A Sed and Awk Micro-Primer

Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# Appendix C. A Sed and Awk Micro-Primer

This is a very brief introduction to the sed and awk text processing utilities. We will deal with only a few basic commands here, but that will suffice for understanding simple sed and awk constructs within shell scripts.

sed: a non-interactive text file editor

awk: a field-oriented pattern processing language with a C-style syntax

For all their differences, the two utilities share a similar invocation syntax, use regular expressions , read input by default from `stdin`, and output to `stdout`. These are well-behaved UNIX tools, and they work together well. The output from one can be piped to the other, and their combined capabilities give shell scripts some of the power of Perl.

| One important difference between the utilities is that while shell scripts can easily pass arguments to sed, it is more cumbersome for awk (see Example 36-5 and Example 28-2).

# C.1. Sed

Sed is a non-interactive [1] stream 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. [2] 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.

Table C-1. Basic sed operators

Operator | Name | Effect
--- | --- | ---
`[address-range]/p` | print | Print [specified address range]
`[address-range]/d` | delete | Delete [specified address range]
`s/pattern1/pattern2/` | substitute | Substitute pattern2 for first instance of pattern1 in a line
`[address-range]/s/pattern1/pattern2/` | substitute | Substitute pattern2 for first instance of pattern1 in a line, over `address-range`
`[address-range]/y/pattern1/pattern2/` | transform | replace any character in pattern1 with the corresponding character in pattern2, over `address-range` (equivalent of tr)
`[address] i pattern Filename` | insert | Insert pattern at address indicated in file Filename. Usually used with `-i` `in-place` option.
`g` | global | Operate on every pattern match within each matched line of input

| 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.

       1 sed -e '/^$/d' $filename
       2 # The -e option causes the next string to be interpreted as an editing instruction.
       3 #  (If passing only a single instruction to sed, the "-e" is optional.)
       4 #  The "strong" quotes ('') protect the RE characters in the instruction
       5 #+ from reinterpretation as special characters by the body of the script.
       6 # (This reserves RE expansion of the instruction for sed.)
       7 #
       8 # Operates on the text contained in file $filename.

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

       1 filename=file1.txt
       2 pattern=BEGIN
       3
       4   sed "/^$pattern/d" "$filename"  # Works as specified.
       5 # sed '/^$pattern/d' "$filename"    has unexpected results.
       6 #        In this instance, with strong quoting (' ... '),
       7 #+      "$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 may be omitted.

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

Table C-2. Examples of sed operators

Notation | Effect
--- | ---
`8d` | Delete 8th line of input.
`/^$/d` | Delete all blank lines.
`1,/^$/d` | Delete from beginning of input up to, and including first blank line.
`/Jones/p` | Print only lines containing "Jones" (with -n option).
`s/Windows/Linux/` | Substitute "Linux" for first instance of "Windows" found in each input line.
`s/BSOD/stability/g` | Substitute "stability" for every instance of "BSOD" found in each input line.
`s/ *$//` | Delete all spaces at the end of every line.
`s/00*/0/g` | Compress all consecutive sequences of zeroes into a single zero.
`echo "Working on it." | sed -e '1i How far are you along?'` | Prints "How far are you along?" as first line, "Working on it" as second.
`5i 'Linux is great.' file.txt` | Inserts 'Linux is great.' at line 5 of the file file.txt.
`/GUI/d` | Delete all lines containing "GUI".
`s/GUI//g` | Delete all instances of "GUI", leaving the remainder of each line intact.

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

results in

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

A backslash forces the sed replacement command to continue on to the next line. This has the effect of using the newline at the end of the first line as the replacement string.

       1 s/^  */\
       2 /g

This substitution replaces line-beginning spaces with a newline. The net result is to replace paragraph indents with a blank line between paragraphs.

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

       1 /[0-9A-Za-z]/,/^$/{
       2 /^$/d
       3 }

This deletes only the first of each set of consecutive blank lines. That might be useful for single-spacing a text file, but retaining the blank line(s) between paragraphs.

| The usual delimiter that sed uses is /. However, sed allows other delimiters, such as %. This is useful when / is part of a replacement string, as in a file pathname. See Example 11-10 and Example 16-32.

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

For illustrative examples of sed within shell scripts, see:

1. Example 36-1

2. Example 36-2

3. Example 16-3

4. Example A-2

5. Example 16-17

6. Example 16-27

7. Example A-12

8. Example A-16

9. Example A-17

10. Example 16-32

11. Example 11-10

12. Example 16-48

13. Example A-1

14. Example 16-14

15. Example 16-12

16. Example A-10

17. Example 19-12

18. Example 16-19

19. Example A-29

20. Example A-31

21. Example A-24

22. Example A-43

23. Example A-55

For a more extensive treatment of sed, refer to the pertinent references in the Bibliography.

### Notes

[1] | Sed executes without user intervention.
[2] | If no address range is specified, the default is all lines.

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