Syntax

sed '[address-range|pattern-range] s/original-string/replacement-string/[substitute-flags]' inputfile

Replace all occurrences of Manager with Director:

sed 's/Manager/Director/' employee.txt

Replace Manager with Director only on lines that contain the keyword 'Sales':

sed '/Sales/s/Manager/Director/' employee.txt

Global Flag (-g)

Without flag -g, only the 1st occurrence of the {original-string} on each line will be replaced.

$ sed 's/a/A/' employee.txt
101, John Doe, CEO
102, J*A*son Smith, IT Manager
103, R*A*j Reddy, Sysadmin
104, An*A*nd Ram, Developer
105, J*A*ne Miller, Sales Manager

With -g, all occurrences of lower case a with upper case A:

$ sed 's/a/A/g' employee.txt
101, John Doe, CEO
102, J*A*son Smith, IT M*A*n*A*ger
103, R*A*j Reddy, Sys*A*dmin
104, An*A*nd Ram, Developer
105, J*A*ne Miller, S*A*les M*A*n*A*ger

Number Flag (1, 2, 3.. flag)

Only the n-th instance of original-string will trigger the substitution.

n can be anything from 1 to 512

Following command will only replace the 2nd of occurrence of 'a' to 'A'.

sed 's/a/A/2' employee.txt

Print Flag (-p)

The sed substitute flag p stands for print. When the substitution is successful, it prints the changed line.

Print only the line that was changed by the substitute command, -n flag to silent the "read" print.

sed -n 's/John/Johnny/p' employee.txt

Change the 2nd instance of “locate” to “find” and print the result:

$ sed -n 's/locate/find/2p' substitute-locate.txt
locate command is used to find files
locate command uses database to find files

Write Flag (-w)

Instead of printing to standard output, -w will write to a file.

sed 's/locate/find/2w output.txt' substitue-locate.txt

Ignore Case Flag (-i)

With -i flag, pattern matching will ignore case.

sed 's/john/Johnny/i' employee.txt

Execute Flag (-e)

-e flag stand for execute. Using the sed -e flag, you can execute whatever is available in the pattern space as a shell command, and the output will be returned to the pattern space.

This is available only in the GNU sed.

Conent in the file:

$ cat files.txt
/etc/passwd
/etc/group

s/^/{text}/ will add text to the start of every line:

$ sed 's/^/ls -l /' file.txt
ls -l /etc/passwd
ls -l /etc/group

Add the command and execute the command:

$ sed 's/^/ls -l /e' files.txt
-rw-r--r-- 1 root root 1547 Oct 27 08:11 /etc/passwd
-rw-r--r-- 1 root root 651 Oct 27 08:11 /etc/group

Combine Sed Substitution Flags

sed -n 's/Manager/Director/gipw output.txt' employee.txt

Sed Substitution Delimiter

If the match pattern contains /, we can use delimiter to differenciate the slash:

sed 's|/usr/local/bin|/usr/bin|' path.txt
sed 's^/usr/local/bin^/usr/bin^' path.txt
sed 's@/usr/local/bin@/usr/bin@' path.txt
sed 's!/usr/local/bin!/usr/bin!' path.txt 

Multiple Sed Commands will be executed as a CHAIN

$ sed '{
s/Developer/IT Manager/
s/Manager/Director/
}' employee.txt
101,John Doe,CEO
102,Jason Smith,IT Director
103,Raj Reddy,Sysadmin
104,Anand Ram,IT Director
105,Jane Miller,Sales Director

Get Matched Pattern, &

$ sed 's/^[0-9][0-9][0-9]/[&]/g' employee.txt
[101],John Doe,CEO
[102],Jason Smith,IT Manager
[103],Raj Reddy,Sysadmin
[104],Anand Ram,Developer
[105],Jane Miller,Sales Manager

Substitution Grouping: \( and \)

In the following example:

• Regular expression \([^,]*\) matches the string up to the 1st comma.
• \1 in the replacement-string replaces the first matched group.
• g is the global substitute flag.

$ sed 's/\([^,]*\).*/\1/g' employee.txt
101
102
103
104
105

This sed example displays only the first field from the /etc/passwd file, i.e. it displays only the username:

sed 's/\([^:]*\).*/\1/' /etc/passwd

The following example encloses the 1st letter in every word inside (), if the 1st character is upper case.

$ echo "The Geek Stuff" | sed 's/\(\b[A-Z]\)/\(\1\)/g'
(T)he (G)eek (S)tuff

Multiple pairs of \( and \) will match multiple groups. Use \1, \2 etc to get those groups.

GNU Sed Only Replacement String Flags \l. \L, \u, \U

These flags are used in {replacement-string}

When you specify \l in the replacement-string part, it treats the character that immediately follows \l as lower case. You already know the following simple example will change John to JOHNNY.

sed 's/John/JOHNNY/' employee.txt

The following example contains \l before H in the replacement-string (i.e. JO\lHNNY). This will change only the character h in JOHNNY to lower case.

$ sed -n 's/John/JO\lHNNY/p' employee.txt
101,JOhNNY Doe,CEO 

When you specify \L in the replacement-string part, it treats the rest of the characters as lower case.

$ sed -n 's/John/JO\LHNNY/p' employee.txt
101,JOhnny Doe,CEO 

\u and \U are for upper case.

\E should be used conjunction with \L and \U, to end the replacements.

These flags are useful because these can be put before grouping.

$ sed 's/\([^,]*\),\([^,]*\),\(.*\).*/\U\2\E,\1,\L\3/g'
employee.txt
JOHN DOE,101,ceo
JASON SMITH,102,it manager
RAJ REDDY,103,sysadmin
ANAND RAM,104,developer
JANE MILLER,105,sales manager