SED

Table of contents
  1. SED
    1. Command Syntax
    2. Sed Scripting Flow
    3. other resources


sed stands for Stream Editor. It is very powerful tool to manipulate, filter, and transform text.

# SAMPLE
# the below employee database contains
# the following fields for every record:
# • Employee Id
# • Employee Name
# • Title


cat << "EOF" > employee.txt
101,John Doe,CEO
102,Jason Smith,IT Manager
103,Raj Reddy,Sysadmin
104,Anand Ram,Developer
105,Jane Miller,Sales Manager
EOF

vi employee.txt

Command Syntax

Basic sed syntax:

Sed reads one line at a time from the {input-file} and executes the {sed-commands} on that particular line.

sed [options] {sed-commands} {input-file}

Basic sed syntax for use with sed-command file:

sed [options] -f {sed-commands-in-a-file} {input-file}

cat << "EOF" > test-script.sed
/^root/ p
/^nobody/ p
EOF

sed -n -f test-script.sed /etc/passwd

Basic sed syntax using -e:

sed [options] -e {sed-command-1} -e {sed-command-2} {input-file}

sed -n -e '/^root/ p' -e '/^nobody/ p' /etc/passwd

Basic sed syntax using { }:

sed [options] '{
sed-command-1
sed-command-2
}' input-file

sed -n '{
/^root/ p
/^nobody/ p
}' /etc/passwd

basic sed alternative

sed -i -e 's|^that starts with|the replace|' filename.ext
sed -i -e 's|^#precedence ::ffff:0:0/96  100|precedence ::ffff:0:0/96  100|' /etc/gai.conf

Sed Scripting Flow

Sed will:

  • Read a line into the pattern space (an internal temporary sed buffer, where it places the line it reads from the input file).
  • Execute the sed command on the line in the sed pattern space. If there are more than one sed commands available, either via a sed script, -e options, or { }, it executes all the sed commands one by one in sequence on the line that is currently in the pattern space.
  • Print the line from the pattern space. After printing this line, the sed pattern space will be empty.
  • Repeat this again until the end of the input file is reached.

other resources