Linux – Command Cheat Sheet

A quick Linux cheat sheet, for those handly little commands you use all the time but sometimes your brain freezes up and a quick reference would be good to have:

File Commands

ls                       -list directory
ls -al                   -formatted listing with hidden files
ls -ltr                  -formatted listing ordered by time
cd dir                   -change to directory
cd -                     -go to directory in previously
cd                       -go to home directory
pwd                      -show current directory path
mkdir dir                -make directory dir
rm file                  -delete file
rm -r dir                -delete directory dir
rm -f file               -force remove file
rm -rf dir               -force remove directory dir
cp file1 file2           -copy file1 to file2
cp -r dir1 dir2          -copy dir1 to dir2; create dir2 if it doesn’t exist
mv file1 file2           -rename/move file1 to file2 if file2 is an existing dir, moves file1
into dir file2
ln -s file link          -create symbolic link link to file
touch file               -create or update file
cat > file               -places standard input into file
more file                -output the contents of file
head file                -output the first 10 lines of file
tail file                -output the last 10 lines of file
tail -f file             -output the contents of file as it grows, starting with the last 10 lines

Process Management

ps                       -display your currently active processes
top                      -display all running processes
kill pid                 -kill process id pid
killall proc             -kill all processes named proc
bg                       -lists stopped or background jobs; resume a stopped job in the background
fg                       -brings the most recent job to foreground
fg n                     -brings job n to the foreground

File Permissions

chmod octal file         -change the permissions of file to octal, which can be found separately for user, group, and world by adding:
● 4 – read (r)
● 2 – write
● 1 – execute (x)
Examples:
chmod 777                -read, write, execute for all
chmod 755                -rwx for owner, rx for group and world For more options, see man chmod.
chown user:group file    -change the owner and group for a file

SSH

ssh user@host            -connect to host as user
ssh -p port user@host    -connect to host on port port as user
ssh-copy-id user@host    -add your key to host for user to enable a keyed or passwordless login

Searching

grep pattern files       -search for pattern in files
grep -r pattern dir      -search recursively for pattern in dir
command | grep pattern   -search for pattern in the output of command
locate file              -find all instances of file
find . file              -find file within current directory
find / file              -find file within whole system

System Info

date                      -show the current date and time
cal                       -show this month’s calendar
uptime                    -show current uptime
w                         -display who is online
whoami                    -who you are logged in as
finger user               -display information about user
uname -a                  -show kernel information
cat /proc/cpuinfo         -cpu information
cat /proc/meminfo         -memory information
man command               -show the manual for command
df                        -show disk usage
du                        -show directory space usage
du -sk | sort -n          -show sum of each directory space usage sorted by number in KB
free                      -show memory and swap usage
whereis app               -show possible locations of app
which app                 -show which app will be run by default

Compression

tar cf file.tar files     – create a tar named file.tar containing files
tar xf file.tar           – extract the files from file.tar
tar czf file.tar.gz files – create a tar with Gzip compression
tar xzf file.tar.gz       – extract a tar using Gzip
tar cjf file.tar.bz2      – create a tar with Bzip2 compression
tar xjf file.tar.bz2      – extract a tar using Bzip2
gzip file                 – compresses file and renames it to file.gz
gzip -d file.gz           – decompresses file.gz back to file

Network

ping host                 – ping host and output results
whois domain              – get whois information for domain
dig domain                – get DNS information for domain
dig -x host               – reverse lookup host
wget file                 – download file
wget -c file              – continue a stopped download

Installation

Install from source:
./configure
make
make install
dpkg -i pkg.deb           – install a package (Debian)
rpm -Uvh pkg.rpm          – install a package (RPM)

Shortcuts

Ctrl+C                    – halts the current command
Ctrl+Z                    – stops the current command, resume with fg in the foreground
or bg in the background
Ctrl+D                    – log out of current session, similar to exit
Ctrl+W                    – erases one word in the current line
Ctrl+U                    – erases the whole line
Ctrl+R                    – type to bring up a recent command
!!                        - repeats the last command
exit                      – log out of current session

Linux – Backup to Tape

One task of a database administrator is to perform backups for recovery purposes. One method of performing backups is to backup files to tape. Tape backups are cheaper for the amount of storage available, and more trustworthy for longer term retention that hard drives. Tapes are also useful for the fact that they can be moved offsite, therefore providing more security. The downside to tape backups is that they are time consuming, however combined with short-term hard-drive backups can provide a good backup regime.

Backup to Remote Tape Device Script

The below script will print the database SID and the current date, before changing to the root directory. The file is then compressed, stored on a remote tape device, and an error log is produced.

#################################################
#  Backup to Remote Tape Device Script
#################################################
(
ps –ef | grep pmon | grep $ORACLE_SID
echo  Backup started at `date`
cd /
#  On Linux gnu tar is run by default so it is not necessary to declare the path explicitly.
#  On Solaris and HP, GNU tar needs to be copied to the path below and referenced
GZIP=--fast
Export GZIP
/usr/local/bin/tar zcvbf 128 [remote server]:[device name] ./[directories]
echo Backup finished at `date`
) > "[error log filename]" 2>&1 &
# End of the script

GZIP=–fast

Sets the speed at which files are compressed. The faster a file is compressed, the lower the amount of compression performed. With the -# option, # can be any number between 1 and 9 where 1 is the fastest and 9 is the slowest. The –fast option is the equivalent to -1 whereas –best is the equivalent to -9. By default gzip performs compression as thought the -6 option was specified.

tar zcvbf

tar compresses files into archive files.
-z :- filter the archive through gzip
-c :- create a new archive
-v :- verbosely list files created
-b :- use record size of Nx512 bytes (default N=20)
-f :- use archive file or device

2>&1 &

The incantation 2>&1 means “Send errors (output stream number 2) to the same place ordinary output (output stream number 1) is going to”.

Backup to Local Tape Script

######################################
#  Backup to script for “$ORACLE_SID”
######################################
mt –t [device name] rewind
(
ps –fe | grep pmon | grep $ORACLE_SID
echo Backup started at `date`
cd /
tar cvbf 128 [device name] ./[directories]
echo Backup finished at `date`
) > “[error log filename]” 2>&1 &
# End of the script

Note about Rewind devices and Norewind devices

The tape device drivers on Linux and Unix allow a tape device to be opened for “rewind on close” or “norewind on close”. You use a slightly different device name to choose one from another, for example:

DLT rewind device: /dev/rmt/0u
DLT norewind device: /dev/rmt/0un

If you choose the rewind device, the driver will place two EOT markers at the end of the tape, and rewind the tape. You cannot get any more backups on the tape.

If you choose the norewind device, the driver will place one EOT marker and then one BOT marker at the end of the tape, and will not rewind the tape. It is possible to add another backup onto the tape. The “mt” command can be used to move the tape forwards and backward between savesets