20 Unix Command Line Tricks

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

Deleting a HUGE file

I had a huge log file 200GB I need to delete on a production web server. My rm 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?

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

1	script my.terminal.session

Type commands:

1
2
3

ls
date
sudo service foo stop

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

exit

To view type:

1
2
3

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

Restoring deleted /tmp folder

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

Locking a directory

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

1	chmod 0000 /downloads

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

1	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
``` 
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:

``` bash 
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, df, du, 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

Show information about known users in the Linux based system

Just type:

## 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:

1 2	cd /var/www/html/ /bin/rm -f "$(tar ztf /path/to/file.tar.gz)"

Confused on a top command output?

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

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 !!

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

1
2
3

!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 !$

Get a reminder you when you have to leave

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

1	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:

1	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

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 ##
``` bash
### List all files or directories on your system

To see all of the directories on your system, run:
``` bash
find / -type d | less
 
# List all directories in your $HOME
find $HOME -type d -ls | less
``` 
To see all of the files, run:
``` bash
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:
``` bash
mkdir -p /jail/{dev,bin,sbin,etc,usr,lib,lib64}
ls -l /jail/
``` 
### Copy file into multiple directories

Instead of running:
``` bash
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:
``` bash
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/

Fig. : Finding differences between folders

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:

1	fmt file.txt

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

1	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:

1	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.

More info: Unix Command