Linux HowTo: Linux Commands

This command bulletin is an unorganized list of command examples in Unix. These examples are not explained in detail. If you have some idea as how the command might look like, you can find it here easily using browser search (Ctrl+f). Some of these commands are bash shell specific.

To find hardware details of linux box: dmesg |less
To find linux version: cat /proc/version
To find cpu info: cat /proc/cpuinfo
To unzip gzip file: gunzip something.tar.gz
To untar tar file: tar xvf something.tar
To unzip and untar in one step: tar xvzf something.tar.gz
To install a software from rpm: rpm -iv myapp_someversion.rpm
To edit your bash shell environment edit ~/.bashrc file
To set the java and ant class path add the following lines in your bashrc file at home directory with appropriate java and ant directories:

export JAVA_HOME=/usr/java/jdk1.5.0_09
export ANT_HOME=/home/oracle/apache-ant-1.7.0
export PATH=$PATH:${JAVA_HOME}/bin:${ANT_HOME}/bin
export CLASSPATH=./:${JAVA_HOME}/lib/tools.jar:${JAVA_HOME}/lib/rt.jar:${ANT_HOME}/lib/ant.jar

To list all the background running oracle process in the system: ps -ef|grep "ora_"|grep -v grep
To give a short name to big command: alias listoraprocess="ps -ef|grep "ora_"|grep -v grep"
To kill all oracle process: ps -ef|grep "ora_"|grep -v grep|grep $ORACLE_SID|awk '{print $2}'|xargs kill -9
To list all files in current directory: find . -print
To search for string "error" in all files in current directory: find . -print|xargs grep -i error
To print time of execution for the bash script:

#!/usr/bin/env bash
curtime=`date +"%s"`
#Your bash script statements here
echo "Time of execution is `expr $(date +'%s') - $curtime` seconds"

To reboot linux in 10 minutes with broadcast message: shutdown -h +10 "scheduled maintenance"
To find number of logged users: who | wc -l
To check the type of a command: type cd , type who
To echo System variables: echo $DISPLAY $HOME $MAIL $OLDPWD $PATH $PWD $SHELL $TERM $USER
To print shell's environment variables: env or printenv
To set command line editing using vi: set -o vi
To execute the last command which was executed begining with p: !p
To execute a command as background job use &: ls -al &
To list all pending jobs: jobs
To create softlink (shortcut, deleting shoftlink doesn't delete original file): ln -s myfile softlink
To create hardlink (deleting hardlink deletes original file):ln -s myfile softlink
To find where the link is pointing to: ls -l linkname
To simply view files use one of these commands:
- cat - View files in their entirety
- more - View files one page at a time
- less - View files one page at a time. Ability to move backwards by pressing b.
- head - View the first lines of a file
- tail - View the last lines of a file
- nl - View files with their lines numbered
To continuously tail a log file: tail -f mylog.log
To update timestamp of a file ot create a empty file: touch filename
To change the default editor set the following environment variables

$EDITOR=emacs
$VISUAL=emacs
$export EDITOR VISUAL

To count number of byte words and lines in a file: wc filename
To count only number of lines in a file: wc -l filename
To list important attributes of a file like size device access : stat filename
To print size of files: du file1 file2
To print all filenames begining with myfile: find / -type f -name myfile* -print
To print all directories begining from current: find . -type d -print
To print lines containg myword in a file myfile: grep myword myfile
To print lines which doesn't contain myword in myfile: grep -v myword myfile
To display only unique lines in sorted order from a file: sort myfile | uniq
To count duplicate lines: sort myfile | uniq -c
To display command out and also write into a file simultaneously use tee command:

who | tee who_output | sort

To print 2nd and 4th word in each line in myfile: awk '{print $2, $4}' myfile
To print all lines shorter than 50 characters in myfile: awk '{length($0) < 50}' myfile
To print contents of myfile with all occurance or red converted to blue: sed 's/red/blue/g' myfile
To print contents of myfile skipping lines 2 to 5: sed '2,3d' myfile
To substitute all occurance of NAME with Kris and AGE with 31 in myfile:

m4 -DNAME=Kris -DAGE=31 myfile

To find size, used space, and free space on all disk partition: df -h
To find size, used space, and free space current directory: df .
To compare two directories recursively: diff -r dir1 dir2
To compare two file ignoring case: diff -i file1 file2
To compare tabular sorted file skipping 2nd column : comm -2 file1 file2
To point out the first diffrence in similar files: cmp file1 file2
To print 32 byte checksum, useful to check downloaded files: md5sum file1
To print name of the terminal device associated with the current shell: tty
To format output display string: printf "User %s is %d years old.\n" kris 31
To enter "y" to a interactive shell program by default: yes | my_interactive_command
To print current year business calendar with day number: cal -y -m
To print current year business calendar with sequential numbers instead of date: cat -y -m -j
To print epoch seconds: date - %s
To print last modified timestamp of a file: date -r myfile
To print Hello every 10 seconds: sleep 10 && echo 'Hello'
To check which files are newly added to current directory ever 20 seconds: watch -n 20 ls -l
To view your own process: ps -ux
To view process started by user1: ps -U user1
To view all occurance of a program name prg: ps -C prg
To view all process in screen width with full command line: ps -efww
To view all process in screen width with threads: ps -efH
To find how long the system is running since last boot: uptime
To display current process of all logged users: w
To monitor most active processes with full command line: top -c [press q to exit from top]
To graphical display of system load (similar to task manager in windows) : xload
To display memory usage in megabyte: free -mt [free -t for kilobytes]
To run system intensive process with controlled priority 7: nice -7 jobname jobargs > outfile [10 is default]
To change the priority of a process id 1834 previously executed with nice: nice -5 -p 1834
To change the priority of a process previously executed by user usr1 with nice: nice -5 -u usr1
To change the priority of already running command by process id from PS: renice +15 23456
To find details about user environment like login id, last login etc:

logname : current login name
whoami : same as logname except if su is used to switch user
finger : user information
last : last login details
printenv : environment details

To find details about your computer: uname -a
To print your host name: hostname
To print details of computer's default network interfaces: ifconfig
To display details of computer's all network interfaces: ifconfig -a
To display details of a hostname from internet directory: whois hotmail.com
To print network path from your machine to a host: traceroute yahoo.com
To scp myfile from current machine to othermachine using id otherid:

scp myfile otherid@remote.example.com:/home/otherhome/

To scp otherfile from other machine with id other id to current machine:

scp otherid@remote.example.com:/home/otherhome/otherfile .

To download a file from internet: wget http://www.yahoo.com/index.htm
To display useful prompt for oracle box:

Put the following in .profile or .bashrc file
PS1="`hostname`*\${ORACLE_SID}-\${PWD}

To display memory segments: ipcs
To display all cpu status: mpstat -P ALL
To display cpu utilization every 5 seconds, 5 times: sar -u 5 5
To run sar command non interactively to check later: nohup sar -o output.file 12 8 >/dev/null 2>&1 &
To display highest CPU utilizing process and user: ps -eo pcpu,pid,user,args | sort -k 1 -r | head -5
To display i/o status every 5 seconds 5 times: iostat -xtc 5 5
To display virtual memory status every 5 seconds: vmstat 5
To find files larger than 10KB: find . -size +10240 –print
To echo quotes: echo 'Single quotes "protect" double quotes' or echo "Well, isn't that \"special\"?"
To display command output in echo: echo "You have `ls | wc -l` files in `pwd`"
To display variable value in echo: echo "The value of \$x is $x"
To sort page and print a file: sort file | pr -3 | lp
To redirect standard output to standard error: echo "Usage error: see administrator" 1>&2
To send command output to one file and error to another file: find / -print > fileOne 2>fileTwo
To remember a directory and use it later:
- pushd /var/dirme
----execute other commands even change directory
- popd
To create a patch file by using diff between files: diff fileOld fileNew > patchfile
To apply patch to a file: patch fileOld patchfile
To create a patch file between directories: diff -Naur prestinedir modifieddir > patchfile
To apply patch to a directory:
- cd prestinedir
- patch -p1 < patchfile
To put the most recent command suspended using Ctrl+z in background: bg
To put a command in background using process number: bg %3
To find background job number: jobs
To bring a background process to foreground: fg %4
To prevent a process being killed when you exit the shell:
- nohup lpr -Phod /var/access.log & (command output goes to nohup.out)
To split a file based on lines:
- split -l 100 long.txt piece
- more piece*
To split a file based on size in KB or MB:
- split -b 10k long piece
- split -b 10m long piece
To add user smith : useradd -d /home/smith -s /bin/bash -g users smith
To delete user smith: userdel smith
To change user home directory: usermod -d /u01/smith smith
To change smith password: passwd smith
To find user details: finger smith
To change name and other finger details of smith: chfn smith
To change shell for smith: chsh smith
To find all the available shells in the system: chsh -l
To switch to super user with directory change to /root: su -l or su -
To switch to super user with current shell and current directory: su
To print group information of smith: groups smith
To add new group: groupadd -f newgroup
To delete group: groupdel group1
To change group name: groupmod -n newgroupname oldgroupname
To set hostname: hostname me.myhost.com
To display current time, number of users logged, how long the system is running and the amount of load into the system: uptime
To reset terminal when backspace and delete keys produce junk:
- stty erase Delete
- stty intr Ctrl+c
To change the group file belongs to: chgrp admingroup imp.tar
To log into new group you belong. This will set the group to new group without running chgrp:
- newgrp admingrp

Vi editor tips:

To delete all occurance of DEBUG. ESC :%s/* DEBUG //g
To delete all occurance of "DEBUG ". ESC :%s/* DEBUG //g
To delete everything between <> including <> in line numbers 1 to 8. ESC :1,8s/<[^>]*>//g
To substitute all occurences of foo on the current line with bar. ESC :.s/foo/bar/g
To subsitute all matches of the word foo in the file with bar. Ignore case in searches.

ESC :%s/foo/bar/g

To subsitute all matches of the microsoft with the linux. ESC :%s;microsoft/;linux;g
To edit files found by grep: vi `grep -l ifdef *.java`
To edit previous command using vi: fc -e vi