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![Linux System Administration Module 3. Work Effectively on the Unix Command Line
3.9 Where Programs are Found
s The location of a program can be specified explicitly:
q ./sample runs the sample program in the current directory
q /bin/ls runs the ls command in the /bin directory
s Otherwise, the shell looks in standard places for the program
q The variable called PATH lists the directories to search in
q Directory names are separated by colon, for example:
$ echo $PATH
/bin:/usr/bin:/usr/local/bin
q So running whoami will run /bin/whoami or /usr/bin/whoami or /usr/local/bin/whoami
(whichever is found first)
3.10 Bash Configuration Variables
s Some variables contain information which Bash itself uses
q The variable called PS1 (Prompt String 1) specifies how to display the shell prompt
s Use the echo command with a $ sign before a varable name to see its value, e.g.
$ echo $PS1
[u@h W]$
s The special characters u, h and W represent shell variables containing, respectively, your
user/login name, machine’s hostname and current working directory, i.e.,
q $USER, $HOSTNAME, $PWD
3.11 Using History
s Previously executed commands can be edited with the Up or Ctrl+P keys
s This allows old commands to be executed again without re-entering
s Bash stores a history of old commands in memory
q Use the built-in command history to display the lines remembered
q History is stored between sessions in the file ˜/.bash_history
s Bash uses the readline library to read input from the user
q Allows Emacs-like editing of the command line
q Left and Right cursor keys and Delete work as expected
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![Linux System Administration Module 4. Process Text Streams Using Text Processing Filters
4.15 Paginating Text Files with pr
s Convert a text file into paginated text, with headers and page fills
s Rarely useful for modern printers
s Options:
q -d double spaced output
q -h header change from the default header to header
q -l lines change the default lines on a page from 66 to lines
q -o width set (‘offset’) the left margin to width
s Example:
$ pr -h "My Thesis" thesis.txt | lpr
4.16 Dividing Files into Chunks with split
s Splits files into equal-sized segments
s Syntax: split [options] [input] [output-prefix]
s Use -l n to split a file into n-line chunks
s Use -b n to split into chunks of n bytes each
s Output files are named using the specified output name with aa, ab, ac, etc., added to the end
of the prefix
s Example: Split essay.txt into 30-line files, and save the output to files short_aa, short_ab, etc:
$ split -l 30 essay.txt short_
4.17 Using split to Span Disks
s If a file is too big to fit on a single floppy, Zip or CD-ROM disk, it can be split into small enough
chunks
s Use the -b option, and with the k and m sufixes to give the chunk size in kilobytes or megabytes
s For example, to split the file database.tar.gz into pieces small enough to fit on Zip disks:
$ split -b 90m database.tar.gz zip-
s Use cat to put the pieces back together:
$ cat zip-* > database.tar.gz
4.18 Reversing Files with tac
s Similar to cat, but in reverse
s Prints the last line of the input first, the penultimate line second, and so on
s Example: show a list of logins and logouts, but with the most recent events at the end:
$ last | tac
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![Linux System Administration Module 5. Perform Basic File Management
5.6 Wildcard Patterns
s Give commands multiple files by specifying patterns
s Use the symbol * to match any part of a filename:
$ ls *.txt
accounts.txt letter.txt report.txt
s Just * produces the names of all files in the current directory
s The wildcard ? matches exactly one character:
$ rm -v data.?
removing data.1
removing data.2
removing data.3
s Note: wildcards are turned into filenames by the shell, so the program you pass them to can’t
tell that those names came from wildcard expansion
5.7 Copying Files with cp
s Syntax: cp [options] source-file destination-file
s Copy multiple files into a directory: cp files directory
s Common options:
q -f, force overwriting of destination files
q -i, interactively prompt before overwriting files
q -a, archive, copy the contents of directories recursively
5.8 Examples of cp
s Copy /etc/smb.conf to the current directory:
$ cp /etc/smb.conf .
s Create an identical copy of a directory called work, and call it work-backup:
$ cp -a work work-backup
s Copy all the GIF and JPEG images in the current directory into images:
$ cp *.gif *.jpeg images/
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![Linux System Administration Module 5. Perform Basic File Management
5.15 Changing Timestamps with touch
s Changes the access and modification times of files
s Creates files that didn’t already exist
s Options:
q -a, change only the access time
q -m, change only the modification time
q -t [YYYY]MMDDhhmm[.ss], set the timestamp of the file to the specified date and
time
q GNU touch has a -d option, which accepts times in a more flexible format
s For example, change the time stamp on homework to January 20 2001, 5:59p.m.
$ touch -t 200101201759 homework
5.16 Exercises
1. a. Use cd to go to your home directory, and create a new directory there called dog.
b. Create another directory within that one called cat, and another within that called mouse.
c. Remove all three directories. You can either remove them one at a time, or all at once.
d. If you can delete directories with rm -r, what is the point of using rmdir for empty directories?
e. Try creating the dog/cat/mouse directory structure with a single command.
2. a. Copy the file /etc/passwd to your home directory, and then use cat to see what’s in it.
b. Rename it to users using the mv command.
c. Make a directory called programs and copy everything from /bin into it.
d. Delete all the files in the programs directory.
e. Delete the empty programs directory and the users file.
3. a. The touch command can be used to create new empty files. Try that now, picking a name for the new
file:
$ touch baked-beans
b. Get details about the file using the ls command:
$ ls -l baked-beans
c. Wait for a minute, and then try the previous two steps again, and see what changes. What happens
when we don’t specify a time to touch?
d. Try setting the timestamp on the file to a value in the future.
e. When you’re finished with it, delete the file.
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![Linux System Administration Module 6. Use Unix Streams, Pipes and Redirects
6.9 Redirection with File Descriptors
s Redirection normally works with stdin and stdout
s Specify different files by putting the file descriptor number before the redirection symbol:
q To redirect the standard error to a file:
$ program 2> file
q To combine standard error with standard output:
$ program > file 2>&1
q To save both output streams:
$ program > stdout.txt 2> stderr.txt
s The descriptors 3–9 can be connected to normal files, and are mainly used in shell scripts
6.10 Running Programs with xargs
s xargs reads pieces of text and runs another program with them as its arguments
q Usually its input is a list of filenames to give to a file processing program
s Syntax: xargs command [initial args]
s Use -l n to use n items each time the command is run
q The default is 1
s xargs is very often used with input piped from find
s Example: if there are too many files in a directory to delete in one go, use xargs to delete them
ten at a time:
$ find /tmp/rubbish/ | xargs -l10 rm -f
6.11 tee
s The tee program makes a ‘T-junction’ in a pipeline
s It copies data from stdin to stdout, and also to a file
s Like > and | combined
s For example, to save details of everyone’s logins, and save Bob’s logins in a separate file:
$ last | tee everyone.txt | grep bob > bob.txt
PIPE PIPE REDIRECT
last tee grep bob.txt
everyone.txt
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![Linux System Administration Module 7. Search Text Files Using Regular Expressions
7.3 Matching Repeated Patterns
s Some regexp special characters are also special to the shell, and so need to be protected with
quotes or backslashes
s We can match a repeating pattern by adding a modifier:
$ grep -i ’continued.*’
s Dot (.) on its own would match any character, so to match an actual dot we escape it with
s The * modifier matches the preceding character zero or more times
s Similarly, the + modifier matches one or more times
7.4 Matching Alternative Patterns
s Multiple subpatterns can be provided as alternatives, separated with |, for example:
$ grep ’fish|chips|pies’ food.txt
s The previous command finds lines which match at least one of the words
s Use (...) to enforce precedence:
$ grep -i ’(cream|fish|birthday) cakes’ delicacies.txt
s Use square brackets to build a character class:
$ grep ’[Jj]oe [Bb]loggs’ staff.txt
s Any single character from the class matches; and ranges of characters can be expressed as
‘a-z’
7.5 Extended Regular Expression Syntax
s egrep runs grep in a different mode
q Same as grep -E
s Special characters don’t have to be marked with
q So + is written +, (...) is written (...), etc
q In extended regexps, + is a literal +
7.6 sed
s sed reads input lines, runs editing-style commands on them, and writes them to stdout
s sed uses regular expressions as patterns in substitutions
q sed regular expressions use the same syntax as grep
s For example, to used sed to put # at the start of each line:
$ sed -e ’s/ˆ/#/’ < input.txt > output.txt
s sed has simple substitution and translation facilities, but can also be used like a programming
language
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![Module 8
Job Control
8.1 Job Control
s Most shells offer job control
q The ability to stop, restart, and background a running process
s The shell lets you put & on the end of a command line to start it in the background
s Or you can hit Ctrl+Z to suspend a running foreground job
s Suspended and backgrounded jobs are given numbers by the shell
s These numbers can be given to shell job-control built-in commands
s Job-control commands include jobs, fg, and bg
8.2 jobs
s The jobs builtin prints a listing of active jobs and their job numbers:
$ jobs
[1]- Stopped vim index.html
[2] Running netscape &
[3]+ Stopped man ls
s Job numbers are given in square brackets
q But when you use them with other job-control builtins, you need to write them with percent
signs, for example %1
s The jobs marked + and - may be accessed as %+ or %- as well as by number
q %+ is the shell’s idea of the current job — the most recently active job
q %- is the previous current job
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![Linux System Administration Module 10. Modify Process Execution Priorities
10.3 renice
s renice changes the niceness of existing processes
s Non-root users are only permitted to increase a process’s niceness
s To set the process with pid 2984 to the maximum niceness (lower priority):
$ renice 20 2984
q The niceness is just a number: no extra - sign
s To set the process with pid 3598 to a lower niceness (higher priority):
$ renice -15 3598
s You can also change the niceness of all a user’s processes:
$ renice 15 -u mikeb
10.4 Exercises
1. a. Create the following shell script, called forever, in your home directory:
#!/bin/sh
while [ 1 ]; do
echo hello... >/dev/null;
done
Make it executable and run it in the background as follows:
$ chmod a+rx forever
$ ./forever &
b. Use ps -l to check the script’s nice level
c. Run the script with nice and give it a niceness of 15. Try running it alongside a less nice version, and
see what the difference is in top
d. Try using nice or renice to make a process’ niceness less than 0
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![Linux System Administration Module 11. Advanced Shell Usage
11.7 Globbing Files Within Directories
s 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
s 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/lpd.8.gz
man1/lpr.1.gz man1/lptest.1.gz man8/lpc.8.gz
11.8 Globbing to Match a Single Character
s * matches any sequence of characters
s To match any single character, use ?:
$ ls ?ouse.txt
Matches mouse.txt and house.txt, but not grouse.txt
s 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
11.9 Globbing to Match Certain Characters
s Instead of matching any single character, we can arrange to match any of a given group of
characters
s *.[ch] matches any file ending in .c or .h
s *[0-9].txt matches any text file with a digit before the dot
s You can use a caret as the first thing in the brackets to match any character that isn’t one of the
listed ones
s [ˆa-z]*.jpg matches any JPEG file that doesn’t begin with a lower-case letter
s To match any hidden file except the . and .. directories: .[ˆ.]*
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![Linux System Administration Module 11. Advanced Shell Usage
11.10 Generating Filenames: {}
s 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}}
s 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}}
s Or combine brace expansion with quoting:
$ echo ’Hello ’{world,Mum}!
Hello world! Hello Mum!
s Braces can be used for generating any strings, not just filenames
s Distinctly different from ordinary glob expansion — the words generated don’t need to be
names of existing files or directories
11.11 Shell Programming
s The shell is designed to be both:
q A convenient environment to type commands into
q A simple programming language
s Any command that can be typed at the command line can be put into a file — and vice versa
s Programming features include variables, loops (including for), and even shell functions
s The Unix component approach makes it very easy to write shell scripts to perform fairly
complex tasks
s Common application domains for shell scripting include:
q Text processing
q Automation of system administration tasks
11.12 Exercises
1. 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.
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![Linux System Administration Module 15. Use File Permissions to Control Access to Files
15.7 Changing File and Directory Permissions: chmod
s The chmod command changes the permissions of a file or directory
q A file’s permissions may be changed only by its owner or by the superuser
s chmod takes an argument describing the new permissions
q Can be specified in many flexible (but correspondingly complex) ways
s Simple example:
$ chmod a+x new-program
adds (+) executable permission (x) for all users (a) on the file new-program
15.8 Specifying Permissions for chmod
s Permissions can be set using letters in the following format:
[ugoa][+=-][rwxX]
s The first letters indicate who to set permissions for:
q u for the file’s owner, g for the group owner, o for other users, or a for all users
s = sets permissions for files, + adds permissions to those already set, and - removes
permissions
s The final letters indicate which of the r, w, x permissions to set
q Or use capital X to set the x permission, but only for directories and already-executable
files
15.9 Changing the Permissions of a Directory and Its Contents
s A common requirement is to change the permissions of a directory and its contents
s chmod accepts a -R option:
$ chmod -R g+rwX,o+rX public-directory
s Mnemonic: ‘recursive’
s Adds rwx permissions on public-directory for the group owner, and adds rx permissions on it
for everyone else
q And any subdirectories, recursively
q Any any contained executable files
q Contained non-executable files have rw permissions added for the group owner, and
r permission for everyone else
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Uploaded byAlessandro Grandi
Cesvip 2010 first_linux_module
This document provides an overview of a Linux system administration training course. It covers 28 chapters on topics like the Unix command line, file management, processes, filesystems, backups and more. Each chapter includes detailed explanations, examples and exercises to help system administrators effectively manage and maintain Linux systems.
