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<input type="hidden" name="sectok" value=""/><input type="hidden" name="id" value="syntax:words"/><input type="hidden" name="rev" value="0"/><input type="hidden" name="date" value="0"/><input type="hidden" name="prefix" value="."/><input type="hidden" name="suffix" value=""/><input type="hidden" name="changecheck" value="e476e521103ce724908eb5986ede1787"/><input type="hidden" name="target" value="section"/><textarea name="wikitext" id="wiki__text" dir="auto" class="edit form-control" cols="80" rows="10" tabindex="1" readonly="readonly">
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====== Words... ======
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{{keywords>bash shell scripting token words split splitting recognition}}
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FIXME This article needs a review, it covers two topics (command line splitting and word splitting) and mixes both a bit too much. But in general, it's still usable to help understand this behaviour, it's "wrong but not wrong".
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One fundamental principle of Bash is to recognize words entered at the command prompt, or under other circumstances like variable-expansion.
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===== Splitting the commandline =====
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Bash scans the command line and splits it into words, usually to put the parameters you enter for a command into the right C-memory (the ''argv'' vector) to later correctly call the command. These words are recognized by splitting the command line at the special character position, **Space** or **Tab** (the manual defines them as **blanks**).
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For example, take the echo program. It displays all its parameters separated by a space. When you enter an echo command at the Bash prompt, Bash will look for those special characters, and use them to separate the parameters.
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You don't know what I'm talking about? I'm talking about this:
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<code>
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$ echo Hello little world
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Hello little world
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</code>
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In other words, something you do (and Bash does) everyday. The characters where Bash splits the command line (SPACE, TAB i.e. blanks) are recognized as delimiters.
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There is no null argument generated when you have 2 or more blanks in the command line. **A sequence of more blank characters is treated as a single blank.**
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Here's an example:
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<code>
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$ echo Hello little world
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Hello little world
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</code>
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Bash splits the command line at the blanks into words, then it calls echo with **each word as an argument**. In this example, echo is called with three arguments: "''Hello''", "''little''" and "''world''"!
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__Does that mean we can't echo more than one Space?__ Of course not!
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Bash treats blanks as special characters, but there are two ways to tell Bash not to treat them special: **Escaping** and **quoting**.
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Escaping a character means, to **take away its special meaning**. Bash will use an escaped character as text, even if it's a special one. Escaping is done by preceeding the character with a backslash:
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<code>
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$ echo Hello\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ little \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ world
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Hello little world
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</code>
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None of the escaped spaces will be used to perform word splitting. Thus, echo is called with one argument: "''Hello little world''".
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Bash has a mechanism to "escape" an entire string: **Quoting**.
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In the context of command-splitting, which this section is about, it doesn't matter which kind of quoting you use: weak quoting or strong quoting, both cause Bash to not treat spaces as special characters:
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<code>
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$ echo "Hello little world"
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Hello little world
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</code>
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<code>
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$ echo 'Hello little world'
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Hello little world
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</code>
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__What is it all about now?__ Well, for example imagine a program that expects a filename as an argument, like cat. Filenames can have spaces in them:
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<code>
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$ ls -l
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total 4
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-rw-r--r-- 1 bonsai bonsai 5 Apr 18 18:16 test file
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$ cat test file
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cat: test: No such file or directory
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cat: file: No such file or directory
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$ cat test\ file
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m00!
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$ cat "test file"
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m00!
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</code>
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If you enter that on the command line with Tab completion, that will take care of the spaces. But Bash also does another type of splitting.
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===== Word splitting =====
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For a more technical description, please read the [[syntax:expansion:wordsplit | article about word splitting]]!
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The first kind of splitting is done to parse the command line into separate tokens. This is what was described above, it's a pure **command line parsing**.
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After the command line has been split into words, Bash will perform expansion, if needed - variables that occur in the command line need to be expanded (substituted by their value), for example. This is where the second type of word splitting comes in - several expansions undergo **word splitting** (but others do not).
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Imagine you have a filename stored in a variable:
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<code>
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MYFILE="test file"
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</code>
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When this variable is used, its occurance will be replaced by its content.
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<code>
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$ cat $MYFILE
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cat: test: No such file or directory
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cat: file: No such file or directory
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</code>
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Though this is another step where spaces make things difficult, **quoting** is used to work around the difficulty. Quotes also affect word splitting:
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<code>
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$ cat "$MYFILE"
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m00!
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</code>
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===== Example =====
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Let's follow an unquoted command through these steps, assuming that the variable is set:
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<code>
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MYFILE="THE FILE.TXT"
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</code>
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and the first review is:
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<code>
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echo The file is named $MYFILE
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</code>
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The parser will scan for blanks and mark the relevant words ("splitting the command line"):
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^Initial command line splitting:^^^^^^
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^Word 1^Word 2^Word 3^Word 4^Word 5^Word 6^
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|''echo''|''The''|''file''|''is''|''named''|''$MYFILE''|
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A [[syntax:pe | parameter/variable expansion]] is part of that command line, Bash will perform the substitution, and the [[syntax:expansion:wordsplit | word splitting]] on the results:
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^Word splitting after substitution:^^^^^^^
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^Word 1^Word 2^Word 3^Word 4^Word 5^Word 6^Word 7^
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|''echo''|''The''|''file''|''is''|''named''|''THE''|''FILE.TXT''|
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Now let's imagine we quoted ''$MYFILE'', the command line now looks like:
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<code>
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echo The file is named "$MYFILE"
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</code>
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^Word splitting after substitution (quoted!):^^^^^^
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^Word 1^Word 2^Word 3^Word 4^Word 5^Word 6^
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|''echo''|''The''|''file''|''is''|''named''|''THE FILE.TXT''|
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**//Bold Text//72i love this world**===== See also =====
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* Internal: [[syntax:quoting | Quoting and character escaping]]
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* Internal: [[syntax:expansion:wordsplit | Word splitting]]
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* Internal: [[syntax:expansion:intro | Introduction to expansions and substitutions]]
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* External: [[http://www.grymoire.com/Unix/Quote.html | Grymore: Shellquoting]]</textarea>
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