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====== SED - Introduction ======

===== Typical Usage =====

^Example Usage^Comment^
|cat filename<nowiki> | </nowiki>sed '10q'|Uses piped input.|
|sed '10q' filename|Same effect, avoids a useless "cat".|
|sed '10q' filename > newfile|Redirects output to disk.|

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===== Quoting Syntax =====

The preceding examples use single quotes **('...')** instead of double quotes **("...")** to enclose editing commands, since sed is typically used on a Unix platform.

Single quotes prevent the Unix shell from interpreting the dollar sign **($)** and back-quotes **(`...`)**, which are expanded by the shell if they are enclosed in double quotes.

Users of the **"csh"** shell and derivatives will also need to quote the exclamation mark **(!)** with the backslash **(i.e., \!)** to properly run the examples listed above, even within single quotes.

Versions of sed written for DOS invariably require double quotes **("...")** instead of single quotes to enclose editing commands.

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===== Use of '\t' in sed scripts =====

For clarity in this documentation, we have used the expression '\t' to indicate a tab character (0x09) in the scripts.

However, most versions of sed do not recognize the '\t' abbreviation, so when typing these scripts from the command line, you should press the TAB key instead.

<WRAP info>
**NOTE:**  '\t' is supported as a regular expression metacharacter in awk, perl, and HHsed, sedmod, and GNU sed v3.02.80.
</WRAP>


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===== Optimizing for speed =====

If execution speed needs to be increased (due to large input files or slow processors or hard disks), substitution will be executed more quickly if the "find" expression is specified before giving the "s/.../.../" instruction. 

Thus:

<code bash>
sed 's/foo/bar/g' filename         # standard replace command
sed '/foo/ s/foo/bar/g' filename   # executes more quickly
sed '/foo/ s//bar/g' filename      # shorthand sed syntax
</code>

On line selection or deletion in which you only need to output lines from the first part of the file, a "quit" command (q) in the script will drastically reduce processing time for large files. 

Thus:

<code bash>
sed -n '45,50p' filename           # print line nos. 45-50 of a file
sed -n '51q;45,50p' filename       # same, but executes much faster
</code>