Linux administration

• The shell actually has three different quoting mechanisms:
  • Single quotes
  • Backslashes
  • Double quotes

• Putting single quotes round something protects it from special interpretation by the shell:

$ xmms 'Tom Lehrer - Poisoning Pigeons in the Park.mp3'
$ rm 'b*lls and whistles'

• But single quotes (obviously) don't protect single quotes themselves
  • So you can't quote something like She said, "Don't go." with only single quotes

• You can put a backslash \ in front of any single character to turn off its special meaning:

$ echo M\&S
$ xmms Suzanne\ Vega\ -\ Tom\'s\ Diner.mp3
$ mail -s C:\\MSDOS.SYS windows-user@example.com

• Putting double quotes round something protects most things within it from interpretation by the shell
  • A dollar sign $ retains its special interpretation
  • As do backticks “
  • ! can't be escaped in double quotes
• A backslash can be used within double quotes to selectively disable the special interpretation of $, ' and \:

$ mail -s "C:\\MSDOS.SYS" windows-user@example.com
$ echo "It cost $price US\$"

• Putting a backslash in front of anything else gives you both characters:

$ echo "\*/"
\*/

• You can build up an argument for a command by combining several chunks of differently-quoted text

• Just put the chunks next to each other with no intervening whitespace:

$ echo "double-quoted"'.single-quoted.'unquoted
double-quoted.single-quoted.unquoted
$ echo 'She said, "Don'\''t go."'
She said, "Don't go."

• Rarely needed - the last example is probably better written as:

$ echo "She said, \"Don't go.\""

• * in a glob pattern can stand for any sequence of characters:

$ ls -l *.txt
-rw-rw-r-1 fred users   108 Nov 16 13:06 report.txt
-rw-rw-r-1 fred users   345 Jan 18 08:56 notes.txt

• * on its own expands to all files in the current directory
• Glob expansion is done by the shell
  • So a program can't tell when the user ran it with a glob as an argument

• You can use globs to get filenames within directories:

$ ls Accounts/199*.txt
Accounts/1997.txt Accounts/1998.txt Accounts/1999.txt
$ ls ../images/*.gif
../images/logo.gif ../images/emblem.gif

• You can also use globs to expand names of intervening directories:

$ cd /usr/man && ls man?/lp*

man1/lpq.1.gz   man1/lprm.1.gz      man4/lp.4.gz    man8/lpc.8.gz
man1/lpr.1.gz   man1/lptest.1.gz    man8/lpd.8.gz

• * matches any sequence of characters
• To match any single character, use ?:

$ ls ?ouse.txt

Matches mouse.txt and house.txt, but not grouse.txt

• Can be useful for making sure that you only match files of at least a certain length:

$ rm ???*.txt

Matches any file ending in .txt that has at least three characters before the dot

• Instead of matching any single character, we can arrange to match any of a given group of characters

• *.[ch] matches any file ending in .c or .h

• *[0-9].txt matches any text file with a digit before the dot

• You can use a caret as the first thing in the brackets to match any character that isn't one of the listed ones

• [ˆa-z]*.jpg matches any JPEG file that doesn't begin with a lower-case letter

• To match any hidden file except the . and .. directories: .[ˆ.]*

• You can use braces {} to generate filenames:

$ mkdir -p Accounts/200{1,2}
$ mkdir Accounts/200{1,2}/{0{1,2,3,4,5,6,7,8,9},1{0,1,2}}

• You could even combine those two lines:

$ mkdir -p Accounts/200{1,2}/{0{1,2,3,4,5,6,7,8,9},1{0,1,2}}

• Or combine brace expansion with quoting:

$ echo 'Hello '{world,Mum}\!
Hello world! Hello Mum!

• Braces can be used for generating any strings, not just filenames
• Distinctly different from ordinary glob expansion — the words generated don't need to be names of existing files or directories

• The shell is designed to be both:
  • A convenient environment to type commands into
  • A simple programming language
• Any command that can be typed at the command line can be put into a file — and vice versa
• Programming features include variables, loops (including for), and even shell functions

• The Unix component approach makes it very easy to write shell scripts to perform fairly complex tasks

• Common application domains for shell scripting include:

  • Text processing
  • Automation of system administration tasks

• a. Print out the following message: *** SALE $$$ ***.
• b. Try escaping the same string using single quotes, double quotes and backslashes.
• c. Echo the message 'quoting isn't simple', escaping the spaces by putting single quotes around it.
• d. Use the glob pattern .[ˆ.]* to list all the hidden files in your home directory.
• e. To find out what shells are available on your system, list the programs in /bin whose names end in sh.
• f. Use [] brackets to list all the files in /usr/bin with names starting with a, b or c.