B.1. Sed

Sed is a non-interactive line editor. It receives text input, whether from stdin or from a file, performs certain operations on specified lines of the input, one line at a time, then outputs the result to stdout or to a file. Within a shell script, sed is usually one of several tool components in a pipe.

Sed determines which lines of its input that it will operate on from the address range passed to it. [1] Specify this address range either by line number or by a pattern to match. For example, 3d signals sed to delete line 3 of the input, and /windows/d tells sed that you want every line of the input containing a match to "windows" deleted.

Of all the operations in the sed toolkit, we will focus primarily on the three most commonly used ones. These are printing (to stdout), deletion, and substitution.

Unless the g (global) operator is appended to a substitute command, the substitution operates only on the first instance of a pattern match within each line.

From the command line and in a shell script, a sed operation may require quoting and certain options.

sed -e '/^$/d' $filename
# The -e option causes the next string to be interpreted as an editing instruction.
#  (If passing only a single instruction to "sed", the "-e" is optional.)
#  The "strong" quotes ('') protect the RE characters in the instruction
#+ from reinterpretation as special characters by the body of the script.
# (This reserves RE expansion of the instruction for sed.)
#
# Operates on the text contained in file $filename.

In certain cases, a sed editing command will not work with single quotes.

filename=file1.txt
pattern=BEGIN

  sed "/^$pattern/d" "$filename"  # Works as specified.
# sed '/^$pattern/d' "$filename"    has unexpected results.
#        In this instance, with strong quoting (' ... '),
#+      "$pattern" will not expand to "BEGIN".

Sed uses the -e option to specify that the following string is an instruction or set of instructions. If there is only a single instruction contained in the string, then this option may be omitted.

sed -n '/xzy/p' $filename
# The -n option tells sed to print only those lines matching the pattern.
# Otherwise all input lines would print.
# The -e option not necessary here since there is only a single editing instruction.

Substituting a zero-length string for another is equivalent to deleting that string within a line of input. This leaves the remainder of the line intact. Applying s/GUI// to the line

The most important parts of any application are its GUI and sound effects

The most important parts of any application are its  and sound effects

The backslash represents a newline as a substitution character. In this special case, the replacement expression continues on the next line.

s/^  */\
/g

An address range followed by one or more operations may require open and closed curly brackets, with appropriate newlines.

/[0-9A-Za-z]/,/^$/{
/^$/d
}

A quick way to double-space a text file is sed G filename.

For illustrative examples of sed within shell scripts, see:

1. Example 34-1

2. Example 34-2

3. Example 12-2

4. Example A-3

5. Example 12-12

6. Example 12-20

7. Example A-13

8. Example A-18

9. Example 12-24

10. Example 10-9

11. Example 12-33

12. Example A-2

13. Example 12-10

14. Example 12-8

15. Example A-11

16. Example 17-11

For a more extensive treatment of sed, check the appropriate references in the Bibliography.