What are the methods available to get the CPU usage in Linux Command line?

As others have said, the best way is probably top. It needs a little tweaking and a little parsing but you can get it to give you the current CPU use as a percentage.

top splits CPU usage between user, system processes and nice processes, we want the sum of the three. So, we can run top in batch mode which allows us to parse its output. However, as explained here, the 1st iteration of top -b returns the percentages since boot, we therefore need at least two iterations (-n 2) to get the current percentage. To speed things up, you can set the delay between iterations to 0.01. Finally, you grep the line containing the CPU percentages and then use gawk to sum user, system and nice processes:

    top -bn 2 -d 0.01 | grep '^%Cpu' | tail -n 1 | gawk '{print $2+$4+$6}'
        -----  ------   -----------    ---------   ----------------------
          |      |           |             |             |------> add the values
          |      |           |             |--> keep only the 2nd iteration
          |      |           |----------------> keep only the CPU use lines
          |      |----------------------------> set the delay between runs
          |-----------------------------------> run twice in batch mode

I thought you could also get this information through ps -o pcpu ax by adding the %use of each running process. Unfortunately, as explained here, ps "returns the percentage of time spent running during the entire lifetime of a process" which is not what you need.

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EDIT

Based on your comment, your version of top is different to mine and you should use this instead:

top -bn 2 -d 0.01 | grep '^Cpu.s.' | tail -n 1 | gawk '{print $2+$4+$6}'

And, to avoid issues with localization, set the locale to C:

LC_ALL=C top -bn 2 -d 0.01 | grep '^Cpu.s.' | tail -n 1 | gawk '{print $2+$4+$6}'

sar is the definitive way to do it. So for instance sar -u will output something like this:

08:30:01 AM       CPU     %user     %nice   %system   %iowait     %idle
08:40:01 AM       all      6.94      0.00      1.77      4.92     86.36
08:50:01 AM       all      5.73      0.00      2.31     12.72     79.24
09:00:01 AM       all      5.95      0.00      2.58     18.36     73.11
09:10:01 AM       all      6.88      0.00      2.22     17.44     73.45
09:20:01 AM       all      8.61      0.00      2.68     27.93     60.78

You don't say which Linux you use but for CentOS/RedHat you need to install the sysstat package, and I think it's the same on Debian/Ubuntu.

You can also use sar to gather statistics ad hoc:

sar -o /tmp/sar.out 60 600

Will gather stats every 60 seconds 600 times, so 600 minutes.