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Regular expressions

SED

Character Description
^ Matches the beginning of the line
$ Matches the end of the line
. Matches any single character
* Will match zero or more occurrences of the previous character
[ ] Matches all the characters inside the [ ]
Regular expression Description
/./ Will match any line that contains at least one character
/../ Will match any line that contains at least two characters
/^#/ Will match any line that begins with a '#'
/^$/ Will match all blank lines
/}$/ Will match any lines that ends with '}' (no spaces)
/} *$/ Will match any line ending with '}' followed by zero or more spaces
/[abc]/ Will match any line that contains a lowercase 'a', 'b', or 'c'
/^[abc]/ Will match any line that begins with an 'a', 'b', or 'c'

SED

Sed Examples
examples
sed -i ’s/Ben/Dave/g’ file.txt # Replace all the words Ben for the word Dave

sed 's/Ben|ben/Dave/g' file.txt # Replace all the words Ben and ben for the word Dave

sed 's/^[ ^t]*//' file.txt # Delete all spaces in front of every line of file.txt

sed 's/[ ^t]*$//' file.txt # Delete all spaces at the end of every line of file.txt

sed -e '/^#/d' file.txt | more # View file without the commented lines

sed -e '/regexp/d' file.txt # delete the word regexp

sed 's/...//' # delete the first 3 characters on every line

AWK
 
awk '!($0 in a){a[$0];print}' # Remove duplicate, nonconsecutive lines

awk '{ print $NF }' # print the last field of each line

awk -F':' '{print $3,$4;}' # show only what is on columns 3 and 4

Find and replace

awk '{gsub(/foo/,"bar")}; 1' # if foo replace by bar

awk '/baz/{gsub(/foo/, "bar")}; 1' # ONLY for lines which contain "baz"

awk '!/baz/{gsub(/foo/, "bar")}; 1' # EXCEPT for lines which contain "baz"

Grep

grep 'word\|logs' file # can contain 2 strings

grep "word1" file | grep "word2" # line must match the 2 strings 

xargs Examplesexamples

locate file* | xargs grep "bob"bob" # find a file and grep a string

locate file* | xargs rm # find a file a del it

CUT example

cut -d " " -f 1 - cut everything after the first word

For loop example

for string in $(cat ips.txt); do ip route add blackhole $string; done

 

 

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