Useful 100+ Linux commands

1. Schedule a queue to run at 9am on March 1st. Note: Ctrl-d to save and exit.

# at 9am March 1

2. Schedule a queue to run after 5 minutes.

# at now +5 minutes

3. Check any jobs pending to run, same as at -l .

# atq

4. Empty out a file.

# cat /dev/null > /path/to/file

5. Change directory, see also pushd and popd.

# cd

6. List run level information for the service type.

# chkconfig --list

7. Change owner recursively.

# chown -R : /path/to/directory

8. Change shell.

# chsh

9. Scan recursively for viruses.

# clamscan -r

10. Compare two files.

# cmp file1 file2

11. Copy keeping the directory structure.

# cp --parent /source/path /destination/path

12. Copy keeping the permissions of the user.

# cp -p

13. Copy recursive.

# cp -r

14. Copy without shell aliasing.

# \cp

15. List crontab for user.

# crontab -u -l

16. Check current date and time.

# date

17. Set current date and time, may need to set the hardware clock to the system time too, `man hwclock`.

# date -s 'Wed May 28 11:35:00 EST 2003'

18. Show disk free in human readable format.

# df -h

19. Configure interface using DHCP protocol.

# dhclient eth0

20. Find context differences between two files.

# diff -c

21. Creating a patch file.

# diff -Naur oldDir/oldFile newDir/newFile > new_patchFile

22. Kernel buffer

# dmesg

23. Show disk used in human readable format.

# du -h /path/to/directory

24. Find files larger than 10MB.

# find /path/to/file -size +10000k

25. Find file permissions with setuids.

find / \( -perm -4000 -o -perm -2000 \) -exec ls -ldb {} \;>> /tmp/suids

26. Search for world writable files and directories.

# find / -perm -002

27. Display information on free and used memory.

# free

28. Grep on word boundaries.

grep -w

29. Count the number of mathces - similar to “wc -l”.

# grep -c

30. Perform timings of device reads for benchmark and comparison purposes.

# hdparm -t /dev/hda1

31. Set the hardware clock to the current system time.

# hwclock --systohc

32. check the ip address

# ifconfig

33. Add an  ip address to eth0.

# ifconfig eth0:x xxx.xxx.xxx.xxx

34. Install loadable kernel module. You can also use `modprobe` to do the same.

# insmod

35. Displays information about your system’s CPU and I/O.

# iostat [ interval [ count ] ]

36. List iptables firewall rules in numeric format.

# iptables -L -n

37. HangUP process so it will re-read the config file.

# killall -HUP

38. Install the boot loader and increase verborsity.

# lilo -v -v

39. Query the boot map.

# lilo -q

40. One time boot to the named kernel.

# lilo -R

41. Create symbolic link to the target file or directory.

# ln -s

42. Configure dynamic linker run-time bindings

# ldconfig

43. List the IPs bound via Ensim

# listaliases

44. Quickly search for indexed files. Run `updatedb` to update the indexed database.

# locate

45. List files.

# ls

46. List loaded kernel modules

# lsmod

47. Create the access.db file database map for sendmail.

# makemap hash /etc/mail/access.db < /etc/mail/access

48. Create/Make a new directory.

# mkdir

49. Generate a random 128 character length password.

# mkpasswd -l 128

50. Read in the contents of your mbox (or the specified file).

# mail -f /var/mail/nameOfFile

51. Print the mail queue

# mailq

52.

# mailstat /path/to/procmail/log

53. Description of the hierarchy directory structure of the system

# man hier

54. Check the MD5 message digest.

# md5sum

55. Mount points check.

# mount

56. Provide information about your systems’ processor.

# mpstat [ interval [ count ] ]

57.

# ncftpget -R -u  -p
hostname /local_dir /remote_dir

58.

# netstat -a | grep -i listen

59. Will show you who is attached to what port.

# netstat -anpe

60.

# netstat -n

61. See which programs are listening on which port

# netstat -lnp

62. Will show you what local TCP ports are open and what programs are running on them.

# netstat -lntpe

63. Will show you what local UDP ports are open and what programs are running on them.

# netstat -lnupe

64. Run a program with modified scheduling priority. (# range between -20 to +20, negative is higher priority)

# nice -n # [command to nice]

65. Scan network

# nmap -v hostname/ip

66. Patch and keep a backup

# patch -p# -b < patch_file

67.

# ps -ecaux

68. Turn off all quotas for users and groups, verbose mode

# quotaoff -augv

69. Check quota for all users and groups interactively, do quotaoff first.

# quotacheck -augmiv

70. Turn on all quotas for users and groups

# quotaon -augv

71. Add host ip to route on a particular device.

# route add -host xxx.xxx.xxx.xxx dev eth0:x

72.

# rdate

73. Remove file

# rm

74. Remove kernel module

# rmmod

75. Display the routing table in numeric.

# route -n

76.

# rpm

77. Uninstall/erase package.

# rpm -e

78. Erase without dependency check.

# rpm -e --nodeps

79. List out installed rpms by date, latest on top.

# rpm -qa --last | less

80. Rebuild rpm database.

# rpm --rebuilddb

81. Find which package owns the file.

# rpm -qf /path/to/file
# rpm -q --whatprovides /path/to/file

82. Verify package.

# rpm -V

or

# rpm -Vf /path/to/file

83. Locate documentation for the package that owns the file.

# rpm -qdf /path/to/file

84. Query information on package.

# rpm -qip

85. Query files installed by package.

# rpm -qlp

86. Gives list of files that will be installed/overwritten.

# rpm -ql

87. Will show the scripts that will be executed.

# rpm -q --scripts

88. Display system activity information

# sar

89. Print a 0 padded sequence of numbers.

# seq -w 1 10

90. Record eveything printed on your terminal screen.

# script -a

Ctrl+D to exit out. `more ` to view.

91. Check the status of a service.

# service status

92. Restart after shutdown and force fsck (fsck may take a while).

# shutdown -rF now

93. Split a file into pieces with numeric suffixes, so it can be burnt to cds.

# split -d -b 640k big_input_filename.gz piece_file_prefix.gz.

To piece it back you can `cat piece_file_prefix.gz.* > original.gz`
94. Determine if a network service binary is linked againt tcp wrapper, libwrap.a

# strings -f | grep hosts_access

96. how to use tar

# tar -cvzf fileName.tar.gz `find /file/path -mtime -1 ! -type d -print`

97.

# tar -xvzpf fileName.tar.gz /path/to/file.txt

98. How to use tcpdump

# tcpdump -i eth0 dst port 80 | more

99. System process status

# top

100. View the full command line.

# top -c

101. Create empty file of 0 byte

# touch

102. Similar to `which` - shows full path to the command.

# type

103. Check the limit of user

# ulimit -a

104. Check the version of kernel running

# uname -a

105. Update package profile with rhn

# up2date -p

106. Install package via up2date.

# up2date -i

107.

# uptime

108.

# usermod

109. Utility reports virtual memory statistics

# vmstat [second interval] [no. of count]

110. Show who is logged on and what they are doing.

# w

111. Periodically watch output of a command in full screen

# watch ''

112. Run and generate the apache reports using webalizer

# webalizer -c /path/to/webalizer.conf

113. Recursive download of a url, converting links, no parent.

# wget -r -k -np

114. Mirror, convert links, backup original, dynamic to html and output a “logFile”.

# wget -m -k -K -E -o [logFile]

115. Locate the binary, source, and manual page files for a command.

# whereis

116. Shows the full path of command.

# which

117. Show who is logged on.

# who

118. Yum package updates

# yum check-update           -- check to see what updates are needed
# yum info     -- show basic information about a package
# yum update   -- update particular package

119. Control jobs:

# Ctrl-z   -- suspend foreground job
# jobs     -- list jobs
# bg       -- send job to background
# fg       -- bring job to foreground