All about SED command

The streamline editor or sed command is a stream editor that reads one or more text files, makes changes or edits according to editing script, and writes the results to standard output. First, we will discuss sed command with respect to search and replace function. Other uses for the sed can also be found in this official guide, but we will discuss them briefly in this chapter as well.

1. Find and replace

Most common use of sed is to substitute text, matching a pattern. The syntax for doing this in sed is as follows:

  • Here, / is the delimiter (you can also use _ (underscore), | (pipe) or : (colon) as delimiter as well)
  • OPERATION specifies the action to be performed (sometimes if a condition is satisfied). The most common and widely used operation is s which does the substitution operation (other useful operators include y for transformation, i for insertion, d for deletion etc.).
  • REGEXP and REPLACEMENT specify search term and the substitution term respectively for the operation that is being performed.
  • FLAGS are additional parameters that control the operation. Some common FLAGS include:
    • g replace all the instances of REGEXP with REPLACEMENT (globally)
    • N where N is any number, to replace Nth instance of the REGEXP with REPLACEMENT
    • p if substitution was made, then prints the new pattern space
    • i ignores case for matching REGEXP
    • w file If substitution was made, write out the result to the given file
    • d when specified without REPLACEMENT, deletes the found REGEXP

Some search and replace examples:

find and replace all chr to chromosome in the file

sed 's/chr/chromosome/g' FILENAME > NEWFILE

find and replace, but only the one instance per line (first occurrence of chr will be changed to chromosome)

sed 's/chr/chromosome/1' FILENAME > NEWFILE

find and replace, but do it directly on the original file

sed -i 's/chr/chromosome/g' FILENAME

find and replace directly, but save a old version too

sed -i.old 's/chr/chromosome/g' FILENAME

find and replace, only if you also find MTF1 in the line

sed '/MTF1/s/chr/chromosome/g' FILENAME > NEWFILE

2. Print specific lines of the file

To print a specific line, you can use the address function, note that by deafault, sed will stream the entire file, so when you are interested in specific lines only, you will have to suppress this feature using the option -n.

print 10th line

sed -n '10p' FILENAME

You can provide any number of additional lines to print using -e option (you can add any number of lines like this)

print 10th and 15th line

sed -n -e '10p' -e '15p' FILENAME

It also accepts range, using ,

print lines 10 to 50

sed -n '10,50p' FILENAME

or you can create specific pattern, like multiple of a number using ~

Every tenth line starting from 10, 20, 30.. to end of the file

sed -n '10~10p' FILENAME

print odd-numbered lines

sed -n '1~2p' FILENAME

Most powerful feature is that you can combine these ranges or multiples in any fashion. Example: fastq files have header on first line and sequence in second, next two lines will have the quality and a blank extra line (four lines make one read). Sometimes you will only need the sequence and header

to print 1,2,5,6,9,10 so on you can use

sed -n '1~4p;2~4p' FASTQ_FILE

pipe this to make a fasta file

sed -n '1~4p;2~4p' FASTQ_FILE | sed 's/^@/>/g' > FASTA_FILE

More combinations:

print 1 to 10, and then multiples of 10

sed -n '1,10~10p'  FILENAME

3. Delete specific lines of the file

All the above address types (specific line, range, multiples), also works with other types of operation, such as deletion and insertion. For deletion, you need to swap p with d

delete first line

sed "1d" FILENAME

delete lines 1 thru 3

sed "1,3d" FILENAME

delete blank lines

sed 's/^$//g' FILENAME

4. Insert specific lines to a file

Here, you use i for inserting text anywhere in the file

put “line to insert” in the second line

sed '2 i line to insert' FILENAME

5. Extract portions of the lines

still to come!

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