ch45_30.htm

the unix power tools

></A
><A
CLASS="indexterm"
NAME="AUTOID-54744"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54747"
></A
>The Bourne shell
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>set</EM
> (<A
CLASS="linkend"
HREF="ch44_19.htm"
TITLE="The Bourne Shell set Command "
>44.19</A
>)</SPAN
>
command can be used to parse a single-line string and
store it in the
<SPAN
CLASS="link"
>command-line parameters (<A
CLASS="linkend"
HREF="ch44_15.htm"
TITLE="Handling Command-Line Arguments in Shell Scripts "
>44.15</A
>)</SPAN
>
<CODE
CLASS="literal"
>&quot;$@&quot;</CODE
>, <CODE
CLASS="literal"
>$*</CODE
>, <CODE
CLASS="literal"
>$1</CODE
>, <CODE
CLASS="literal"
>$2</CODE
>, and so on.
Then you can also loop through the words with a
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>for</EM
> loop (<A
CLASS="linkend"
HREF="ch44_16.htm"
TITLE="Handling Command-Line Arguments with a for Loop "
>44.16</A
>)</SPAN
>
and use everything else the shell has for dealing with command-line
parameters.
Also, you can set the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>IFS</EM
> variable (<A
CLASS="linkend"
HREF="ch35_21.htm"
TITLE="Using IFS to Split Strings "
>35.21</A
>)</SPAN
>
to control how the shell splits
the string.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-232-SECT-1.4"
>45.30.4 Using sed </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54765"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54768"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54771"
></A
>The UNIX
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>sed</EM
> (<A
CLASS="linkend"
HREF="ch34_24.htm"
TITLE="Quick Reference: sed "
>34.24</A
>)</SPAN
>
utility is good at parsing input that you may or may not be able to
split into words otherwise, at finding a single line of text in a group and
outputting it, and many other things.
In this example, I want to get the percentage-used of the filesystem
mounted on <EM
CLASS="emphasis"
>/home</EM
>.
That information is buried in the output of the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>df</EM
> (<A
CLASS="linkend"
HREF="ch24_09.htm"
TITLE="How Much Disk Space? "
>24.9</A
>)</SPAN
>
command.
On my system, <EM
CLASS="emphasis"
>df</EM
> output looks like:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>df</B
></CODE
>
Filesystem            kbytes    used   avail capacity  Mounted on
   ...
/dev/sd3c            1294854  914230  251139    78%    /work
/dev/sd4c             597759  534123    3861    99%    /home
   ...</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54784"
></A
>I want the number <EM
CLASS="emphasis"
>99</EM
> from the line ending with <EM
CLASS="emphasis"
>/home</EM
>.
The <EM
CLASS="emphasis"
>sed</EM
> address <CODE
CLASS="literal"
>/&nbsp;\/home$/</CODE
> will find that line (including a
space before the <EM
CLASS="emphasis"
>/home</EM
> makes sure the address doesn't match a
line ending with <EM
CLASS="emphasis"
>/something/home</EM
>).
The <EM
CLASS="emphasis"
>-n</EM
> option keeps <EM
CLASS="emphasis"
>sed</EM
> from printing any lines except
the line we ask it to print (with its <EM
CLASS="emphasis"
>p</EM
> command).
I know that the &quot;capacity&quot; is the only word on the line that ends
with a percent sign (<CODE
CLASS="literal"
>%</CODE
>).
A space after the first <CODE
CLASS="literal"
>.*</CODE
> makes sure that <CODE
CLASS="literal"
>.*</CODE
> doesn't
&quot;eat&quot; the first digit of the number that we want to match by <CODE
CLASS="literal"
>[0-9]</CODE
>.
The <EM
CLASS="emphasis"
>sed</EM
>
<SPAN
CLASS="link"
>escaped-parenthesis operators (<A
CLASS="linkend"
HREF="ch34_10.htm"
TITLE="Referencing Portions of a Search String "
>34.10</A
>)</SPAN
>
grab that number.
Here goes:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>usage=`df | sed -n '/ \/home$/s/.* \([0-9][0-9]*\)%.*/\1/p'`</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54805"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54808"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54811"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54814"
></A
>Combining <EM
CLASS="emphasis"
>sed</EM
> with
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>eval</EM
> (<A
CLASS="linkend"
HREF="ch08_10.htm"
TITLE="eval: When You Need Another Chance "
>8.10</A
>)</SPAN
>
lets you set several shell variables
at once from parts of the same line.
Here's a command line that sets two shell variables from the <EM
CLASS="emphasis"
>df</EM
>
output:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>eval `df |
sed -n '/ \/home$/s/^[^ ]*  *\([0-9]*\)  *\([0-9]*\).*/kb=\1 u=\2/p'`</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>The left-hand side of that substitution command has a regular expression
that uses <EM
CLASS="emphasis"
>sed</EM
>'s escaped parenthesis operators.
They grab the &quot;kbytes&quot; and &quot;used&quot; columns from the <EM
CLASS="emphasis"
>df</EM
>
output.
The right-hand side outputs the two <EM
CLASS="emphasis"
>df</EM
> values with Bourne shell
variable-assignment

commands to set the <EM
CLASS="emphasis"
>kb</EM
> and <EM
CLASS="emphasis"
>u</EM
> variables.
After <EM
CLASS="emphasis"
>sed</EM
> finishes, the resulting command line looks like this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>eval kb=597759 u=534123</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Now <CODE
CLASS="literal"
>$kb</CODE
> will give you <EM
CLASS="emphasis"
>597759</EM
>
and <CODE
CLASS="literal"
>$u</CODE
> contains <EM
CLASS="emphasis"
>534123</EM
>.</P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
></DIV
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