3.1 gnu and unix commands v4

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System Administration

Key Knowledge Areas
Use single shell commands and one line command sequences to perform basic tasks on the
command line.
Use and modify the shell environment including defining, referencing and exporting
environment variables.
Use and edit command history.
Invoke commands inside and outside the defined path.
Unix Commands
Work on the command line
Terms and Utilities
. Bash echo
env exec Export
pwd set unset
man uname history
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GNU and Unix Commands
Syntax:
$ command options arguments
Using the Command Line
3
$ ls # simple commandEx:
$ ls –F # command with options
$ ls -F /etc # execute command on other directory
$ ls -F /etc ; ls -F /usr # first command will complete before next command is started
$ ls -F /etc # ignore Enter key and treat all commands as though they are on the same cmdline.
ls -F /usr

Basic Commands
4
$ who #Lists currently logged in users
$ uptime #Statistics about machine usage and run time
$ echo #Prints the given arguments to the screen
$ date #Print current date and time
$ exit #Terminate current shell session
$ reset #Reset terminal state to default settings
Ex:

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Command completion:
bash shell includes a feature called command completion.
This enables to type first few letters of the command at the prompt, hit the Tab key, and have the system
complete the command.
$ dm <press tab key>Ex:
$ dmesg
• If there is more than one possible match, the system will simply beep.
• Pressing Tab key again will display all possible command matches.
• Pressing Esc twice performs same action as pressing the Tab key.

Command Line Working
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1. Hierarchie Organization
•Data is stored in files
•Files are grouped and organized in Directories, creating a tree structure
•Filesystem begins at root, represented as: /
•The Standard Hierarchy provides basic organization
2. Working Directory
•Operations within shell generally gather input from files and output information
to files, so the shell tracks a “working directory” to ease file specifications, and
have a default location to output files if one is not provided.
$ pwd # Print Working DirectoryEx:
$ cd # Change [working] Directory

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3. Pathnames
•A pathname specifies exact location of a file or directory within the filesystem.
There are two types of pathnames: absolute and relative
 Absolute pathname
An absolute pathname uses root of the filesystem as starting location for path search.
Ex: /etc/passwd
Starting from /, descend into etc folder, then locate file named passwd
The key is the leading slash - exactly giving the starting point
 Relative pathname
Relative pathnames only specify a file’s location with respect to a working directory.
Path is relative to current working directory. Relative pathnames never start with /.
Ex: memo/january.txt

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4. Pathname Wildcards
Wildcards are a set of metacharacters which provide a shorthand notation for
specifying large groups of files. There are 3 basic pathname wildcards:
* Match 0 or more characters. Any characters. Ex: * ; a* ; *.txt
? Matches exactly 1 character. Can be any character, but there must be exactly 1.
Ex: file?.txt ; log-???? ; ????*
[set] Match exactly 1 character, character must be from the set. Great flexibility in specifying the set.
Useful when filenames are following a specific pattern.
Ex: log-2009-1.[012]-* ; [a-zA-Z]*
•Each desired character can be directly typed into the set: [012345]
•Ranges are acceptable. Starting point must be “less” than ending point. Starting/ending case must match
for letters: [0-5] ; [d-h] ; [N-Z]
•Can Mix and match: [0-9a-zA-Z].
•If a hyphen is needed to be part of the set, specify it first: [-acg0-4]
•You can specify an “anti” set. Anything listed in set will not match: start set with !:[!0-9]

In order to master the shell, you have to understand it’s inner workings
concept of The Big Loop:
Shells
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1. Print prompt, await user input
2. Parse and verify input; on error, loop
Syntax checking, command identification, metacharacter substitutions and operations
3. Perform requested operation ( execute command, built-in )
4. Loop

The shell allows users to enter commands and then interprets these commands into
instructions for Linux operating system.
Linux allows to use a number of different shells; default shell installed with Linux is bash shell.
default shell is specified in /etc/passwd file and can be changed there on a per user basis.
Shells
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To change to a different shell, you simply type the full path along with the command name of
the new shell. Ex. /bin/sh
To return to the default login shell, type exit or press CTRL-D.
The default login shell is contained in the environment variable SHELL.
Each shell has corresponding resource configuration file - rc file, located in /etc directory
It contains global settings for the shell. For the bash shell the file is /etc/bashrc.
Related information to environment goes to /etc/profile
To know which default shell is currently configured, type: echo $SHELL

The available shells and paths are listed in /etc/shells
Shells
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Shell Explanation
/bin/bash Bourne-Again Shell that is compatible with the sh shell, which includes features of both the
Korn and C shells. This may be a link to the /bin/bash2 file on some distributions.
/bin/sh The Bourne Shell. On many systems this file is linked to /bin/bash.
/bin/ash A System V version of the sh shell.
/bin/bsh This file is linked to /bin/ash.
/bin/bash2 Bourne-Again Shell version 2.
/bin/csh The Berkeley UNIX C shell.
/bin/tcsh An enhanced version of the Berkeley UNIX C shell.
chsh command is used to change the default shell for the user
Options Alternate Function
-s --shells Specifies the default login shell for this user.
-l --list Lists the shells specified in the /etc/shells file.
-u --help Displays the options for the chsh command.
-v --version Displays version information for the shell.

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Editing shell commands with the Readline Library:
Key Combination Function
Ctrl-b Move back one character.
Ctrl-f Move forward one character.
Del Delete the character to the left of the cursor.
Ctrl-d Delete the character underneath the cursor.
Ctrl-a Move to the start of the line.
Ctrl-e Move to the end of the line.
Esc-f Move forward a word.
Esc-b Move backward a word.
Ctrl-l Clear the screen, reprinting the current line at the top.
Ctrl-k Kill (delete) the text from the current cursor position to the end of the line.
Esc-d Kill from the cursor to the end of the current word, or if between words, to the end of the next word.
Esc-Del Kill from the cursor the start of the previous word, or if between words, to the start of the previous word.
Ctrl-w Kill from the cursor to the previous white space.
This differs from Esc-Del because the boundaries separating words differ.
•Editor can be configured globally using /etc/inputrc file for global changes.
•To make changes for specific user edit .inputrc file located in user’s home directory.
This file can be used to change key mappings for Readline Library editor, and map keys to commonly used commands.
•To view keyboard bindings type the command: bind -v

<command> [options] [arguments]
•Everything is separated with white space
•Options are just a special interpretation of arguments, generally identified with a
prefixed hyphen
•POSIX options (long options) use double hyphen prefix
often spell out option with a word rather than just a letter ( --verbose instead of -v )
Command Line Syntax
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- Arguments are separated with whitespace
- sometimes whitespace needs to be part of the argument itself (ex: spaces in filenames)
 command filename with spaces
Without any guidance, the shell will interpret this input as a command with various arguments.
Quoting is the easiest way to guide the shell in this matter.
There are two forms...

Single quotes: command ‘filename with spaces’
-Quotes let shell know where an argument starts and stops (quotes not included);
-It doesn’t bother with what’s between markers - its interpreted strictly as data;
this line would be interpreted as a command with one argument: filename with spaces
Command Quoting
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Double quotes: command “filename with spaces” - equals single syntax, but interpret differently:
-Quotes let the shell know where an argument starts and stops,
-but data between is loosely examined for metacharacters.
this line would also be interpreted as a command with one argument: filename with spaces
Metacharacter any character that has more than one meaning or interpretation.
Ex: single and double quotes.
-In normal context, they denote endpoints for arguments, not actual quote characters;
-But if you need a quote in your argument value (ex: filename smith’s) use the
backslash: escape metacharacter command (ex: filename smith’s)
tells shell to interpret character following backslash as normal character, not a metacharacter
This allows you to use metacharacters as regular characters

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History file
history file contains list of commands issued at command prompt.
Number of commands stored - Is specified by HISTSIZE environment variable in:
/etc/profile or ~/.profile file - Default setting is 1,000 entries.
The command history displays all entries in the history file, which is ~/.bash_history.
HISTCMD
variable is used to provide history index number of command currently being run.
HISTFILE
variable specifies file used to contain the cmd history – default /home/user/.bash_history.
HISTFILESIZE
variable specifies maximum number of lines contained in the HISTFILE.

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fc
provides another option for editing commands in history file before running them.
fc utility opens the command into default editor, to edit and save it before rerunning the command.
fc utility allows to specify number of history events to edit, which enables to edit a range of cmds at once.
# fc –l 1020 1025
1020 cd /etc
1021 ls -al
1022 vi services
1023 vi hosts
1024 man ls
1025 ls -al s*
Ex: Example uses fc utility to list the history events1020-1025.

Environment variables
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Command substitution $(...) and `...`
Some commands say interesting things to include as parameters on other commands.
command substitution operators allows to include output of a command in another command.
foo:~ $ whoami
Gerald
foo:~ $ grep gerald /etc/passwd
gerald:x:500:500:G Smith:/home/gerald:/bin/bash
It's much easier like using substitution:
foo:~ $ grep `whoami` /etc/passwd
gerald:x:500:500: G Smith:/home/gerald:/bin/bash
foo:~ $ grep $( whoami ) /etc/passwd
gerald:x:500:500: G Smith:/home/gerald:/bin/bash
Ex: # echo `pwd`
Ex:

Environment
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Every piece of running software has it’s own environment
environment is a collection of KEY->value pairs
1.The KEY is [traditionally capitalized] letters, numbers and symbols to uniquely identify the variable
2.The value is a string
Ex: PATH=/usr/local/bin:/usr/bin:/bin:/sbin
HOME=/home/bob
TOTAL=348
To create a new variable (or change an existing one):
TOTAL=100 Type the name of the variable, an equals sign, and the value. Quoting if needed.
Once a variable is created, view it’s value with the $ metacharacter.
-The easiest way is to use echo: echo $TOTAL equals echo 100
-$ metacharacter asks shell to look for value of named variable, and replace everything with that value.

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Prompt
The shell prompt can be configured by the user to display a variety of information.
[godmode@zeus godmode]$ _Ex:
Godmode:login name; zeus: name of computer; 2nd
godmode: current working directory; _: represents cursor.
prompt is set by environmental variable called PS1.
To display the current settings use command echo $PS1
System-wide setting of prompt (for all users) is in file /etc/bashrc
[godmode@zeus godmode]$ cat /etc/bashrcEx:
To customize prompt, edit file /etc/bashrc (as root) and insert almost any text inside the quotes.

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Prompt build-in for handling settings
Settings Used To Configure the Prompt
Setting Meaning
u User name of the current user.
h The name of the computer running the shell (hostname).
set.
w The full name of the current working directory.
$ Displays “$” for normal users and “#” for the root.
! History number of the current command.
# Number of the current command.
d Current date.
t Current time.
s Name of the shell.
n New line.
Backslash.
[ Begins a sequence of nonprintable characters.
] Ends a sequence of nonprintable characters.
nnn The ASCII character corresponding to the octal number nnn.
$(date) Output from the date command (or any other command for that matter).

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$PATH
When you enter a command, bash searches for executable program in a number of directories.
The places bash searches are specified in the PATH environment variable.
foo:~ $ echo $PATH
/home/user/bin:/bin:/usr/bin:/usr/X11R6/bin:/usr/local/bin
Ex:
One of places that bash does not search by default is the current directory – usually.
Some distributions do include the current directory in the path (spot the difference).
linux:~ > echo $PATH
/bin:/usr/bin:/usr/X11R6/bin:/usr/local/bin:/home/joe/bin:
Ex:
Because of PATH environment variable, you can enter cmds in any working directory.
you can use the ls cmd even when you are not in the /bin directory which contains the ls program.
linux:/usr/bin $ cd /bin
linux:/bin $ ls -la ls
-rwxr-xr-x 1 root root 90811 Apr 20 16:32 ls
linux:/bin $ cd /usr/bin
linux:/usr/bin $ ls -la ls
/bin/ls: ls: No such file or directory
Ex:

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The shell environment contains values called environment variables which can be displayed
and changed.
To change value of an environment variable, use the syntax: VARIABLENAME='value'
commands related to setting environment variables:
foo:~ $ set | less
foo:~ $ echo PATH
foo:~ $ echo $PATH
foo:~ $ echo HOME
foo:~ $ echo $HOME
foo:~ $ echo $HOSTNAME
foo:~ $ echo $PS1
foo:~ $ PS1="# "
foo:~ $ PS1=“u: w $ “
foo:~ $ env | less # search for PS1 in env by typing /PS1
foo:~ $ HOME=/var
foo:~ $ cd
foo:~ $ pwd
foo:~ $ HOME=~
Ex:

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command env displays list of all the exported environment variables.
These are the variables that are passed from shell to apps when the applications are executed
foo:~ $ env | head
KDE_MULTIHEAD=false
SSH_AGENT_PID=511
HOSTNAME=foo.ledge.co.za
TERM=xterm
SHELL=/bin/bash
XDM_MANAGED=/var/run/xdmctl/xdmctl-:0,maysd,mayfn,sched
HISTSIZE=1000
GTK_RC_FILES=/etc/gtk/gtkrc:/home/joe/.gtkrc
GS_LIB=/home/joe/.kde/share/fonts
QTDIR=/usr/lib/qt3-gcc3.4
Ex:

Environment
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Environment variables are local to the containing process
Its possible to mark variables “exported” - allows to be passed down to sub-processes (child processes)
-Once a variable is created, to mark it exported:
export TOTAL (no $ metacharacter)
-Stop exporting:
export -n TOTAL
set: Displays all environment variables and values
env: Displays exported environment variables and values
To remove a variable completely:
unset TOTAL

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The behaviour of many commands can be customised by setting (and exporting) shell
variables as listed on the man pages of the application.
foo:~ $ echo PRINTER value is $PRINTER
PRINTER value is
foo:~ $ lpq
lp0 is ready
no entries
foo:~ $ PRINTER=lp1
foo:~ $ export PRINTER
foo:~ $ lpq
lp1 is not ready
no entries
Ex: lpq command shows print queue for the current printer.
current printer is set using the PRINTER environment variable.

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To remove a variable, use the command unset.
foo:~ $ echo SSH_ASKPASS is $SSH_ASKPASS
SSH_ASKPASS is /usr/libexec/openssh/gnome-ssh-askpass
Ex:
foo:~ $ unset SSH_ASKPASS
foo:~ $ echo SSH_ASKPASS is $SSH_ASKPASS
SSH_ASKPASS is
foo:~ $

Bash session
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During a login session, bash does a number of special things:
• At beginning of session, file ~/.profile is run automatically.
Any special commands placed in this file are run whenever logged
• At end of the session, all commands entered are added to the file ~/.bash_history.
exec /usr/bin/pine # when pine exits, the session is over
If this is put in a user’s ~/.profile, that user will automatically run pine when logging.
contents of ~/.profile can include a executable command to replace the shell
Ex:

Recursive commands
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Recursion in terms of Linux commands refers to applying a command to a all the files in a
directory, and all the files in all the subdirectories (and subdirectories of subdirectories).
$ find /etc -type f -exec cat {} ; # find with find, then cat each one
Ex:
Some commands (ls, chown, chmod, cp, rm, grep) support switches – R or – r .
commands without recursive mode, combine the cmd with find to achieve your results.
find ... Exec can execute a specific command each time a file is found:
$ cat `find -type f /etc` # cat whatever find says
$ find /etc -type f | xargs cat # run cat with parameters from find
The output of find can list file names on the command line:
Using parameter -type f to find ensures that we only consider regular files.
find can use other criteria to identify files - name, permissions, modification, date, etc.

man pages
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The primary (and traditional) source of documentation is the manual pages, which
can be accessed using command: man command or man section command
Manual pages are usually installed when you install a package, so if you do not have a package installed,
you probably won't have a manual page for it.
Manpages can be written to cover any topic, but generally are available for commands, libraries, function
calls, kernel modules and configuration files.
Manual pages have:
•Heading with the name of the command followed by its section number in parentheses
•The name of the command and any related commands that are described on the same man page
•Synopsis of the options and parameters applicable to the command
•Short description of the command
•Detailed information on each of the options
There might be other sections on usage, how to report bugs, author information, and a list of related commands.
Ex: man page for man tells us that related commands (and their manual sections) are: apropos(1), whatis(1), less(1), groff(1),
and man.conf(5).

man pages
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There are 8 common manual page sections:
1. User commands (env, ls, echo, mkdir, tty)
2. System calls or kernel functions (link, sethostname, mkdir)
3. Library routines (acosh, asctime, btree, locale, XML::Parser)
4. Device related information (isdn_audio, mouse, tty, zero)
5. File format descriptions (keymaps, motd, wvdial.conf)
6. Games (note that many games are now graphical and have graphical help outside the man page system)
7. Miscellaneous (arp, boot, regex, unix utf8)
8. System administration (debugfs, fdisk, fsck, mount, renice, rpm)
Other sections could include:
9.for Linux kernel documentation,
n. new documentation,
o. old documentation,
l. local documentation.
2 important commands related to man are whatis and apropos.
whatis - searches man pages for the name given and displays name info from appropriate manual pages.
apropos - does a keyword search of manual pages and lists ones containing the keyword.

infopages
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There is some movement to convert the aging manpage system into a newer format, the
infopage system.
Info system provides a more advanced interface, supporting links, split windows and more.
Accessing infopages is the same as man:
info <topic>
Once within the info system, type ? for help on the interface

commands
. The current directory
bash What you bash your commands into
echo echo
env Show environment variables
exec Run and don’t return
export Add a variable to the export list
man Manual pages
pwd Print working directory
set Show environment settings
unset Clear an environment variable
~/.bash_history The last n commands you typed
~/.profile What runs when you login interactively
Test commands
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Fim de sessão
33

3.1 gnu and unix commands v4

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