Quick references to some frequently used Linux commands...

Submitted by sandip on Wed, 10/05/2005 - 23:54

Linux

This list has been compiled over time as reference to some of the frequently used linux commands that I use. I hope it will help some new-comers ;)

Schedule a queue to run at 9am on March 1st. Note: Ctrl-d to save and exit.
```
$ at 9am March 1
```
Schedule a queue to run after 5 minutes.
```
$ at now +5 minutes
```
Check any jobs pending to run, same as at -l .
```
$ atq
```
Empty out a file.
```
$ cat /dev/null > /path/to/file
```
Change directory, see also pushd and popd.
```
$ cd
```
List run level information for the service type.
```
chkconfig --list <nameOfService>
```

Change owner recursively.

$ chown -R <username>:<groupname> /path/to/directory

Change shell.
```
$ chsh
```
Scan recursively for viruses.
```
clamscan -r
```
Format input into multiple columns.
```
mount | column -t
```
Compare two files.
```
cmp file1 file2
```

Copy keeping the directory structure.

$ cp --parent /source/path /destination/path

Copy keeping the permissions of the user.
```
$ cp -p <source> <destination>
```
Copy recursive.
```
$ cp -r <source> <destination>
```
Copy without shell aliasing.
```
$ \cp <source> <destination>
```

Force copy new data over old data.

$ cp -a --reply=yes /path/to/new_data/* /path/to/old_data/

List crontab for user.
```
$ crontab -u <user> -l
```
Check current date and time.
```
$ date
```
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'
```
Show disk free in human readable format.
```
$ df -h
```
Configure interface using DHCP protocol.
```
$ dhclient eth0
```
Find context differences between two files.
```
$ diff -c <from-file> <to-file>
```

Creating a patch file.

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

Kernel buffer
```
$ dmesg
```
Show disk used in human readable format.
```
$ du -h /path/to/directory
```
Find files larger than 10MB.
```
$ find /path/to/file -size +10000k
```

Find file permissions with setuids.

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

Search for world writable files and directories.
```
$ find / -perm -002
```
Display information on free and used memory.
```
$ free
```
Grep on word boundaries.
```
grep -w <word>
```
Count the number of mathces - similar to "wc -l".
```
$ grep -c <match expression>
```
Perform timings of device reads for benchmark and comparison purposes.
```
$ hdparm -t /dev/hda1
```
Set the hardware clock to the current system time.
```
$ hwclock --systohc
```
Print user identity.
```
$ id <user>
```
Configure network interface.
```
$ ifconfig
```
Add an additional ip to eth0.
```
$ ifconfig eth0:x xxx.xxx.xxx.xxx
```
Install loadable kernel module. You can also use `modprobe` to do the same.
```
$ insmod
```
Displays information about your system's CPU and I/O.
```
$ iostat [ interval [ count ] ]
```
List iptables firewall rules in numeric format.
```
$ iptables -L -n
```
HangUP process so it will re-read the config file.
```
$ killall -HUP <serviceName>
```
Install the boot loader and increase verborsity.
```
$ lilo -v -v
```
Query the boot map.
```
$ lilo -q
```
One time boot to the named kernel.
```
$ lilo -R <kernelName>
```
Create symbolic link to the target file or directory.
```
$ ln -s <target> <linkName>
```
Configure dynamic linker run-time bindings
```
$ ldconfig
```
List the IPs bound via Ensim
```
$ listaliases
```
Quickly search for indexed files. Run `updatedb` to update the indexed database.
```
$ locate
```
List files.
```
$ ls
```
List directories.
```
$ ls -d */
```
List all files reverse sorted by modification time-stamp in human-readable format.
```
$ ls -altrh
```
List all files reverse sorted by file size in human-readable format.
```
$ ls -alSrh
```
List loaded kernel modules
```
$ lsmod
```

Create the access.db file database map for sendmail.

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

Create/Make a new directory.
```
$ mkdir
```
Generate a random 128 character length password.
```
$ mkpasswd -l 128
```
Read in the contents of your mbox (or the specified file).
```
$ mail -f /var/mail/nameOfFile
```
Print the mail queue
```
$ mailq
```
```
$ mailstat /path/to/procmail/log
```
Description of the hierarchy directory structure of the system
```
$ man hier
```
Check the MD5 message digest.
```
$ md5sum 
```
Mount points check.
```
$ mount
```
Provide information about your systems' processor.
```
$ mpstat [ interval [ count ] ]
```

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

```
$ netstat -a | grep -i listen
```
Will show you who is attached to what port.
```
$ netstat -anpe
```
```
$ netstat -n
```
See which programs are listening on which port
```
$ netstat -lnp
```
Will show you what local TCP ports are open and what programs are running on them.
```
$ netstat -lntpe
```
Will show you what local UDP ports are open and what programs are running on them.
```
$ netstat -lnupe
```
Run a program with modified scheduling priority. (# range between -20 to +20, negative is higher prio
rity)
```
$ nice -n # [command to nice] 
```
Scan network
```
$ nmap -v hostname/ip
```
Query a list of the NTP peers known to the server as well as a summary of their state.
```
# ntpq -p localhost
```
merge lines of files
```
paste file1 file2
```
Patch and keep a backup
```
$ patch -p# -b < patch_file
```
```
$ ps -ecaux
```
Turn off all quotas for users and groups, verbose mode
```
$ quotaoff -augv
```
Check quota for all users and groups interactively, do quotaoff first.
```
$ quotacheck -augmiv
```
Turn on all quotas for users and groups
```
$ quotaon -augv
```
Display the routing table in numeric.
```
$ route -n
```

Add host ip to route on a particular device.

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

Add network to route on a particular device.

# route add -net 192.168.0.0 netmask 255.255.255.0 eth0

```
$ rdate
```
```
$ rm
```
Remove kernel module
```
$ rmmod <kernelModule>
```
```
$ rpm
```
Uninstall/erase package.
```
$ rpm -e <package>
```
Erase without dependency check.
```
$ rpm -e --nodeps <package>
```
List out installed rpms by date, latest on top.
```
$ rpm -qa --last | less
```
Rebuild rpm database.
```
$ rpm --rebuilddb
```

Find which package owns the file.

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

Verify package.

$ rpm -V <package>

$ rpm -Vf /path/to/file

Locate documentation for the package that owns the file.
```
$ rpm -qdf /path/to/file
```
Query information on package.
```
$ rpm -qip <package.rpm>
```
Query files installed by package.
```
$ rpm -qlp <package.rpm>
```
Gives list of files that will be installed/overwritten.
```
$ rpm -ql <rpmname>
```
Will show the scripts that will be executed.
```
$ rpm -q --scripts <rpmname>
```
Display system activity information
```
$ sar
```
Print a 0 padded sequence of numbers.
```
$ seq -w 1 10
```
Record eveything printed on your terminal screen.
```
$ script -a <filename>
```
Ctrl+D to exit out. `more <filename>` to view.
Check the status of a service.
```
$ service <name of service> status
```
Restart after shutdown and force fsck (fsck may take a while).
```
$ shutdown -rF now
```
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`
Determine if a network service binary is linked againt tcp wrapper, libwrap.a
```
$ strings -f <binary file name> | grep hosts_access
```
```
$ tar
```

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

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

```
$ tcpdump -i eth0 dst port 80 | more
```
```
$ top
```
View the full command line.
```
$ top -c
```
```
$ touch
```
Similar to `which` - shows full path to the command.
```
$ type <command>
```
```
$ ulimit -a
```
```
$ uname
```
Update package profile with rhn
```
$ up2date -p
```
Install package via up2date.
```
$ up2date -i <packageName>
```
```
$ uptime
```
```
$ usermod
```

Utility reports virtual memory statistics

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

Show who is logged on and what they are doing.
```
$ w
```
Periodically watch output of a command in full screen
```
$ watch '<command>'
```
```
$ webalizer -c /path/to/webalizer.conf
```
Recursive download of a url, converting links, no parent.
```
$ wget -r -k -np <URL> 
```
Mirror, convert links, backup original, dynamic to html and output a "logFile".
```
$ wget -m -k -K -E <URL> -o [logFile]
```
Locate the binary, source, and manual page files for a command.
```
$ whereis <command>
```
Shows the full path of command.
```
$ which <command>
```
Show who is logged on.
```
$ who
```

Yum package updates

  $ yum check-update           -- check to see what updates are needed
  $ yum info <package name>    -- show basic information about a package
  $ yum update <package name>  -- update particular package

Control jobs:

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

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Quick references to some frequently used Linux commands...

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