Pathname Expansion:
[me@linuxbox me]$ echo [[:upper:]]*
Desktop Documents Music Pictures Public Templates Videos
Tilde Expansion:
[me@linuxbox me]$ echo ~
/home/me
Arithmetic Expansion:
[me@linuxbox me]$ echo $((2 + 2))
4
which takes the form:
$((expression))
Arithmetic expansion only supports integers (whole numbers, no decimals).
Braces Expansion:
Maybe this is the strangest expansion. you can create multiple strings with a pattern of braces. such as:
[me@linuxbox me]$ echo Front-{A,B,C}-Back
Front-A-Back Front-B-Back Front-C-Back
The content in the braces may contain a preamble and a postscript.
[me@linuxbox me]$ echo Number_{1..5}
Number_1 Number_2 Number_3 Number_4 Number_5
It can also be nested:
[me@linuxbox me]$ echo a{A{1,2},B{3,4}}b
aA1b aA2b aB3b aB4b
So what is this good for? Consider you have a bunch of folders to be created:
[me@linuxbox me]$ mkdir Photos [me@linuxbox me]$ cd Photos [me@linuxbox Photos]$ mkdir {2007..2009}-0{1..9} {2007..2009}-{10..12} [me@linuxbox Photos]$ ls 2007-01 2007-07 2008-01 2008-07 2009-01 2009-07 2007-02 2007-08 2008-02 2008-08 2009-02 2009-08 2007-03 2007-09 2008-03 2008-09 2009-03 2009-09 2007-04 2007-10 2008-04 2008-10 2009-04 2009-10 2007-05 2007-11 2008-05 2008-11 2009-05 2009-11 2007-06 2007-12 2008-06 2008-12 2009-06 2009-12
Parameter Expansion:
[me@linuxbox me]$ echo $USER me
To see available variables:
[me@linuxbox me]$ printenv | less
Command Substituion
Command Substitution allows us to use the output of a command as an expansion. e.g.
[me@linuxbox me]$ echo $(ls)
Desktop Documents ls-output.txt Music Pictures Public Templates Videos
Another one:
me@linuxbox me]$ ls -l $(which cp)
-rwxr-xr-x 1 root root 71516 2007-12-05 08:58 /bin/cp
The alternative syntax is to use back-quotes instead of the dollar sign and parenthesis:
[me@linuxbox me]$ ls -l `which cp`
-rwxr-xr-x 1 root root 71516 2007-12-05 08:58 /bin/cp
Double Quotes:
All special characters used by the shell lose their special meaning and are treated as ordinary characters. The exceptions are "$", "" and "`". This means word-splitting, pathname expansion, tilde expansion, and brace expansion are all suppressed. but parameter expansion, arithmetic expansion, ad command substitution are still carried out.
[me@linuxbox me]$ echo "$USER $((2+2)) $(cal)" me 4 February 2008 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
The fact that newlines are considered delimiters by the word-splitting mechanism causes an interesting, albeit subtle, effect on command substitution. Consider the followings:
[me@linuxbox me]$ echo $(cal) February 2008 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 [me@linuxbox me]$ echo "$(cal)" February 2008 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Single Quotes
To suppress all expansions, use single quotes:
[me@linuxbox me]$ echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
text /home/me/ls-output.txt a b foo 4 me
[me@linuxbox me]$ echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER"
text ~/*.txt {a,b} foo 4 me
[me@linuxbox me]$ echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER'
text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
Escaping Characters:
Backslash is used as the escape character. It's usually used to selectively prevent an expansion:
[me@linuxbox me]$ echo "The balance for user $USER is: $5.00" The balance for user me is: $5.00
More about backslash
Use backslash to get the shell to ignore the newline like this:
ls -l --reverse --human-readable --full-time
Backslash escape characters:
[me@linuxbox me]$ echo -e "Inserting several blank lines " Inserting several blank lines [me@linuxbox me]$ echo -e "Words separated by horizontal tabs." Words separated by horizontal tabs [me@linuxbox me]$ echo -e "aMy computer went "beep"." My computer went "beep". [me@linuxbox me]$ echo -e "DEL C:\WIN2K\LEGACY_OS.EXE" DEL C:WIN2KLEGACY_OS.EXE
From: http://linuxcommand.org/lc3_lts0080.php