Commonly Used for awk

awk mainly deals with data in the field of each row, and the default field separator is blank key or tab key.

We can use last to retrieve the information of the login person, and the result is as follows(take out only the first 5 lines):

$ last -n 5
jimmy      unix:0.0                        Tue Nov  9 12:41   still logged in
jimmy      unix:0.0                        Tue Nov  9 11:54 - 12:40  (00:46)
boot time                                  Tue Nov  9 11:52
shutdown time                              Tue Nov  9 01:15
jimmy      unix:0.0                        Mon Nov  8 22:46 - 01:15  (02:29)

I want to retrieve the account number and the day of the week, and the account number and the day of the week are separated by tab key:

$ last -n 5 | awk '{print $1 "\t" $3}'
jimmy    Tue
jimmy    Tue
boot    Tue
shutdown    Tue
jimmy    Mon

 

The $0 in the first line represents the line "jimmy      unix:0.0                        Tue Nov  9 12:41   still logged in".

 

The processing flow of awk is:

1. Read in the first row, and fill in the data of the first row into variables such as $0, $1, $2....

2. According to the restriction of condition type, determine whether the following action is needed.

3. Finish all actions and condition types.

4. If there are subsequent line data, repeat the steps 1~3 above until all the data have been read.

Built-in variables of awk:

NF :  The total number of fields in each row ($0).

NR :  Currently, what awk is processing is the number of rows data.

FS  :  The current separator character, the default is the space bar.

Example:

$ last -n 5| awk '{print $1 "\t lines: " NR "\t columns: " NF}'
jimmy     lines: 1     columns: 9
jimmy     lines: 2     columns: 9
boot     lines: 3     columns: 6
shutdown     lines: 4     columns: 6
jimmy     lines: 5     columns: 9

awk logical operation characters:

>    :  Greater than

<    :  Less than

>= :  Greater than or equal to

<= :  Less than or equal to

== :  Equal

!=  :  Not equal to

 

Example: Suppose I want to check the data in the third column less than 10, and only list the account number and the third column. 

cat /etc/passwd | awk '{FS=":"} $3 < 10 {print $1 "\t" $3}'

or

cat /etc/passwd | awk 'BEGIN {FS=":"} $3 < 10 {print $1 "\t " $3}'

 

 

The content of pay.txt is as follows:
Name    1st     2nd     3th
VBird   23000   24000   25000
DMTsai  21000   20000   23000
Bird2   43000   42000   41000

I wan to calculate the total of each person and also format the output.

$ cat pay.txt |\
> awk 'NR==1{printf "%10s %10s %10s %10s %10s\n",$1,$2,$3,$4,"Total"}
> NR>=2{total = $2 + $3 + $4
> printf "%10s %10d %10d %10d %10.2f\n",$1,$2,$3,$4,total}'
      Name        1st        2nd        3th      Total
     VBird      23000      24000      25000   72000.00
    DMTsai      21000      20000      23000   64000.00
     Bird2      43000      42000      41000  126000.00

The {} in the action of awk also supports if (condition) as follows:

$ cat pay.txt |\
> awk '{if(NR==1) printf "%10s %10s %10s %10s %10s\n",$1,$2,$3,$4,"Total"}
> NR>=2{total = $2 + $3 + $4
> printf "%10s %10d %10d %10d %10.2f\n",$1,$2,$3,$4,total}'
      Name        1st        2nd        3th      Total
     VBird      23000      24000      25000   72000.00
    DMTsai      21000      20000      23000   64000.00
     Bird2      43000      42000      41000  126000.00

About printf.