Grep and Sed Commands

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Complete Overview on Grep and Sed command

Created By - Ashutosh Priyadarshi

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GREP

Grep is the frequently used command in Unix (or Linux). Most of us use grep just for finding

the words in a file. The power of grep comes with using its options and regular expressions.

You can analyze large sets of log files with the help of grep command. Grep stands for Global

search for Regular Expressions and Print.

Contents of grep_test file.

-A

This option print the lines after the match in short print NUM lines of trailing context

after matching lines.


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-B

This option print the lines before the match in short print NUM lines of leading context

before matching lines.

-C

This option print the lines before and after the match in short print NUM lines between

contiguous groups of matches

-b

This option print the byte offset within the input file before each line of output.


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--color

Surround the matching string with the marker find in GREP_COLOR environment

variable. three option are = never, always or auto.

-c

Print the total number of lines in which pattern had been found, when it is used with -v

option it print the total number of lines which doesnt contain the search pattern

-f

Obtain patterns from FILE, one per line. The empty file contains zero patterns, and

therefore matches nothing.


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-F

-e and-E

-e is strictly the flag for indicating the pattern you want to match against. -E controls

whether you need to escape certain special characters.

More Explanation :

grep understands three different versions of regular expression syntax: basic,

extended and perl. In GNU grep, there is no difference in available functionality

between basic and extended syntaxes. In other implementations, basic regular

expressions are less powerful. The following description applies to extended regular

expressions; differences for basic regular expressions are summarized afterwards. Perl

regular expressions give additional functionality, and are documented in pcresyntax(3)

and pcrepattern(3), but may not be available on every system.

-e stand for basic, -E stands for extened and -P for perl


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--label=LABEL

-H

This option prints the filename for each match.

-h

This option Suppress the prefixing of filenames on output when multiple files are

searched.

-I

Process a binary file as if it did not contain matching data.


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-i

Ignore case distinctions in both the PATTERN and the input files

-L

Suppress normal output, instead print the name of each input file from which no output

would normally have been printed. Also print the directory name if there in no match

inside it.

-l

Prints the name of the file where proper match will be found, it will not print the

pattern, it will just print the file name.


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-m

Stop reading a file after NUM matching lines. When the -v or --invert-match option is

also used, grep stops after NUM non-matching lines

-n

This option prints line number and line for particular pattern match.

-o

This option prints the pattern not the whole line.


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-q

Do not write anything to standard output. Exit immediately with zero status if any

match is found, even if an error was detected.

-r , -R, --recursive

Read all files under each directory, recursively; this is equivalent to the -d recurse

option.

-s

This option suppresses error messages about nonexistent or unreadable files.

Without using -s option


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With -s option

-v

This option inverts the sense of matching, to select non-matching lines.

-w

Select only those lines containing matches that form whole words. The test is that the

matching substring must either be at the beginning of the line, or preceded by a non-


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word constituent char acter.

This can be a example for -s option, check both grep command in above screen shots.

-x, --line-regexp

Select only those matches that exactly match the whole line.


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SED COMMAND

Def :: sed command is used as stream editor for filtering and transforming text

Content of file.txt

Replacing or substituting string

sed 's/unix/linux/' file.txt

Replacing the nth occurrence of a pattern in a line

sed 's/unix/linux/2' file.txt

Replacing all the occurrence of the pattern in a line.

sed 's/unix/linux/g' file.txt

Replacing from nth occurrence to all occurrences in a line.

Use the combination of /1, /2 etc and /g to replace all the patterns from the nth

occurrence of a pattern in a line.


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sed 's/unix/linux/3g' file.txt

Changing the slash (/) delimiter

Content of sed_test.dat file.

sed 's/https:\/\//www./' sed_test.dat

In this case the url consists the delimiter character which we used. In that case you have

to escape the slash with backslash character, otherwise the substitution won't

work.Using too many backslashes makes the sed command look awkward. In this case

we can change the delimiter to another character as shown in the below example.

sed 's_https://_www._' sed_test.dat

sed 's|https://|www.|' sed_test.dat

Using & as the matched string

There might be cases where you want to search for the pattern and replace that pattern

by adding some extra characters to it. In such cases & comes in handy. The &

represents the matched string.


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sed 's/unix/{&}/' file.txt

sed 's/unix/{&&}/' file.txt

Using \1,\2 and so on to \9

Duplicating the replaced line with /p flag

sed 's/unix/linux/p' file.txt

Printing only the replaced lines

sed -n 's/unix/linux/p' file.txt

If you use -n alone without /p, then the sed does not print anything.

Running multiple sed commands

sed 's/unix/linux/' file.txt| sed 's/os/system/'

Sed provides -e option to run multiple sed commands in a single sed command


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sed -e 's/unix/linux/' -e 's/os/system/' file.txt

Replacing string on a specific line number

sed '3 s/unix/linux/' file.txt

Replacing string on a range of lines

sed '1,3 s/unix/linux/' file.txt

sed '2,$ s/unix/linux/' file.txt

Replace on a lines which matches a pattern

sed '/linux/ s/unix/centos/' file.txt

Here the sed command first looks for the lines which has the pattern "linux" and then

replaces the word "unix" with "centos".

How to delete lines from a file.

sed '2 d' file.txt

sed '5,$ d' file.txt


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How to Duplicate lines

sed 'p' file.txt

How to use sed as grep command

grep 'unix' file.txt

sed -n '/unix/ p' file.txt

You can also make the sed command to work as grep -v, just by using the reversing the

sed with NOT (!).

grep -v 'unix' file.txt

sed -n '/unix/ !p' file.txt

Add a line after a match.

sed '/unix/ a "Add a new line"' file.txt

How to change a line after match

sed '/unix/ c "Change line"' file.txt


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Transform like tr command

sed 'y/ul/UL/' file.txt

Display specific lines (based on line number) of a file using sed command

View only the specific lines mentioned by line numbers.

sed -n -e Xp -e Yp FILENAME

sed : sed command, which will print all the lines by default.

-n : Suppresses output.

-e CMD : Command to be executed

Xp: Print line number X

Yp: Print line number Y

FILENAME : name of the file to be processed.

How to view the content of file from starting line number to ending line number

sed -n 101,110p File_Name


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How to add line at first and last in a file

sed '1 i first_line' FILE_NAME

sed '$ a last_line' FILE_NAME

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Grep and Sed Commands