GNU Sed

When an extension is provided as an argument to -i option, the original contents of the input file gets preserved as per the extension given. For example, if the input file is ip.txt and -i.orig is used, the backup file will be named as ip.txt.orig

sed -i.bkp 's/blue/green/' examples/sed/colors.txt

Sometimes backups are not desirable. Using -i option on its own will prevent creating backups. Be careful though, as changes made connot be undone. In such cases, tets the command with sample input before using -i option on actual file. You could also use the option with backup, compare the differences with a diff program and then delete the backup.

sed -i 's/an/AN/g' examples/sed/fruits.txt

Multiple input files are treated individually and the changes are written back to repective files.

sed -i.bkp 's/bad/good/' examples/sed/f1.txt examples/sed/f2.txt

Using wildcard and globbing

sed -i.bkp 's/bad/good/' examples/sed/f?.*

A * character in the argumetn to -i option is special. It will get replaced with the input filename. This is helpful if you need to use a prefix instead of suffix for the backup filename. Or any other combination that may be needed.

sed -i'bkp.*' 's/green/yellow/' examples/sed/colors.txt

The * trick can also be used to place the backups in another directory instead of the parent directory of input files. The backup directory should already exist for this to work.

sed -i'backups/*' 's/good/nice' examples/sed/f?.*

As seen earlier, the syntax for substitute command is s/REGEXP/REPLACEMENT/FLAGS. The /REGEXP/FLAGS portion can be used as a conditional expression to allow commands to execute only for the lines matching the pattern.

printf '1,2,3,4\na,b,c,d\n' | sed '/2/ s/,/-/g'

Use /REGEXP/FLAGS! to act upon lines other than the matching ones.

printf '1,2,3,4\na,b,c,d\n' | sed '/2/! s/,/-/g'

To delete the filtered lines, use the d command. Recall that all input lines are printed by default.

printf 'sea\neat\ndrop\n' | sed '/at/d'

# same as: grep -v 'at'
printf 'sea\neat\ndrop\n' | sed '/at/!d'

To print the filtered lines, use the p command. But, recall that all input lines are printed by default. So, this command is typically used in combination with -n command line option, which would turn off the default printing.

# same as: grep 'twice' examples/sed/programming_quotes.txt
sed -n '/twice/p' examples/sed/programming_quotes.txt

# same as: grep 'e th' examples/sed/programming_quotes.txt
sed -n '/e th/p' examples/sed/programming_quotes.txt

The substitue command provides p as a flag. In such a case, the modified line would be printed only if the substituion succeeded.

# same as grep '1' programming_quotes.txt | sed 's/1/one/g'
sed -n 's/1/one/gp' examples/sed/programming_quotes.txt

# filter + substitution + p combination
# same as grep 'not' programming_qoutes.txt | sed 's/in/**/g'
sed -n '/not/ s/in/**/gp' examples/sed/programming_quotes.txt

Using !p with -n option will be equivalent to using d command.

# same as: sed '/at/d'
printf 'sea\neat\ndrop\n' | sed -n '/at/!p'

Using q command wil exit sed immediately, without any further processing.

# quits after an input line containing 'if' is found
sed '/if/q' examples/sed/programming_quotes.txt 

Q command is similar to q but won't print the matching line.

sed '/if/Q' examples/sed/programming_quotes.txt 

Use tac to get all lines starting from last occurrence of the search string with respect to entire file content.

tac examples/sed/programming_quotes.txt | sed '/not/q' | tac

You can optionally provides an exit status (from 0 to 255) along with the quit commands.

printf 'sea\neat\ndrop\n' | sed '/at/q2'

Line numbers can also be used as a filtering criteria.

# here, 3 represents the address for the print command
# same as: head -n3 programming_quotes.txt | tail -n1 and sed '3!d'
sed -n '3p' examples/sed/programming_quotes.txt
echo '--------------------'
# print 2nd and 5th line
sed -n '2p; 5p' examples/sed/programming_quotes.txt
echo '--------------------'
# substitution only on 2nd line
printf 'gates\nnot\nused\n' | sed '2 s/t/*/g'

As a special case, $ indicates the last line of the input

# same as: tail -n1 programming_quotes.txt
sed -n '$p' examples/sed/programming_quotes.txt

For large input files, use q command to avoid processing unnecessary input lines.

seq 3542 4623452 | sed -n '2452{p; q}'
echo '--------------------'
seq 3542 4623452 | sed -n '250p; 2452{p; q}'
echo '--------------------'
# here is a sample time comparison
time seq 3542 4623452 | sed -n '2452{p; q}' > examples/sed/f1
echo '--------------------'
time seq 3542 4623452 | sed -n '2452p' > examples/sed/f2

Mimicking head command using line addressing and the q command.

# same as: seq 23 45 | head -n5
seq 23 45 | sed '5q'

The = command will display the line numbers of matching lines.

# gives both line number and matching line
grep -n 'not' examples/sed/programming_quotes.txt

# gives only line number of matching line
sed -n '/not/=' examples/sed/programming_quotes.txt 

If needed, matching line can also be printed. But there will be a newline character between the matching line and line number.

sed -n '/off/{=; p}' examples/sed/programming_quotes.txt
echo '--------------------'
sed -n '/off/{p; =}' examples/sed/programming_quotes.txt