Linux mini guide
22 Feb 2011

Free disk cache.

More info at

 free -m
 echo 3 | sudo tee /proc/sys/vm/drop_caches
 free -m

Display VmSize used by all apache httpd processes.

 ps aux |grep httpd| cut -f 4 -d " "| xargs -n 1 -I {} cat /proc/{}/status|grep "VmSize"

Reverse tunnel ssh

 ssh -p35 -N -f -C -g -R 8022:localhost:22 [email protected]

Create and apply a patch

diff -uN  patchtest.txt patchtest1.txt > patchtest.patch
patch patchtest.txt < patchtest.patch

Print the matched line, along with the 3 lines after it. case insensetive search.

grep -A 3 -i "elif" *

Start SSH tunnel

ssh -N -L 2080:localhost:80 home

Start webserver in current directory

python -m SimpleHTTPServer

Insert many rows into one file

cat >> /tmp/file << EOF
row 1
row 2

Replace string in a file, write change to file.

sed -i 's/SEARCH/REPLACE/g' /etc/passwd

Replace string in a file, show change on stdout.

sed 's/SEARCH/REPLACE/g' /etc/passwd

Find all files except .svn that includes @todo

find . -name .svn -prune -o -type f -exec grep -Hn "@todo” {} \;

Find all files including the ip and delete them.

find -exec grep {} \;|awk ‘{print $3}' |xargs -L 1 rm

Find all 404 errors in apache error logs.

find -exec grep 404 {} \; >ERR

Setup ssh login without password

If you like to login from computer A to computer B with ssh, without using any password, follow the below script code. This is useful if you have some automated scripts that do things on other computers.

Generate authentication keys, login to computer A as user a. Do not enter a passphrase:

[email protected]:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa):
Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 [email protected]
[email protected]:~$ ssh [email protected] 'mkdir -p .;chmod 700 .ssh; \
touch .ssh/authorized_keys;chmod 640 .ssh/authorized_keys'
[email protected]'s password:
[email protected]:~> cat .ssh/ | ssh [email protected] 'cat >> .ssh/authorized_keys'
[email protected]'s password:

Now it's possible to login to B from A with user a and no password.

[email protected]:~> ssh [email protected] hostname

Setup nfs

Click here or here for external link.

Find all ips on a network

nmap -v -sP 192.168.0.*

View the arp cache (ip vs mac)

arp -na

Enable ip-forarding

sysctl -w net.ipv4.ip_forward=1

Setup dar backup

Click here for a mini howto about dar backups. Click here to download a dar backup script or here to download my version.

Linux commands

Click here or here for mini linux command howto.

Temp on ati GPU

aticonfig –odgt | grep Temp | cut -b 43-47
for (( i=0;i<10;i++ )) ;
  echo -n `date`
  aticonfig –odgt | grep Temp| cut -b 42-47;
  sleep 1;

Test network speed

On server: ./iperf -s -K 1M
On Client: ./iperf -c -i 1

Centos kernel rescue

If the kernel is damaged, or as in my case you like to move a hard drive from an intel to an amd computer. You need to rebuild/reinstall the kernel. boot the centos installation cd/dvd Type "linux rescue” in the boot menu Click next, next, next (connect to the network) When in the shell type chroot /mnt/sysimage yum install kernel reboot

Create and mount an lvm logical volume

vgcreate vg_data /dev/hdd
lvcreate -n VolData -L 50G vg_data
mke2fs -j /dev/vg_data/VolData
mount /dev/vg_data/VolData /opt
echo "/dev/vg_data/VolData  /opt  ext3  defaults  1 2″ >> /etc/fstab
Execute a program periodically, showing output fullscreen
watch "ps aux|grep mysql”

Watch changeable data continuously

watch -n.1 'cat /proc/interrupts'

Log all selinux errors

semodule -DB

Create selinux rules

yum install -y policycoreutils-python setroubleshoot-server
sealert -a /var/log/audit/audit.log
grep tftpdir_rw_t /var/log/audit/audit.log | audit2allow -m sycocobbler > sycocobbler.te
cat sycocobbler.te
checkmodule -M -m -o sycocobbler.mod sycocobbler.te
semodule_package -o sycocobbler.pp -m sycocobbler.mod
semodule -i sycocobbler.pp

How is centos packages build?

yumdownloader --source openldap-servers
rpm -Uvh openldap-servers*
less rpmbuild/SPECS/openldap.spec

Extract info from an rpm.

rpm2cpio foo.rpm | cpio -idmv --no-absolute-filenames

Install an older kernel

yum install kernel-2.6.18-238.9.1.el5

# Make sure the older kernel will boot first.
cat /etc/grup.conf


# This might be done before reboot, to make sure that the newer kernel doesn't boot.
rpm -q kernel
yum remove kernel-2.6.18-274.7.1.el5

Update everything except kernel

sed -i '$exclude=kernel kernel-devel kernel-headers' /etc/yum.conf
yum update

Add a module to /etc/sysconfig/iptables-config

# Line 1: Remove all old existing module X
# Line 2: Add module X
# Line 3: Only one whitespace between each module
# Line 4: No whitespaces before last "
# Line 5: No whitespaces before firstt "
sed "/IPTABLES_MODULES=/s/ip_conntrack_tftp\( \|\"\)/\1/g;
/IPTABLES_MODULES=/s/\"/\"ip_conntrack_tftp /;
/IPTABLES_MODULES=/s/\( \)\+/ /g;
" iptables-config

Links to useful commands