Linux useful commands

# Allow a command to continue execution after logout
nohup mycommand &

# Display one particular line from a file
gzip -dc mytarball.tar.gz | tar xvf -

# Display current runlevel
who -r

# Split lines of text into n-char lines
echo "some line of chars" | fold -w 3

# Insert spaces at beginning of each line in vi
:5,16s/^/   /    # for lines 5 through 16, for example

# Display last 10 system reboots
last reboot | head

# Check which processes are listening on which ports
lsof -i | grep LISTEN

# Print range of lines from a file
sed -n '100,200 p' filename

# Move last file modified
mv `ls -1 -t *.txt | head -1` whatever.txt

# Bourne-type shells: Find a file’s hard links
inode=`ls -li /path/and/filename | awk '{print $1}'`
find / -inum $inode -exec ls -li {} \;

# Sybase – Check status of databases
# su - sybase
Password:
> isql -Usa -Ppassword -w200
1> use master
2> go
1> select status, name from sysdatabases
2> go

# Convert single column into two columns line by line
$ cat foofile
a
b
c
d
e
f
$ paste -d: - - < foofile
a:b
c:d
e:f

# gunzip and untar with one pipe
gzip -dc SOMEFILE.tar.gz | tar xvf -
# or with GNU tar
tar xvzf SOMEFILE.tar.gz

# Quickly copy a directory tree with tar
cd /path/to/source
tar cvf - ./* | ( cd /dest; tar xvf - )

# Access the local shell from an ftp session
# note - this depends on your ftp client
ftp> !
$ echo "Now in local shell"

# iconv convert between codesets
iconv -f UTF-8 -t iso85591 somefile

# recursively change permissions on directories only
find . -type d -exec chmod 755 {} \; -print 2>/dev/null

# set MTU value for an interface
ifconfig interface mtu n
# e.g.
ifconfig hme0 mtu 1500
# place in hostname.interface file for persistence

# ping using alternative interface
ping -i bge1 somehost

# shutdown your system
init 0
shutdown -h now
# reboot
init 6
shutdown -r now

# make a file immutable
chattr +i somefile

# show inode information for all files in a directory
find . -type f -exec stat {} \;

# Linux screenshots
xwd -display servername:0 -root > myfile.dmp
xwd | xwud

# perform a reverse DNS lookup
host xxx.xxx.xxx.xxx dns_server_ip

# check mouse is operational
cat /dev/mouse       # then move the mouse

# is SELinux enabled?
selinuxenabled && echo Yes
# or
# getenforce
Enforcing

# Display SELinux context in ls listings
ls -lZ /some/dir

# tcpdump port 123 (ntp)
tcpdump port 123

# disable HTTP TRACE in apache
# vi /etc/httpd/conf/httpd.conf
# grep Rewrite /etc/httpd/conf/httpd.conf
RewriteEngine On
RewriteCond %{REQUEST_METHOD} ^TRACE
RewriteRule .* - [F]
# apachectl restart

# check TCPD rules in /etc/hosts.{allow,deny}
grep -v "^#" /etc/hosts.allow /etc/hosts.deny | grep -v "^[ 	]*$"

# common vi(m) options
# ignore case
:set ic
# line numbers
:set num
:set nonum
# turn off search highlighting (VIM)
:nohl
# syntax highlight on (VIM)
:sy on
# show unprintable characters
:set list

# remove files older than one week
/usr/bin/find /some/dir -name "*whatever*" -mtime +7 | xargs rm

# find world writable files
/usr/bin/find / -perm -0777 -exec /usr/bin/ls -ld {} \;
# symbolic links will always be 77 so we can ignore them
/usr/bin/find -perm -0777 ! -type l -exec /usr/bin/ls -ld {} \;

# gzip and tar in one line
tar cvf - ./* | gzip -c > foo.tar.gz
# untar
tar xzvf foo.tar.gz

# find ip address for an interface
/sbin/ifconfig interface | grep inet
# e.g.
/sbin/ifconfig bge0 | grep inet

# filename completion ksh (vi mode)
ESC-\

# bourne based shells – show current environment variables
env
# show all variables
set
# show shell options
set -o
# current shell options (set via set -x, etc)
echo $-
# bash
shopt
# show current shell
echo $0

# stop people logging in
touch /etc/nologin

# sed to replace \ with \\
sed -e 's_\\_\\\\_g'

# recursive grep
find . -type f -exec grep "somestring" {} /dev/null \;

# show subdirectories only in an ls listing
ls -l | grep "^d"
# recursively
find . -type d -exec ls -ld {} \;

# create soft link
ln -s file_to_link link_name
# hard link
ln file_to_link link_name

# searching for a literal $ in a file
grep '\$' filename
# or fgrep / grep -F
fgrep '$' filename

# correct permissions on /tmp and /var/tmp
chmod 1777 /var/tmp
# or
chmod a=rwx,+t /var/tmp

# display unprintable filenames
ls -lb somedir

# find files larger than 300k
find / -size +300k -ls 2> /dev/null