of a system, you need to learn to write and modify Bourne shell scripts.. Furthermore, and this is what this tutorial is all about, you can put commands in a file and execute them all at once
There are several things to note here: first of all, comments begin with a hash (#) and continue to the end of the line (the first line is special, and we’ll cover that in just a moment).. Secondly, the script itself is just a series of commands
Sources
- https://www.ibm.com/docs/SSLTBW_2.2.0/com.ibm.zos.v2r2.bpxa400/std.htm#:~:text=Using%20the%20shell%3A%20In%20the,for%20the%20standard%20output%20file.
- https://www.tutorialspoint.com/understanding-stdin-stderr-and-stdout-in-linux#:~:text=stdin%20%E2%88%92%20It%20stands%20for%20standard,It%20stands%20for%20standard%20error.
- https://riptutorial.com/bash/example/7602/stdin–stdout-and-stderr-explained#:~:text=Standard%20input%20is%20used%20to,read%20a%20line%20from%20STDIN.)&text=Standard%20output%20is%20generally%20used,files%20are%20sent%20to%20STDOUT.
- https://www.learnlinux.org.za/courses/build/shell-scripting/ch01s04.html#:~:text=Generally%20standard%20input%2C%20referred%20to,(a%20standard%20input%20terminal).
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- https://stackoverflow.com/questions/73393441/unpack-arguments-into-a-command-line-flags-using-shell-script
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- https://www.redhat.com/sysadmin/redirect-shell-command-script-output
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