You can prevent the shell from interpreting a character by placing a Quoting a single character with the backslash Quote, and double quote, and the backslash. The three quoting mechanisms you can use are the single Suppose you had a file in your directory that had a "*" in the filename? How could you list, or remove, just that file? You have to quote the "*" to get it past the shell. The "ls" command never sees the "*" character. The shell sees the "*" character, which it considers to be special, and it changes "*" to match the filed in the current directory, and then it calls the "ls" comand with those arguments. If you want to understand how the shell interprets meta-characters, put an "echo" in front of the command.įor instance, to understand what happens when you type The first thing to learn is the "echo" command built into the Unix shell. Let's get back to the Unix quoting mechanisms. While I'm here, I'll breifly mention that newer POSIX shells denegrate the backticks cor command substitution, and now use this form: That character is used for command substitution, where the characters between them are executed by the shell and the results is inserted on that line. The back quote is not used for quoting characters. The first two are used for quoting phrases in Unix. It looks like the single quote and some times people get them confused in shell scripts. (more properly called backtick or grave) character: "`". The third quotation mark is the back quote Two of them use marks used for quotations in English usage, and are sometimes called the single quote and double quote. There are three different "quotation" marks on the keyboard.
Shell is interpreting the special characters, so you can pinpoint I will also discuss how you can verify the What is important is a solid understanding of the characters that have I am not going to cover each one of these special meta-characters.
Output to new file, ignore error if not there Separates Values in environment variablesĪppend to file, ignore error if not there List of Special Characters and what they mean Character I just wanted you to realize this stuff can be complicated.Īnd if aren't sure if something is a special character or not, it's a good idea to use quotes just in case. There is a lot of detail on this chart, and it's far from complete.įrankly, skip over this for now. I have also included several combinations of characters just to make this table Here is a chart of the meta-characters the Bourne and C shell know There are some tricky bits, but I will mention them later when I cover the different shells. In other words, it doesn't matter which shell you use - because they all understand the three methods of quoting/escaping meta-characters - more or less. That is, what I describe here holds for all of the major varieties of shells, including the dreaded C shell. I wrote this tutorial in a shell-agnostic manner.
To quote that character, so that it is passed to the program without If you need a regular expression, you must know if any of theĬharacters of the expression are meta-characters, and must know
It could be the beginning of a variable name, or it could be When you quote a character, you ask the shell to leave it alone - and pass it on unchanged to the utility. In other works, you put quotes around the meta-characters to inticate to the shell that they are not special - as far as the shell is concerned. I want the utility to treat them special, not you!" The mechanism to do this is called quoting. Or to put it another way, you want to say "Hey Shell.
#CMD C QUOTES HOW TO#
Some characters are special to the Unix shell, and some are special to a utility like sed or grep.īefore you study regular expressions, it is important that you understand how to use the shell to send regular expressions to a program. The actual meaning depends on the program that sees this meta-character. This is part of my Unix tutorials series.įirst thing you have to understand is that certain characters, called meta-characters, have special meanings.
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