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