Command Substitution in Linux

In Linux, we do a lot of onliners on the shell. Something like,

Code:

root@bt > date
Tue Jan  3 01:19:19 IST 2012

Now if we want to store this date into a variable, what do we do?

There are 2 ways in which we can achieve this i.e. command substitution

One way, using backticks i.e. `

Code:

root@bt > var=`date`
root@bt > echo $var
Tue Jan 3 01:20:47 IST 2012

Other way is using $(...)

Code:

root@bt > var=$(date)
root@bt > echo $var
Tue Jan 3 01:21:18 IST 2012

Command line or script, you can use it anywhere you want. I prefer the second way!

Note, there is no space between var = and the command. Why so? Because that is how it should be.
That is the syntax. If you have a space in between you will see strange errors.
Let us try it once with a space inbetween,

Code:

root@bt > var =$(date)
No command 'var' found, did you mean:
 Command 'jar' from package 'openjdk-6-jdk' (main)
 ...
 Command 'par' from package 'par' (universe)
var: command not found

Oops, we don't want that now, do we?

Good Day!

"which" command wasn't working in Linux

That was weird, or is that how it works?

I had this executable "db_linux" in "/usr/local/bin" and this was included in the env variable PATH. But there was no execute permission for this executable.And when I did a "which db_linux", it didn't show me anything. I was checking the file, the path everything was proper. The I thought fine lets execute it with absolute path. In the process I gave execute permission for the file and executed it. Later when I tried "which db_linux" again, it was showing. May be that is how it works.

BTW, I use Fedora Core 13 v 2.6.33.3

Code:

[root@ahamed bin]# ls -lrt
total 4
-rw-r--r--. 1 root root 63 Nov 16 16:33 db_linux

[root@ahamed bin]# which db_linux
/usr/bin/which: no db_linux in (/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:
/usr/loca/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin)

[root@ahamed bin]# chmod 744 db_linux
[root@ahamed bin]# ls -lrt
total 4
-rwxr--r--. 1 root root 63 Nov 16 16:33 db_linux

[root@ahamed bin]# which db_linux
/usr/local/bin/db_linux

Have a good day!