3.8 Commands Specific to GNU sed

These commands are specific to GNU sed, so you must use them with care and only when you are sure that hindering portability is not evil. They allow you to check for GNU sed extensions or to do tasks that are required quite often, yet are unsupported by standard seds.

e [command]

This command allows one to pipe input from a shell command into pattern space. Without parameters, the e command executes the command that is found in pattern space and replaces the pattern space with the output; a trailing newline is suppressed.

If a parameter is specified, instead, the e command interprets it as a command and sends its output to the output stream. The command can run across multiple lines, all but the last ending with a back-slash.

In both cases, the results are undefined if the command to be executed contains a NUL character.

Note that, unlike the r command, the output of the command will be printed immediately; the r command instead delays the output to the end of the current cycle.

F

Print out the file name of the current input file (with a trailing newline).

L n

This GNU sed extension fills and joins lines in pattern space to produce output lines of (at most) n characters, like fmt does; if n is omitted, the default as specified on the command line is used. This command is considered a failed experiment and unless there is enough request (which seems unlikely) will be removed in future versions.

Q [exit-code]

This command only accepts a single address.

This command is the same as q, but will not print the contents of pattern space. Like q, it provides the ability to return an exit code to the caller.

This command can be useful because the only alternative ways to accomplish this apparently trivial function are to use the ‘-n’ option (which can unnecessarily complicate your script) or resorting to the following snippet, which wastes time by reading the whole file without any visible effect:

:eat
$d       Quit silently on the last line
N        Read another line, silently
g        Overwrite pattern space each time to save memory
b eat

R filename

Queue a line of filename to be read and inserted into the output stream at the end of the current cycle, or when the next input line is read. Note that if filename cannot be read, or if its end is reached, no line is appended, without any error indication.

As with the r command, the special value ‘/dev/stdin’ is supported for the file name, which reads a line from the standard input.

T label

Branch to label only if there have been no successful substitutions since the last input line was read or conditional branch was taken. The label may be omitted, in which case the next cycle is started.

v version

This command does nothing, but makes sed fail if GNU sed extensions are not supported, simply because other versions of sed do not implement it. In addition, you can specify the version of sed that your script requires, such as 4.0.5. The default is 4.0 because that is the first version that implemented this command.

This command enables all GNU extensions even if POSIXLY_CORRECT is set in the environment.

W filename

Write to the given filename the portion of the pattern space up to the first newline. Everything said under the w command about file handling holds here too.

z

This command empties the content of pattern space. It is usually the same as ‘s/.*//’, but is more efficient and works in the presence of invalid multibyte sequences in the input stream. POSIX mandates that such sequences are not matched by ‘.’, so that there is no portable way to clear sed’s buffers in the middle of the script in most multibyte locales (including UTF-8 locales).

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