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01-16-2008, 05:21 PM
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 alias substitution fails Hello All, Can anyone advise me regarding the following problem/bug? I have used an example below... On the command line......I set up the following... $alias test='/usr/bin/ls' $ $ export LIST="test /home/myhome" If I now type in $ set -x $ test /home/myhome + /usr/bin/ls /home/myhome .......... ..........ls ouput .......... But, $ $LIST + test /home/myhome $ Why in the second case even though the variable $LIST is expanded out does the shell not realise that 'test' is an alias? Is this a bug? Everything is still set okay but it doesn't substitute 'test'.      |
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01-16-2008, 05:21 PM
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| Re: alias substitution fails "J" <snakes_boi@yahoo.co.uk> wrote in message news:1068195165.588181@ernani.logica.co.uk... > Hello All, > Can anyone advise me regarding the following problem/bug? > I have used an example below... > > On the command line......I set up the following... > > $alias test='/usr/bin/ls' > $ > $ export LIST="test /home/myhome" .... > $ $LIST > + test /home/myhome > $ > > Why in the second case even though the variable $LIST is expanded out does > the shell not realise that 'test' is an alias? Is this a bug? Everything is > still set okay but it doesn't substitute 'test'. The shell is an interpreter that reads each line and pre-processes it before executing it. The preprocessing is done in the following order: - alias substitution - variable substitution - .... The shell reads "$LIST" and cannot find an alias. It then finds a variable with that name and substitutes "test". However, it does not go back to the first stage to check whether an alias has been used. To let the shell retrace it's steps you can use the eval command. eval $LIST will give you the result you wanted. Siem Korteweg __________________________________________________ _______ Siem Korteweg Open Challenge www.open-challenge.nl/scc/index.html Havenweg 24A telefoon: 0347 324 130 4131 NM Vianen fax : 0347 324 120 __________________________________________________ _______ |
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01-16-2008, 05:21 PM
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| Re: alias substitution fails Thanks, This works fine now - I'd forgotten that the shell did things in a certain order..... Cheers, Jon  "Siem Korteweg" <NOsiemkSPAM@open-challenge.nl> wrote in message news:3fab6e15$0$58714$e4fe514c@news.xs4all.nl... > > "J" <snakes_boi@yahoo.co.uk> wrote in message > news:1068195165.588181@ernani.logica.co.uk... > > Hello All, > > Can anyone advise me regarding the following problem/bug? > > I have used an example below... > > > > On the command line......I set up the following... > > > > $alias test='/usr/bin/ls' > > $ > > $ export LIST="test /home/myhome" > ... > > $ $LIST > > + test /home/myhome > > $ > > > > Why in the second case even though the variable $LIST is expanded out does > > the shell not realise that 'test' is an alias? Is this a bug? Everything > is > > still set okay but it doesn't substitute 'test'. > > The shell is an interpreter that reads each line and pre-processes it before > executing it. The preprocessing is done in the following order: > - alias substitution > - variable substitution > - .... > The shell reads "$LIST" and cannot find an alias. It then finds a variable > with that name and substitutes "test". However, it does not go back to the > first stage to check whether an alias has been used. To let the shell > retrace it's steps you can use the eval command. > > eval $LIST > > will give you the result you wanted. > > Siem Korteweg > __________________________________________________ _______ > Siem Korteweg > Open Challenge www.open-challenge.nl/scc/index.html > Havenweg 24A telefoon: 0347 324 130 > 4131 NM Vianen fax : 0347 324 120 > __________________________________________________ _______ > > |
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01-16-2008, 05:21 PM
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| Re: alias substitution fails "J" <snakes_boi@yahoo.co.uk> schreef in bericht news:1068210975.734412@ernani.logica.co.uk... > Thanks, > > This works fine now - I'd forgotten that the shell did things in a certain > order..... > > Cheers, > > Jon > "Siem Korteweg" <NOsiemkSPAM@open-challenge.nl> wrote in message > news:3fab6e15$0$58714$e4fe514c@news.xs4all.nl... > > > > "J" <snakes_boi@yahoo.co.uk> wrote in message > > news:1068195165.588181@ernani.logica.co.uk... > > > Hello All, > > > Can anyone advise me regarding the following problem/bug? > > > I have used an example below... > > > > > > On the command line......I set up the following... > > > > > > $alias test='/usr/bin/ls' > > > $ > > > $ export LIST="test /home/myhome" > > ... > > > $ $LIST > > > + test /home/myhome > > > $ > > > > > > Why in the second case even though the variable $LIST is expanded out > does > > > the shell not realise that 'test' is an alias? Is this a bug? Everything > > is > > > still set okay but it doesn't substitute 'test'. > > > > The shell is an interpreter that reads each line and pre-processes it > before > > executing it. The preprocessing is done in the following order: > > - alias substitution > > - variable substitution > > - .... > > The shell reads "$LIST" and cannot find an alias. It then finds a variable > > with that name and substitutes "test". However, it does not go back to the > > first stage to check whether an alias has been used. To let the shell > > retrace it's steps you can use the eval command. > > > > eval $LIST > > > > will give you the result you wanted. > > > > Siem Korteweg > > __________________________________________________ _______ > > Siem Korteweg > > Open Challenge www.open-challenge.nl/scc/index.html > > Havenweg 24A telefoon: 0347 324 130 > > 4131 NM Vianen fax : 0347 324 120 > > __________________________________________________ _______ > > > > > > Jon, Be carefull with using the name test for aliases, shell-scripts or programms. Do you realise what you do by using test? You overwrite the test command that is used in a lot of if statement of shell scripts. So some shell scripts may not work, if your alias would be put in a .profile of a user or /etc/profile. Regards, Jan Gerrit |
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01-16-2008, 05:22 PM
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| Re: alias substitution fails Ah yes - this is true indeed but this was done as example in the posting only rather than use the 'real' alias names etc - but thanks for the warning anyway! "Jan Gerrit Kootstra" <jan.gerrit@kootstra.org.uk> wrote in message news:3fabf29e$0$79334$5fc3050@dreader2.news.tiscal i.nl... > "J" <snakes_boi@yahoo.co.uk> schreef in bericht > news:1068210975.734412@ernani.logica.co.uk... > > Thanks, > > > > This works fine now - I'd forgotten that the shell did things in a certain > > order..... > > > > Cheers, > > > > Jon > > "Siem Korteweg" <NOsiemkSPAM@open-challenge.nl> wrote in message > > news:3fab6e15$0$58714$e4fe514c@news.xs4all.nl... > > > > > > "J" <snakes_boi@yahoo.co.uk> wrote in message > > > news:1068195165.588181@ernani.logica.co.uk... > > > > Hello All, > > > > Can anyone advise me regarding the following problem/bug? > > > > I have used an example below... > > > > > > > > On the command line......I set up the following... > > > > > > > > $alias test='/usr/bin/ls' > > > > $ > > > > $ export LIST="test /home/myhome" > > > ... > > > > $ $LIST > > > > + test /home/myhome > > > > $ > > > > > > > > Why in the second case even though the variable $LIST is expanded out > > does > > > > the shell not realise that 'test' is an alias? Is this a bug? > Everything > > > is > > > > still set okay but it doesn't substitute 'test'. > > > > > > The shell is an interpreter that reads each line and pre-processes it > > before > > > executing it. The preprocessing is done in the following order: > > > - alias substitution > > > - variable substitution > > > - .... > > > The shell reads "$LIST" and cannot find an alias. It then finds a > variable > > > with that name and substitutes "test". However, it does not go back to > the > > > first stage to check whether an alias has been used. To let the shell > > > retrace it's steps you can use the eval command. > > > > > > eval $LIST > > > > > > will give you the result you wanted. > > > > > > Siem Korteweg > > > __________________________________________________ _______ > > > Siem Korteweg > > > Open Challenge www.open-challenge.nl/scc/index.html > > > Havenweg 24A telefoon: 0347 324 130 > > > 4131 NM Vianen fax : 0347 324 120 > > > __________________________________________________ _______ > > > > > > > > > > > > Jon, > > > Be carefull with using the name test for aliases, shell-scripts or > programms. > > Do you realise what you do by using test? > > You overwrite the test command that is used in a lot of if statement of > shell scripts. > > So some shell scripts may not work, if your alias would be put in a ..profile > of a user or /etc/profile. > > > Regards, > > > Jan Gerrit > > |
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