sed command

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