Linux Examples

postfix/qmgr status=deferred (delivery temporarily suspended: transport is unavailable)

admin — Wed, 22 Feb 2012 02:44:46 +0000

I have a crontab job to check something every day and send me the results to me via email,but today no email received,and I found below error messages in mail log files

postfix/qmgr status=deferred (delivery temporarily suspended: transport is unavailable)

Then I just try to restart postfix service ,then I also got below error messages

postfix/local[4470]: fatal: open database /etc/postfix/aliases.db: No such file or directory

So the reason was found ,aliases.db file was missing,also my /etc/postfix/aliase file was missing,then I just did below steps to fix the issue

service postfix stop cp /etc/aliases /etc/postfix/aliases newaliases service postfix start

Hope this can help you ,good luck.

linux chkconfig command example

admin — Tue, 21 Feb 2012 04:46:58 +0000

Linux command chkconfig is used to updates and queries runlevel information for system services. SYNOPSIS

chkconfig --list [name] chkconfig --add name chkconfig --del name chkconfig [--level levels] name <on|off|reset> chkconfig [--level levels] name

EXAMPLES: 1.Example to list all the serviced that chkconfig knows

chkconfig --list

2.Example to list a specified service's status ,below command list the status of service

bash-3.2$ chkconfig --list xinetd[root@server001 ~]# chkconfig --list mysqld mysqld 0:off 1:off 2:off 3:off 4:off 5:off 6:off

3.Example to enable a specified service,below command just enable service mysqld

[root@server001 ~]# chkconfig mysqld on [root@server001 ~]# chkconfig --list mysqld mysqld 0:off 1:off 2:on 3:on 4:on 5:on 6:off [root@server001 ~]#

4.Example to enable a service on at specified levels,below command enable mysqld on level 3 and 5

[root@server001 ~]# chkconfig --list mysqld mysqld 0:off 1:off 2:off 3:off 4:off 5:off 6:off [root@server001 ~]# chkconfig --level 35 mysqld on [root@server001 ~]# chkconfig --list mysqld mysqld 0:off 1:off 2:off 3:on 4:off 5:on 6:off [root@server001 ~]#

5.To delete a service from chkconfig management

chconfig --del service

6.To add a service for management by chkconfig(Make sure your service's script locates under /etc/init.d)

chkconfig --add service

Note:About linux run levels

0: shutdown 1:single mode 2:multiple user mode without network 3.multiple user mode with network 4.Non-used,reserved for future 5.multiple user mode with network and gui mode 6.Reboot

linux ssh automatic login without password prompt

admin — Mon, 20 Feb 2012 05:30:46 +0000

If you need to use ssh to login another host every day,maybe you need to know how to use ssh automatic login to other host without password prompt,here we go 1.On the source host,run below command and hit ENTER 3 times to create rsa keys

user001@client[/home/user001]122% ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/home/user001/.ssh/id_rsa): Created directory '/home/user001/.ssh'. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/user001/.ssh/id_rsa. Your public key has been saved in /home/user001/.ssh/id_rsa.pub. The key fingerprint is: e1:b8:f0:db:09:02:8d:d6:30:59:66:0c:4e:9d:db:8d user001@client

2.ssh to the server with user user002 and create another .ssh directory under user002's home directory

user001@client[/home/user001]125% ssh user002@server mkdir -p .ssh user001@server's password: user001@client[/home/user001]126%

3.Copy the content of .ssh//id_rsa.pub to /home/user002/.ssh and rename to authorized_keys(if existed,just append)

user001@client[/home/user001]3% cat .ssh/id_rsa.pub | ssh user002@server 'cat >>! .ssh/authorized_keys' user001@server's password: user001@client[/home/user001]4%

4.Done.Now you can ssh automatic login without password prompt Note:If the above steps not working,pls try: 1.Change permission of your .ssh directory to 770 using chmod command "chmod 700 .ssh" 2.Change the parent directory of .ssh to 750 3.Replace authorized_keys with authorized_keys2

regular expression examples

admin — Fri, 17 Feb 2012 08:49:12 +0000

Metacharacter regular expression examples

. (dot)

match any single character except line break characters \r and \n

bash-3.2$ cat file.txt
abc
acc
add
bash-3.2$ sed 's/a.c/relaced/' file.txt
relaced
relaced
add
bash-3.2$

[ ]

A bracket expression. Matches a single character that is contained within the brackets

user1@server1[/home/user1/examples]104% ls [a-c]*
a   b   c
user1@server1[/home/user1/examples]105% ls [0-9]*
1   2   3
user1@server1[/home/user1/examples]106% ls [a-c0-9]*
1   2   3   a   b   c
user1@server1[/home/user1/examples]107%

[^ ]

A bracket expression. Matches a single character that is not contained within the brackets

user1@server1[/home/user1/examples]107% ls
./   ../  1    2    3    a    b    c
user1@server1[/home/user1/examples]108% ls [^0-9]*
a   b   c
user1@server1[/home/user1/examples]109% ls [^a-z]*
1   2   3
user1@sdc-master[/home/user1/examples]110% ls [^0-9a-z]*
ls: No match.
user1@server1[/home/user1/examples]111%

^

Matches at the start of the string the regex pattern is applied to, for examples ^# matches comment lines

user1@server1[/home/user1/examples]120% cat examples.txt
This is the first line

#This is the 3rd line
This is the last line
user1@server1[/home/user1/examples]121% grep -v ^# examples.txt
This is the first line

This is the last line
user1@server1[/home/user1/examples]122%

$

Matches at the end of the string the regex pattern is applied to ,for examples ^$ matches empty lines

user1@server1[/home/user/examples]122% grep -v ^$ examples.txt
This is the first line
#This is the 3rd line
This is the last line
user1@server1[/home/user1/examples]123%

*

Matches previous item zero or more times

{n,m}

Matches the preceding element at least m and not more than n times

bash-3.2$ ls
a  aa  aaa  aaaa  aaaaa  examples.txt
bash-3.2$ ls |grep 'a\{4,5\}'
aaaa
aaaaa
bash-3.2$ ls |grep 'a\{1,5\}'
a
aa
aaa
aaaa
aaaaa
examples.txt
bash-3.2$ ls |grep 'a\{2,5\}'
aa
aaa
aaaa
aaaaa
bash-3.2$

Linux sed command examples

admin — Fri, 17 Feb 2012 06:58:24 +0000

Linux sed command is a powerful stream editor for filtering and transforming text.Below will show you several examples,after which you should can use sed to finish most basic tasks. Below file will be used to do the tests in below examples

bash-3.2$ cat example.txt
This is the first line
This is the second line
#This is the 3rd line

This is the 5th line
bash-3.2$

Use sed command to replace matched text with replacement 1.The simplest replace,below example replace the first "is" to "IS" of every line in file example.txt

bash-3.2$ sed 's/is/IS/' example.txt
ThIS is the first line
ThIS is the second line
#ThIS is the 3rd line

ThIS is the 5th line
bash-3.2$

2.Same as above example,but replace all "is" to "IS"

bash-3.2$ sed 's/is/IS/g' example.txt
ThIS IS the first line
ThIS IS the second line
#ThIS IS the 3rd line

ThIS IS the 5th line
bash-3.2$

3.Example to do replace twice.(-e option is needed )

bash-3.2$ sed -e 's/This/THIS/g' -e 's/the/THE/g' example.txt
THIS is THE first line
THIS is THE second line
#THIS is THE 3rd line

THIS is THE 5th line
bash-3.2$

Use sed command to delete the lines matched 1.Remove comments lines

bash-3.2$ sed '/^#/d' example.txt
This is the first line
This is the second line

This is the 5th line
bash-3.2$

2.Remove empty lines

bash-3.2$ sed '/^$/d' example.txt
This is the first line
This is the second line
#This is the 3rd line
This is the 5th line
bash-3.2$

Use sed command to instert append modify

bash-3.2$ sed -e '/first/a\ THIS IS APPENED' example.txt
This is the first line
THIS IS APPENED
This is the second line
#This is the 3rd line

This is the 5th line
bash-3.2$ sed -e '/first/i\ THIS IS INSERTED' example.txt
THIS IS INSERTED
This is the first line
This is the second line
#This is the 3rd line

This is the 5th line
bash-3.2$ sed -e '/first/c\ THIS IS CHANGED' example.txt
THIS IS CHANGED
This is the second line
#This is the 3rd line

This is the 5th line
bash-3.2$

Use sed command to transfer characters Below example will transfer c to x ,d to y ,e to z

bash-3.2$ sed  'y/cde/xyz/' example.txt
This is thz first linz
This is thz szxony linz
#This is thz 3ry linz

This is thz 5th linz
bash-3.2$

Linux ln command examples

admin — Sun, 12 Feb 2012 08:19:12 +0000

We can use linux ln command to make link between files,and there are 2 kinds of link,hard link and symbolic link. Difference between hard link and symbolic link 1.Hard link can't be used for directory or cross file system.Symbolic link can do this 2.Remove the original of the hard link does not remove the actual data,we can still access the data by the hard link,but if the original file or directory of a symbolic link was removed ,then the actual data lost,and we can't access the data anyway 3.Hard link won't use a new block(inode),but symbolic link will.So in theory symbolic link will occupy more disk space(inodes) 4.Symbolic link is used more than hard link Examples: 1.Create a hard link use ln command

j@J:~$ touch a.txt
j@J:~$ echo "Test..." > a.txt
j@J:~$ cat a.txt
Test...
j@J:~$ ln a.txt hardlink.txt
j@J:~$ ls -il *.txt
35226 -rw-rw-r-- 2 j j 8 2012-02-12 16:08 a.txt
35226 -rw-rw-r-- 2 j j 8 2012-02-12 16:08 hardlink.txt
j@J:~$ cat hardlink.txt
Test...
j@J:~$ rm -f a.txt
j@J:~$ cat hardlink.txt
Test...
j@J:~$

The above examples make a hard link hardlink.txt to a.txt,we can see the 2 files use the same inode,also if you remove the original file a.txt,we still can access hardlink.txt and the data is still there 2.Create a symblic file with option -s

j@J:~$ touch a.txt
j@J:~$ echo "Test..." > a.txt
j@J:~$ cat a.txt
Test...
j@J:~$ ln a.txt hardlink.txt
j@J:~$ ls -il *.txt
35226 -rw-rw-r-- 2 j j 8 2012-02-12 16:08 a.txt
35226 -rw-rw-r-- 2 j j 8 2012-02-12 16:08 hardlink.txt
j@J:~$ cat hardlink.txt
Test...
j@J:~$ rm -f a.txt
j@J:~$ cat hardlink.txt
Test...
j@J:~$

The above example create a symbolic link to a.txt ,and we also can see the 2 files use the different inode (block),and if we remove the original file,the symbolic link will be unaccessable. 3.Create a symbolic link for a directory.

j@J:~$ ln -s /var/log .

The above example just make a symbolic link for directory /var/log in current directory 4.To remove a link ,just use "rm linkname"

j@J:~$ ls -l log
lrwxrwxrwx 1 j j 8 2012-02-12 16:14 log -> /var/log
j@J:~$ rm log
j@J:~$

Linux mount command examples

admin — Thu, 09 Feb 2012 09:07:50 +0000

We can use mount command in linux to mount a filesystem ,for example, ext3 /ext4 file system or nfs or iso or cdrom or usb disk. Pls see below examples to know how to mount these file system. 1.Mount a nfs file system mount remotehostname:sharepoint /mountpoint eg:

# mount server2:/u/data /mnt

2.Mount an ISO image file in linux

# mount -o loop disk1.iso /mnt

3.Mount usb disk in linux a)use command "fdisk -l" to check the detail path of your usb disk,eg:we get the path is /dev/sdb1

#fdisk -l

b) then use mount command to mount it ,eg:mount to /mnt

#mount /dev/sdb1 /mnt

4.Mount cd or dvd rom (Please intert your cd or dvd firstly)

#mount -t iso9660 -o ro /dev/cdrom /cdrom

5.Umount a file system

# umount DIR

Linux kill command examples

admin — Thu, 09 Feb 2012 08:22:10 +0000

We can use linux command kill to terminate a process ,below will show you several examples: Usage: kill [ -s signal | -p ] [ -a ] [ -- ] pid ... Examples: 1.Kill xclock process using command kill ,firstly we use command ps to find the process id of xclock then kill it

bash-3.2$ ps aux |grep xclock
user01   28369  0.0  0.0 105424  3164 pts/421  S+   15:56   0:00 xclock
user01   28856  0.0  0.0  61140   760 pts/427  R+   15:56   0:00 grep xclock
bash-3.2$ kill 28369
bash-3.2$ ps aux |grep xclock
user01   29288  0.0  0.0  61140   760 pts/427  R+   15:57   0:00 grep xclock
bash-3.2$

2.Sometimes the kill could not kill the process normally and we can add option "-9" to kill it forcely

bash-3.2$ ps aux |grep xclock
user01    6954  0.0  0.0 105424  3164 pts/57   S+   16:00   0:00 xclock
user01    7172  0.0  0.0 105424  3164 pts/421  S+   16:00   0:00 xclock
user01    7207  0.0  0.0  61144   768 pts/427  S+   16:00   0:00 grep xclock
bash-3.2$ kill -9 6954 7172
bash-3.2$

3.Use kill -HUP to let process to reload it's configurations.Examples,if we changed /etc/xinetd.conf ,we can use below command to let process xinetd to reload the configuration file with terminating the process and start again.

bash-3.2$ ps -ef |grep xinetd
root      3325     1  0  2011 ?        00:00:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
user01   11435 10695  0 16:06 pts/427  00:00:00 grep xinetd
bash-3.2$ kill -HUP 3325
bash-3.2$

4.Kill all your own processes with option "-1 -1" (Both are number one)

bash-3.2$ kill -1 -1

5.Use command killall the kill process by spcecify it's process name but not process id

bash-3.2$ killall -HUP xinetd

Linux find command examples

admin — Thu, 09 Feb 2012 07:03:35 +0000

Linux find command could be used to search for files in a directory hierarchy. SYNOPSIS

find [-H] [-L] [-P] [path...] [expression]

Examples:(From find manpage)

find /tmp -name core -type f -print | xargs /bin/rm -f

Find files named core in or below the directory /tmp and delete them. Note that this will work incorrectly if there are any filenames containing newlines, single or double quotes, or spaces.

find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f

Find files named core in or below the directory /tmp and delete them, processing filenames in such a way that file or directory names containing single or double quotes, spaces or newlines are correctly handled. The -name test comes before the -type test in order to avoid having to call stat(2) on every file.

find . -type f -exec file {} \;

Runs file on every file in or below the current directory. Notice that the braces are enclosed in single quote marks to protect them from interpretation as shell script punctuation. The semicolon is similarly pro- tected by the use of a backslash, though ; could have been used in that case also.

find /    \( -perm -4000 -fprintf /root/suid.txt %#m %u %p\n \) , \
\( -size +100M -fprintf /root/big.txt %-10s %p\n \)

Traverse the filesystem just once, listing setuid files and directories into /root/suid.txt and large files into /root/big.txt.

find $HOME -mtime 0

Search for files in your home directory which have been modified in the last twenty-four hours. This command works this way because the time since each file was last modified is divided by 24 hours and any remainder is discarded. That means that to match -mtime 0, a file will have to have a modification in the past which is less than 24 hours ago.

find . -perm 664

Search for files which have read and write permission for their owner, and group, but which other users can read but not write to. Files which meet these criteria but have other permissions bits set (for example if someone can execute the file) will not be matched.

find . -perm -664

Search for files which have read and write permission for their owner and group, and which other users can read, without regard to the presence of any extra permission bits (for example the executable bit). This will match a file which has mode 0777, for example.

find . -perm /222

Search for files which are writable by somebody (their owner, or their group, or anybody else).

find . -perm /220
find . -perm /u+w,g+w
find . -perm /u=w,g=w

All three of these commands do the same thing, but the first one uses the octal representation of the file mode, and the other two use the symbolic form. These commands all search for files which are writable by either their owner or their group. The files dont have to be writable by both the owner and group to be matched; either will do.

find . -perm -220
find . -perm -g+w,u+w

Both these commands do the same thing; search for files which are writable by both their owner and their group.

find . -perm -444 -perm /222 ! -perm /111
find . -perm -a+r -perm /a+w ! -perm /a+x

These two commands both search for files that are readable for everybody (-perm -444 or -perm -a+r), have at least on write bit set (-perm /222 or -perm /a+w) but are not executable for anybody (! -perm /111 and ! -perm /a+x respectively)

gzip command examples in linux

admin — Thu, 09 Feb 2012 06:25:13 +0000

Linux command gzip is used to compress files or extract .gz files,below will show you some examples. 1.Compress file01 to file01.gz with gzip command

bash-3.2$ ls
dir1  file01
bash-3.2$ gzip file01
bash-3.2$ ls
dir1  file01.gz
bash-3.2$

2.Compress file01 to file01.gz with option -c to keep original file file01 untouched

bash-3.2$ ls
dir1  file01
bash-3.2$ gzip -c file01 > file01.gz
bash-3.2$ ls
dir1  file01  file01.gz
bash-3.2$

3.Regulate the speed of compression using the specified digit number(1-9),where -1 or --fast indicates the fastest compression method (less compression) and -9 or --best indicates the slowest compression method (best compression). The default compression level is -6 (that is, biased towards high compression at expense of speed).

bash-3.2$ ls
dir1  file01
bash-3.2$ ls -lh file01
-rw-r----- 1 user1 group1 71K Feb  9 14:04 file01
bash-3.2$ gzip -c file01 > file01-default.gz
bash-3.2$ gzip -c -1 file01 > file01-1.gz
bash-3.2$ gzip -c -9 file01 > file01-9.gz
bash-3.2$ ls -lh file01*
-rw-r----- 1 user1 group1  71K Feb  9 14:04 file01
-rw-r----- 1 user1 group1  10K Feb  9 14:11 file01-1.gz
-rw-r----- 1 user1 group1 7.2K Feb  9 14:11 file01-9.gz
-rw-r----- 1 user1 group1 7.2K Feb  9 14:11 file01-default.gz
bash-3.2$ ls -l file01*
-rw-r----- 1 user1 group1 72393 Feb  9 14:04 file01
-rw-r----- 1 user1 group1 10195 Feb  9 14:11 file01-1.gz
-rw-r----- 1 user1 group1  7322 Feb  9 14:11 file01-9.gz
-rw-r----- 1 user1 group1  7365 Feb  9 14:11 file01-default.gz
bash-3.2$

4.Using command zcat the see the content of the .gz file

bash-3.2$ ls
dir1  file01.gz
bash-3.2$ zcat file01.gz
...etc.

5.Using command zcat the calculate the true size(in Bytes) of the uncompressed files

bash-3.2$ zcat file01.gz|wc -c
72393
bash-3.2$

6.Extract a .gz file using command gzip with option -d (of course you can use command gunzip directly)

bash-3.2$ ls
dir1  file01.gz
bash-3.2$ gzip -d file01.gz
bash-3.2$ ls
dir1  file01
bash-3.2$