Awk

awk

pattern {action}
pattern {action}

$ awk '{print $2}' /etc/fstab

to filter out comment lines - starting with #

$ awk '/^[^#]/ {print $2}' /etc/fstab

print first field from password file using field seperator ':'

$ awk -F: '{print$1}' /etc/passwd

print only user ID > 500

$ awk -F: '$3>=500 {print$1}' /etc/passwd

display average of numbers entereed on a line:

$ awk '{ sum=0; for (i=1; i<=NF; i++) sum +=$i; print sum/NF; }'

NF	number of fields in the current line
NR	the current record number (line number)
FS	input field seperator (space by default)
RS	record seperator (newline by default)
Example	Example
Example	Example
Example	Example
Example	Example
Example	Example

show the 5th record on the second line:

df  /tmp | awk 'NR==2 {print$5 }'

use awk with a program

the file maxuid:

BEGIN {maxuid = 0; FS=":" }
{ if ($3 > maxuid) maxuid = $3 }
END { print" the largest UID is ", maxuid }

then use awk like this:

$ awk -f maxuid /etc/passwd

to ignore user nobody:

BEGIN {maxuid = 0; FS=":" }
'''$1 != "nobody"''' { if ($3 > maxuid) maxuid = $3 }
END { print" the largest UID is ", maxuid }