LINUX HACK 29

Chapter 5: PS1, PS2, PS3, PS4 and

PROMPT_COMMAND

Hack 29. PS1 - Default Interaction Prompt

The default interactive prompt on your Linux can be modified as shown below to something useful and informative. In the following example, the default PS1 was “\s-\v\$”, which displays the shell name and the version number. Let us change this default behavior to display the username, hostname and current working directory name as shown below.

-bash-3.2$    export  PS1="\u@\h  \w>  "

ramesh@dev-db  ~>  cd  /etc/mail

ramesh@dev-db  /etc/mail>

[Note: Prompt changed to "username@hostname current-dir>" format]

Following PS1 codes are used in this example:

o    \u – Username

o    \h – Hostname

o    \w - Full pathname of current directory. Please note that when you are in the home directory, this will display only ~ as shown above

Note that there is a space at the end in the value of PS1. Personally, I prefer a space at the end of the prompt for better readability.

Make this setting permanent by adding export PS1=”\u@\h \w> ” to either

.bash_profile (or) .bashrc as shown below.

ramesh@dev-db  ~>  vi  ~/.bash_profile

ramesh@dev-db  ~>  vi  ~/.bashrc

[Note: Add export PS1="\u@\h \w> " to one of the above files]

Refer to the next chapter for several practical examples of PS1 usage in detail.

LINUX HACK 28

Hack 28. 
Display total connect time of users
Ac command will display the statistics about the user’s connect time.
Connect time for the current logged in user
With the option –d, it will break down the output for the individual days. In this example, I’ve been logged in to the system for more than 6 hours today. On Dec 1st, I was logged in for about 1 hour.
$ ac –d
Dec 1 total 1.08
Dec 2 total 0.99
Dec 3 total 3.39
Dec 4 total 4.50
Today total 6.10
 
Connect time for all the users
To display connect time for all the users use –p as shown below. Please note that this indicates the cumulative connect time for the individual users.
$ ac -p
john 3.64
madison 0.06
sanjay 88.17
nisha 105.92
ramesh 111.42
total 309.21
 
Connect time for a specific user
To get a connect time report for a specific user, execute the following:
$ ac -d sanjay
Jul 2 total 12.85
Aug 25 total 5.05
Sep 3 total 1.03
Sep 4 total 5.37
Dec 24 total 8.15
Dec 29 total 1.42
Today total 2.95

LINUX HACK 27

Hack 27. Diff Command

diff command compares two different files and reports the difference. The output is very cryptic and not straight forward to read.

Syntax:  diff  [options]  file1  file2

What was modified in my new file when compare to my old file?

The option -w in the diff command will ignore the white space while performing the comparison.

In the following diff output:

o    The lines above ---, indicates the changes happened in first file in the diff command (i.e name_list.txt).

o    The lines below ---, indicates the changes happened to the second file in the diff command (i.e name_list_new.txt). The lines that belong to the first file starts with < and the lines of second file starts with >.

#  diff  -w  name_list.txt  name_list_new.txt 

 

2c2,3

<  John  Doe

---

>  John  M  Doe

>  Jason  Bourne

LINUX HACK 26

Hack 26. Stat Command

Stat command can be used either to check the status/properties of a single file or the filesystem.

Display statistics of a file or directory.

$  stat  /etc/my.cnf
	File:	`/etc/my.cnf'
	Size:	346  Blocks:  16  IO  Block:  4096			regular  file
Device:		801h/2049d	Inode:  279856		Links:  1
Access:		(0644/-rw-r--r--)    Uid:  (		0/	root)	Gid:
(	0/	root)
Access:		2009-01-01  02:58:30.000000000  -0800
Modify:		2006-06-01  20:42:27.000000000  -0700
Change:		2007-02-02  14:17:27.000000000  -0800
$  stat  /home/ramesh
	File:	`/home/ramesh'
	Size:	4096	Blocks:  8		IO  Block:
4096		directory
Device:		803h/2051d	Inode:  5521409		Links:  7
Access:		(0755/drwxr-xr-x)    Uid:  (    401/ramesh)				Gid:  (
401/ramesh)

Access:  2009-01-01  12:17:42.000000000  -0800

Modify:  2009-01-01  12:07:33.000000000  -0800

Change:  2009-01-09  12:07:33.000000000  -0800

Display the status of the filesystem using option –f
$  stat  -f  /
File:	"/"
ID:	0	Namelen:	255	Type:  ext2/ext3
Blocks:	Total:  2579457		Free:  2008027		Available:
1876998		Size:  4096
Inodes:	Total:  1310720		Free:  1215892

LINUX HACK 25

Hack 25. Cut Command

Cut command can be used to display only specific columns from a text file or other command outputs.

Following are some of the examples.

Display the 1^st  field (employee name) from a colon delimited file

$  cut  -d:  -f  1  names.txt

Emma  Thomas

Alex  Jason

Madison  Randy

Sanjay  Gupta

Nisha  Singh

Display 1^st  and 3^rd  field from a colon delimited file

$  cut  -d:  -f  1,3  names.txt

Emma  Thomas:Marketing

Alex  Jason:Sales

Madison  Randy:Product  Development

Sanjay  Gupta:Support

Nisha  Singh:Sales

Display only the first 8 characters of every line in a file

$  cut  -c  1-8  names.txt

Emma  Tho

Alex  Jas

Madison

Sanjay  G

Nisha  Si

Misc Cut command examples

o    cut -d: -f1 /etc/passwd Displays the unix login names for all the users in the system.

o    free | tr -s ' ' | sed '/^Mem/!d' | cut -d" " -f2 Displays the total memory available on the system.

LINUX HACK 24

Hack 24. Uniq Command

Uniq command is mostly used in combination with sort command, as uniq removes duplicates only from a sorted file. i.e In order for uniq to work, all the duplicate entries should be in the adjacent lines. Following are some common examples.

1. When you have an employee file with duplicate entries, you can do the following to remove duplicates.

$  sort  namesd.txt  |  uniq

$  sort  –u  namesd.txt

2. If you want to know how many lines are duplicates, do the following. The first field in the following examples indicates how many duplicates where found for that particular line. So, in this example the lines beginning with Alex and Emma were found twice in the namesd.txt file.

$  sort  namesd.txt  |  uniq  –c

2  Alex  Jason:200:Sales

2  Emma  Thomas:100:Marketing

1  Madison  Randy:300:Product  Development

1  Nisha  Singh:500:Sales

1  Sanjay  Gupta:400:Support

3. The following displays only the entries that are duplicates. $ sort namesd.txt | uniq –cd

2  Alex  Jason:200:Sales

2  Emma  Thomas:100:Marketing

LINUX HACK 23

Hack 23. Sort Command



Sort command sorts the lines of a text file. Following are several practical examples on how to use the sort command based on the following sample text file that has employee information in the format:


employee_name:employee_id:department_name.



$ cat names.txt

Emma Thomas:100:Marketing


Alex Jason:200:Sales


Madison Randy:300:Product Development


Sanjay Gupta:400:Support


Nisha Singh:500:Sales


Sort a text file in ascending order



$ sort names.txt

Alex Jason:200:Sales


Emma Thomas:100:Marketing


Madison Randy:300:Product Development


Nisha Singh:500:Sales


Sanjay Gupta:400:Support



Sort a text file in descending order


$ sort -r names.txt

Sanjay Gupta:400:Support


Nisha Singh:500:Sales


Madison Randy:300:Product Development


Emma Thomas:100:Marketing


Alex Jason:200:Sales



Sort a colon delimited text file on 2nd field (employee_id)


$ sort -t: -k 2 names.txt

Emma Thomas:100:Marketing


Alex Jason:200:Sales


Madison Randy:300:Product Development


Sanjay Gupta:400:Support


Nisha Singh:500:Sales



Sort a tab delimited text file on 3rd field (department_name) and suppress duplicates


$ sort -t: -u -k 3 names.txt

Emma Thomas:100:Marketing

Madison Randy:300:Product Development


Alex Jason:200:Sales


Sanjay Gupta:400:Support


Sort the passwd file by the 3rd field (numeric userid)


$ sort -t: -k 3n /etc/passwd | more

root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin


Sort /etc/hosts file by ip-addres


$ sort -t . -k 1,1n -k 2,2n -k 3,3n -k 4,4n /etc/hosts

127.0.0.1 localhost.localdomain localhost 192.168.100.101 dev-db.thegeekstuff.com dev-db 192.168.100.102 prod-db.thegeekstuff.com prod-db 192.168.101.20 dev-web.thegeekstuff.com dev-web 192.168.101.21 prod-web.thegeekstuff.com prod-web


Combine sort with other commands



o ps –ef | sort : Sort the output of process list


o ls -al | sort +4n : List the files in the ascending order of the file-size. i.e sorted by 5th filed and displaying smallest files first.


ls -al | sort +4nr : List the files in the descending order of the file-size. i.e sorted by 5th filed and displaying largest files first.

LINUX HACK 22

Hack 22. Xargs Command

xargs is a very powerful command that takes output of a command and pass it as argument of another command. Following are some practical examples on how to use xargs effectively.

1. When you are trying to delete too many files using rm, you may get error message: /bin/rm Argument list too long – Linux. Use xargs to avoid this problem.

find  ~  -name  ‘*.log’  -print0  |  xargs  -0  rm  -f

2. Get a list of all the *.conf file under /etc/. There are different ways to get the same result. Following example is only to demonstrate the use of xargs. The output of the find command in this example is passed to the ls –l one by one using xargs.

#  find  /etc  -name  "*.conf"  |  xargs  ls  –l

4.  If you have a file with list of URLs that you would like to download, you can use xargs as shown below.

#  cat  url-list.txt  |  xargs  wget  –c

5.  Find out all the jpg images and archive it.

#  find / -name *.jpg -type f -print | xargs tar -cvzf images.tar.gz

6.  Copy all the images to an external hard-drive.

#  ls *.jpg | xargs -n1 -i cp {} /external-hard-drive/directory