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Previous: 44.18 Standard Command-Line Parsing Chapter 44
Shell Programming for the Uninitiated
Next: 44.20 test: Testing Files and Strings
 

44.19 The Bourne Shell set Command

[Most of this article, except IFS and -- , also applies to the C shell. -JP]

The Bourne shell command line can have options like -e (exit if any command returns non-zero status). It can also have other arguments; these are passed to shell scripts. You can set new command-line parameters while you're typing interactive commands (at a shell prompt) or in a shell script.

To reset the command-line parameters, just type set followed by the new parameters. So, for example, to ask the shell to show expanded versions of command lines after you type them, set the -v (verbose) option ( 8.17 ) :

$ 

set -v


$ 

mail $group1 < message


mail andy ellen heather steve wilma < message
$ 

mail $group2 < message


mail jpeek@jpeek.com randy@xyz.edu yori@mongo.medfly.com < message
$ 

set +v

Typing set +v cancels the v option on many Bourne shells.

You can put filenames or any other strings in the command-line parameters interactively or from a shell script. That's handy for storing and parsing the output of a UNIX command with backquotes ( 9.16 ) . For example, you can get a list of all logged-in users from the parameters $1 , $2 , and so on. Use users if your system has it. Otherwise, use who ( 51.4 ) - and cut ( 35.14 ) to strip off everything but the usernames:




for
 



$ 

set `users`


$ 

set `who | cut -c1-8`


$ 

for u


> 

do


> 
...do something with each user ($u)...

> 

done

You can save the original parameters in another variable and reset them later:

oldparms="$*"
set 

something new


    
...use new settings...

set $oldparms

If the first parameter you set starts with a dash, like -e , the shell will treat it as its own option instead of as a string to put into the command-line parameters. To avoid this, use -- (two dashes) as the first argument to set . In this example, $1 gets -e , and the filenames expanded from the wildcard pattern go into $2 , $3 , etc.:

set -- -e file*

Because the shell parses and scans the new parameters before it stores them, wildcards ( 15.2 ) and other special characters ( 8.19 ) will be interpreted - watch your quoting ( 8.14 ) . You can take advantage of this to parse lines of text into pieces that aren't separated with the usual spaces and TABs - for instance, a line from a database with colon-separated fields - by setting the IFS ( 35.21 ) variable before the set command.

If you want to save any special quoting on the original command line, be careful; the quoting will be lost unless you're clever. For example, if $1 used to be John Smith , it'll be split after it's restored: $1 will have John and $2 will be Smith . A better solution might be to use a subshell ( 13.7 ) for the part of the script where you need to reset the command-line parameters:

# reset command-line parameters during subshell only:
(set 

some new parameters


   ...
do something with new parameters
...
)
# original parameters aren't affected from here on...

One last note: set won't set $0 , the name of the script file.

- JP


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