sed Command

Purpose

A stream editor.

Syntax

sed [  -n ] Script [ File ... ]

sed [  -n ] [  -e Script ] ... [  -f ScriptFile ] ... [ File ... ]

Description

The sed command modifies lines from the specified File parameter according to an edit script and writes them to standard output. The sed command includes many features for selecting lines to be modified and making changes only to the selected lines.

The sed command uses two work spaces for holding the line being modified: the pattern space, where the selected line is held; and the hold space, where a line can be stored temporarily.

An edit script consists of individual subcommands, each one on a separate line. The general form of sed subcommands is the following:

[address-range] function[modifiers]

The sed command processes each input File parameter by reading an input line into a pattern space, applying all sed subcommands in sequence whose addresses select that line, and writing the pattern space to standard output. It then clears the pattern space and repeats this process for each line specified in the input File parameter. Some of the sed subcommands use a hold space to save all or part of the pattern space for subsequent retrieval.

When a command includes an address (either a line number or a search pattern), only the addressed line or lines are affected by the command. Otherwise, the command is applied to all lines.

An address is either a decimal line number, a $ (dollar sign), which addresses the last line of input, or a context address. A context address is a regular expression similar to those used in the ed command except for the following differences:

• You can select the character delimiter for patterns. The general form of the expression is:

  \?pattern?

  where ? (question mark) is a selectable character delimiter. You can select any character from the current locale except for the space or new-line character. The \ (backslash) character is required only for the first occurrence of the ? (question mark).

  The default form for the pattern is the following:

  /pattern/

  A \ (backslash) character is not necessary.

• The \n sequence matches a new-line character in the pattern space, except the terminating new-line character.
• A . (period) matches any character except a terminating new-line character. That is, unlike the ed command, which cannot match a new-line character in the middle of a line, the sed command can match a new-line character in the pattern space.

Certain commands called addressed commands allow you to specify one line or a range of lines to which the command should be applied. The following rules apply to addressed commands:

• A command line without an address selects every line.
• A command line with one address, expressed in context form, selects each line that matches the address.
• A command line with two addresses separated by commas selects the entire range from the first line that matches the first address through the next line that matches the second. (If the second address is a number less than or equal to the line number first selected, only one line is selected.) Thereafter, the process is repeated, looking again for the first address.

Flags

sed Subcommands

The sed command contains the following sed script subcommands. The number in parentheses preceding a subcommand indicates the maximum number of permissible addresses for the subcommand.

1. The Text variable accompanying the a\, c\, and i\ subcommands can continue onto more than one line, provided all lines but the last end with a \ (backslash) to quote the new-line character. Backslashes in text are treated like backslashes in the replacement string of an s command and can be used to protect initial blanks and tabs against the stripping that is done on every script line. The RFile and WFile variables must end the command line and must be preceded by exactly one blank. Each WFile variable is created before processing begins.
2. The sed command can process up to 999 subcommands in a pattern file.

Exit Status

This command returns the following exit values:

Examples

1. To perform a global change, enter:

   sed  "s/happy/enchanted/g" chap1

   This command sequence replaces each occurrence of the word happy found in the file chap1 with the word enchanted. The g character at the end of the s subcommand tells the sed command to make as many substitutions as possible on each line. Without the g character, the sed command replaces only the first occurrence of the word happy on a line.

   The sed command operates as a filter. It reads text from standard input or from the files named on the command line (chap1 in this example), modifies this text, and writes it to standard output. Unlike most editors, it does not replace the original file. This makes the sed command a powerful command when used in pipelines.

2. To use the sed command as a filter in a pipeline, enter:

   pr  chap2 | sed "s/Page *[0-9]*$/(&)/" | enq

   This command sequence encloses the page numbers in parentheses before printing the file chap2. The pr command puts a heading and page number at the top of each page, then the sed command puts the page numbers in parentheses, and the enq command prints the edited listing.

   The sed command pattern /Page *[0-9]*$/ matches page numbers that appear at the end of a line. The s subcommand changes this to (&), where the & stands for the page number that was matched.

3. To display selected lines of a file, enter:

   sed  -n  "/food/p" chap3

   The sed -n displays each line in the file chap3 that contains the word food. Normally, the sed command copies every line to standard output after it is edited. The -n flag stops the sed command from doing this. You then use subcommands like p to write specific parts of the text. Without the -n flag, this example displays all the lines in the file chap3, and it shows each line containing food twice.

4. To perform complex editing, enter:

   sed  -f  script.sed  chap4

   This command sequence creates a sed script file when you want to do anything complex. You can then test and modify your script before using it. You can also reuse your script to edit other files. Create the script file with an interactive text editor.

5. A sample sed script file:

   :join
   /\\$/{N
   s/\\\n//
   b join
   }

   This sed script joins each line that ends with a \ (backslash) to the line that follows it. First, the pattern /\\$/ selects a line that ends with a \ for the group of commands enclosed in {} (braces). The N subcommand then appends the next line, embedding a new-line character. The s/\\\n// deletes the \ and embedded new-line character. Finally, b join branches back to the label :join to check for a \ at the end of the newly joined line. Without the branch, the sed command writes the joined line and reads the next one before checking for a second \.

   Note:
   The N subcommand causes the sed command to stop immediately if there are no more lines of input (that is, if the N subcommand reads an end-of-file character). It does not copy the pattern space to standard output before stopping. This means that if the last line of the input ends with a \, it is not copied to the output.
6. To copy an existing file (oldfile) to a new file (newfile) and replace all occurrences of the testpattern text string with the contents of the $REPL shell variable, enter:

   cat oldfile | sed -e "s/testpattern/$REPL/g" | tee newfile

7. To replace all occurrences of A with a, B with b, C with c, and all occurrences of newlines with character Z in the input file, enter:

   $ sed -f command.file input.file

   where command.file is the script file and input.file is the input file.

      $cat command.file
      y/ABC\n/abcZ/
   

   Alternatively, the following command can also be executed for the same function:

   sed "y/ABC\n/abcZ/" input.file

Related Information

The awk command and the grep command.