20 Unix/Linux Command Line Tricks – Part I

Let us start new year with these Unix and Linux command line tricks to increase productivity at the Terminal. I have found them over the years, and I am now going to share with you.



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Deleting a HUGE file on Unix like system

I had a huge log file 200GB I need to delete on a production web server. My rm command and ls command was crashed and I was afraid that the system to a crawl with huge disk I/O load. To remove a HUGE file, enter:

> /path/to/file.log # or use the following syntax : > /path/to/file.log # finally delete it rm /path/to/file.log

Want to cache console output on the CLI?

Try the script command line utility to create a typescript of everything printed on your terminal.

script my.terminal.session script my.terminal.session

Type commands:

ls date sudo service foo stop ls date sudo service foo stop

To exit (to end script session) type exit or logout or press control-D

exit exit

To view type:

more my.terminal.session less my.terminal.session cat my.terminal.session more my.terminal.session less my.terminal.session cat my.terminal.session

Restoring deleted /tmp folder on Linux

As my journey continues with Linux and Unix shell, I made a few mistakes. I accidentally deleted /tmp folder. To restore it all you have to do is:

mkdir / tmp chmod 1777 / tmp chown root:root / tmp ls -ld / tmp mkdir /tmp chmod 1777 /tmp chown root:root /tmp ls -ld /tmp

Locking a directory

For privacy of my data I wanted to lock down /downloads on my file server. So I ran:

chmod 0000 / downloads chmod 0000 /downloads

The root user can still has access and ls and cd commands will not work. To go back:

chmod 0755 / downloads chmod 0755 /downloads

Password protecting file in vim text editor

Afraid that root user or someone may snoop into your personal text files? Try password protection to a file in vim, type:

vim +X filename vim +X filename

Or, before quitting in vim use :X vim command to encrypt your file and vim will prompt for a password.

Clear gibberish all over the screen

Just type the following command to fix the display and console gibberish:

## terminal initialization ## reset ## terminal initialization ## reset

Becoming human

Pass the -h or -H (and other options) command line option to GNU or BSD utilities to get output of command commands like ls command, df command, du command, in human-understandable formats:

ls -lh # print sizes in human readable format (e.g., 1K 234M 2G) df -h df -k # show output in bytes, KB, MB, or GB free -b free -k free -m free -g # print sizes in human readable format (e.g., 1K 234M 2G) du -h # get file system perms in human readable format stat -c % A / boot # compare human readable numbers sort -h -a file # display the CPU information in human readable format on a Linux lscpu lscpu -e lscpu -e =cpu,node # Show the size of each file but in a more human readable way tree -h tree -h / boot ls -lh # print sizes in human readable format (e.g., 1K 234M 2G) df -h df -k # show output in bytes, KB, MB, or GB free -b free -k free -m free -g # print sizes in human readable format (e.g., 1K 234M 2G) du -h # get file system perms in human readable format stat -c %A /boot # compare human readable numbers sort -h -a file # display the CPU information in human readable format on a Linux lscpu lscpu -e lscpu -e=cpu,node # Show the size of each file but in a more human readable way tree -h tree -h /boot

Show information about known users in the Linux based system

Just type:

## linux version ## lslogins ## *BSD version ## logins ## linux version ## lslogins ## *BSD version ## logins

Sample outputs:

UID USER PWD-LOCK PWD-DENY LAST-LOGIN GECOS 0 root 0 0 22:37:59 root 1 bin 0 1 bin 2 daemon 0 1 daemon 3 adm 0 1 adm 4 lp 0 1 lp 5 sync 0 1 sync 6 shutdown 0 1 2014-Dec17 shutdown 7 halt 0 1 halt 8 mail 0 1 mail 10 uucp 0 1 uucp 11 operator 0 1 operator 12 games 0 1 games 13 gopher 0 1 gopher 14 ftp 0 1 FTP User 27 mysql 0 1 MySQL Server 38 ntp 0 1 48 apache 0 1 Apache 68 haldaemon 0 1 HAL daemon 69 vcsa 0 1 virtual console memory owner 72 tcpdump 0 1 74 sshd 0 1 Privilege-separated SSH 81 dbus 0 1 System message bus 89 postfix 0 1 99 nobody 0 1 Nobody 173 abrt 0 1 497 vnstat 0 1 vnStat user 498 nginx 0 1 nginx user 499 saslauth 0 1 "Saslauthd user"

How do I fix mess created by accidentally untarred files in the current dir?

So I accidentally untar a tarball in /var/www/html/ directory instead of /home/projects/www/current. It created mess in /var/www/html/. The easiest way to fix this mess:

cd / var / www / html / / bin / rm -f " $(tar ztf /path/to/file.tar.gz) " ## or better ## tar ztf / path / to / file.tar.gz | xargs -d '

' rm -v cd /var/www/html/ /bin/rm -f "$(tar ztf /path/to/file.tar.gz)" ## or better ## tar ztf /path/to/file.tar.gz | xargs -d'

' rm -v

Confused on a top command output?

Seriously, you need to try out htop instead of top:

sudo htop sudo htop

Want to run the same command again?

Just type !! . For example:

/ myhome / dir / script / name arg1 arg2 # To run the same command again !! ## To run the last command again as root user sudo !! /myhome/dir/script/name arg1 arg2 # To run the same command again !! ## To run the last command again as root user sudo !!

The !! repeats the most recent command. To run the most recent command beginning with “foo”:

! foo # Run the most recent command beginning with "service" as root sudo ! service !foo # Run the most recent command beginning with "service" as root sudo !service

The !$ use to run command with the last argument of the most recent command:

# Edit nginx.conf sudo vi / etc / nginx / nginx.conf # Test nginx.conf for errors / sbin / nginx -t -c / etc / nginx / nginx.conf # After testing a file with "/sbin/nginx -t -c /etc/nginx/nginx.conf", you # can edit file again with vi sudo vi ! $ # Edit nginx.conf sudo vi /etc/nginx/nginx.conf # Test nginx.conf for errors /sbin/nginx -t -c /etc/nginx/nginx.conf # After testing a file with "/sbin/nginx -t -c /etc/nginx/nginx.conf", you # can edit file again with vi sudo vi !$

Get a reminder you when you have to leave

If you need a reminder to leave your terminal, type the following command:

leave +hhmm leave +hhmm

Where,

hhmm – The time of day is in the form hhmm where hh is a time in hours (on a 12 or 24 hour clock), and mm are minutes. All times are converted to a 12 hour clock, and assumed to be in the next 12 hours.

Home sweet home

Want to go the directory you were just in? Run:

cd -

Need to quickly return to your home directory? Enter:

cd

The variable CDPATH defines the search path for the directory containing directories:

export CDPATH = / var / www: / nas10 export CDPATH=/var/www:/nas10

Now, instead of typing cd /var/www/html/ I can simply type the following to cd into /var/www/html path:

cd html cd html

Editing a file being viewed with less pager

To edit a file being viewed with less pager, press v . You will have the file for edit under $EDITOR:

less * .c less foo.html ## Press v to edit file ## ## Quit from editor and you would return to the less pager again ## less *.c less foo.html ## Press v to edit file ## ## Quit from editor and you would return to the less pager again ##

List all files or directories on your system

To see all of the directories on your system, run find command:

find / -type d | less # List all directories in your $HOME find $HOME -type d -ls | less find / -type d | less # List all directories in your $HOME find $HOME -type d -ls | less

To see all of the files, run:

find / -type f | less # List all files in your $HOME find $HOME -type f -ls | less find / -type f | less # List all files in your $HOME find $HOME -type f -ls | less

Build directory trees in a single command

You can create directory trees one at a time using mkdir command by passing the -p option:

mkdir -p / jail / { dev,bin,sbin,etc,usr,lib,lib64 } ls -l / jail / mkdir -p /jail/{dev,bin,sbin,etc,usr,lib,lib64} ls -l /jail/

Copy file into multiple directories

Instead of running cp command as follows:

cp / path / to / file / usr / dir1 cp / path / to / file / var / dir2 cp / path / to / file / nas / dir3 cp /path/to/file /usr/dir1 cp /path/to/file /var/dir2 cp /path/to/file /nas/dir3

Run the following command to copy file into multiple dirs:

echo / usr / dir1 / var / dir2 / nas / dir3 | xargs -n 1 cp -v / path / to / file echo /usr/dir1 /var/dir2 /nas/dir3 | xargs -n 1 cp -v /path/to/file

Creating a shell function is left as an exercise for the reader

Quickly find differences between two directories

The diff command compare files line by line. It can also compare two directories:

ls -l / tmp / r ls -l / tmp / s # Compare two folders using diff ## diff / tmp / r / / tmp / s / ls -l /tmp/r ls -l /tmp/s # Compare two folders using diff ## diff /tmp/r/ /tmp/s/

Text formatting

You can reformat each paragraph with fmt command. In this example, I’m going to reformat file by wrapping overlong lines and filling short lines:

fmt file.txt fmt file.txt

You can also split long lines, but do not refill i.e. wrap overlong lines, but do not fill short lines:

fmt -s file.txt fmt -s file.txt

See the output and write it to a file

Use the tee command as follows to see the output on screen and also write to a log file named my.log:

mycoolapp arg1 arg2 input.file | tee my.log mycoolapp arg1 arg2 input.file | tee my.log

The tee command ensures that you will see mycoolapp output on on the screen and to a file same time. We can also use the tee to append text to a file when using sudo command:

cat foo | sudo tee -a bar.txt > / dev / null find / path / to / dir1 / -iname "*.py" -ls | sudo tee -a lists.txt > / dev / null cat foo | sudo tee -a bar.txt > /dev/null find /path/to/dir1/ -iname "*.py" -ls | sudo tee -a lists.txt > /dev/null

Conlcusion

We cover some basic and useful Unix and Linux command line tips and tricks to increase your productivity at the bash shell prompt for fun and profit.