Eval Command in Linux

Eval is built in unix command – it’s used to execute arguments as shell commands. It’s useful when you have command stored in variable and you want to execute it.

[root@server Files]# mycommand="ls -ltr"

[root@server Files]# echo $mycommand
ls -ltr

[root@server Files]# eval $mycommand
total 0
-rw-r--r-- 1 root root 0 Nov  5 07:40 file1.txt
-rw-r--r-- 1 root root 0 Nov  5 07:40 file3.txt
-rw-r--r-- 1 root root 0 Nov  5 07:40 file2.txt
[root@server Files]#

Look at the example above, I have stored the command ls – ltr in the variable mycommand so when I do echo it’s just prints what in the variable. But when I do eval followed with the variable – it will actually execute the command.

Eval is powerful command but it’s Evil – it has security issues as well.
You can find the details here

Avoid Confirmation while Copying Files

While copying files, using cp command you would have noticed that it always asks for the confirmation overwriting files like below.

[root@server ~]# cp -v test.txt example.txt
cp: overwrite ‘example.txt’?

Will see how you can avoid that – but before that let’s see why it happens.

This happens because of alias set in the .bashrc file.

[root@server ~]# cat .bashrc
# .bashrc

# User specific aliases and functions

alias rm='rm -i'
alias cp='cp -i'
alias mv='mv -i'

you can remove alias in .bashrc file – and you won’t be asked for confirmation ever again.

but if you want to skip confirmation for time being then you can call the cp command directly from bin – Example.

[root@server ~]# /bin/cp -v test.txt example.txt
‘test.txt’ -> ‘example.txt’
[root@server ~]#

Rsync command examples

So what is Rsync?
Rsync is a Linux tool which helps is syncing a file from a server A to server B and vice versa. Rsync is powerful tool which numerous options and we will see some of the examples below.

Rsync Command to copy files in the destination.

So the basic command would look like this.

[root@server ~]#  rsync [option] user@host:[file/directory origin] [destination]

[root@server ~]# rsync -avzh /home/user/backups/ user@backup01:/home/backup/

So above command will copy contents in the backups folder to the remote destination – Important note if you don’t put trailing / in the /home/user/backups it will copy whole folder instead of just contents in it.

Further to explain above command:-
-a stands for copy files recursively and preserve ownership of files when files are copied
-v stands for runs the verbose version of the command; all of the processes that are run will be written out for the user to read
-z stands for compress the data synced
– h stands for produce easily readable output, not just numbers

Show Progress:-

If you are doing a large transfers and want to see progress, you can use –progress option.

[root@server ~]# rsync -avzh --progress /home/user/backups/ user@backup01:/home/backup/


Exclude Files/folders from syncing.

Create a file call exclude.txt and add all the files and folders you want to exclude e.g:-
test/file.txt
foldername

rsync -avzh --exclude-from 'exclude.txt' sync/ 5.7.7.2:/home/

Basic AWK command examples.

Along with SED command Linux – AWK is powerful command line tool which would help you to perform a lot of actions on your data. It can help you to sort, print and process your data in the way you like. You can use AWK to read and edit the file.

AWK is mostly used in the Shell Scripts. Lets check out Awk without wasting more time 🙂

Let’s create a file called data.txt with the following contents – so we can learn AWK command better with examples.

Bob America Married 50
Jay India Unmarried 25
Abdul UAE married 33
Mona London Married 28
John America Unmarried 18
Neha India Unmarried 16

Run the command below which will print the contents in the file as it is.

[root@server ~]# awk '{print}' data.txt

Bob America Married 50
Jay India Unmarried 25
Abdul UAE married 33
Mona London Married 28

Print all the lines Matching with America.

[root@server ~]# awk '/America/ {print}' data.txt
Bob America Married 50
John America Unmarried 18

Only Print age from the file. As you can see that Age is written in the 4 column so we will use the command below.

[root@server ~]# awk '{print $4}' data.txt
50
25
33
28
18
16
[root@server ~]#

So now if you want to print Name and Age.

[root@server ~]# awk '{print $1, $4}' data.txt
Bob 50
Jay 25
Abdul 33
Mona 28
John 18
Neha 16

You can print age first by changing the order.


[root@server ~]# awk '{print $4, $1}' data.txt
50 Bob
25 Jay
33 Abdul
28 Mona
18 John
16 Neha
[root@server ~]#

Conditional Printing

I would like to print the names of the people who live in America. So here you will use If Condition in awk. In the example below it will check if the America is present in the column 2 it will print the column 1

[root@server ~]# awk '{ if ($2 == "America") print $1;}' data.txt
Bob
John

In the above example, if you just put awk ‘{ if ($2 == “America”) print;}’ data.txt it will print the complete line.

AWK can do much more that what I have specified above. I will come up with Advanced AWK article soon.