topic Re: Parameter checking in scripts in Operating System - Linux

Parameter checking in scripts

PJJ — Tue, 02 May 2006 05:08:44 GMT

Hi,
I need to write a script which performes some action on files which are 4 weeks old (no parameters specified), unless another timeframe is specified (in parameter $1).
I can't figure out how to check if $1 is specified or not.
Any help appreciated.
Pete.

Re: Parameter checking in scripts

Peter Godron — Tue, 02 May 2006 05:16:27 GMT

Pete,
you could use the length of $1:
a=`expr length "$1"`
or
if [ -z "$1" ]

Re: Parameter checking in scripts

Hemmetter — Tue, 02 May 2006 05:19:25 GMT

Hi Peter

see:
man ksh(1) Section "Parameter Substitution."

<...>
${parameter:=word} If parameter is not set or is null, set it to
word; then substitute the value of the
parameter. Positional parameters cannot be
assigned in this way.
<...>
my be what you're searching for.

rgds
HGH

Re: Parameter checking in scripts

James R. Ferguson — Tue, 02 May 2006 05:20:12 GMT

Hi Pete:

One way is simply to check the number of arguments passed to the script:

# cat .mysh
#!/usr/bin/sh
if [ $# -eq 0 ]; then
echo "nothing passed"
else
echo "I passed $@"
fi

# ./mysh 123
I passed 123

# ./mysh
nothing passed

Regards!

...JRF...

Re: Parameter checking in scripts

Peter Nikitka — Tue, 02 May 2006 05:56:16 GMT

Hi,

set your script variable conditionally:

...
timespec=${1:-28}
...
find .. -mtime +$timespec ...

mfG Peter

Re: Parameter checking in scripts

PJJ — Tue, 02 May 2006 06:14:42 GMT

Thanks all for the quick replies.
James's solution seems elegant. But when I run this script with parameters it gives correct output. Running a 2nd time without parameters it gives the same output as the 1st run.
Somehow it seems to conserve the parameters???

Re: Parameter checking in scripts

PJJ — Tue, 02 May 2006 06:17:38 GMT

Sorry, I shpuld have supplied output:
$ . script.sh a b c
I passed a b c
$ . script.sh
I passed a b c

Re: Parameter checking in scripts

James R. Ferguson — Tue, 02 May 2006 06:35:28 GMT

Hi (again) Pete:

Well, that's quite impossible for the script to retain state between executions. I suspect that your terminal settings are to blame. I suspect that you ran the script with arguments; recalled the command line; and "backspaced" over the arguments. If the backspace truly didn't erase, then you would see the behavior you did.

Regards!

...JRF...

Re: Parameter checking in scripts

PJJ — Tue, 02 May 2006 06:41:38 GMT

Hi James,

I didn't recall... Is it the way I call the script?
$ ./script.sh
nothing passed
$ ./script.sh a b c
I passed a b c
$ ./script.sh
nothing passed
$ . script.sh
I passed a b c
$ . script.sh d e f
I passed d e f
$ . script.sh
I passed d e f

This last run not recalled!

Re: Parameter checking in scripts

Hein van den Heuvel — Tue, 02 May 2006 06:59:40 GMT

Peter,

Yes your are 'sourcing' the script instead of simply running it.
Is that deliberate? (why?)
What shell are you in while executing this?
Did you use the exact script as provided, or did you already augment it?
This does not reproduce for me.

From the Posix shell man page:
"The dot (.) special command, as in . file, reads the entire file before any commands are executed. Therefore, alias and unalias commands in the file will not apply to any functions defined in the file."

Groetjes,
Hein.

Re: Parameter checking in scripts

Peter Nikitka — Tue, 02 May 2006 07:00:49 GMT

Hi,

your error is using
. myscript

This is executed directly in the running shell.
Check this:
. myscript a b c

echo $1
a

You have to call the script 'in normal way':

myscript a b c

without leading '. '.

mfG Peter

Re: Parameter checking in scripts

James R. Ferguson — Tue, 02 May 2006 07:10:25 GMT

Hi Pete:

Both Hein and Peter N. are correct. A dot character followed by a space followed by the script name means *read* or 'source' the file.

This is used to bring environmental variables into your current environment (shell). For instance, do this:

# cat /tmp/env
#!/usr/bin/sh
export VAR1=jrf
export VAR2=hein
export VAR3=peter

Now, compare:

# /tmp/env
echo "${VAR1}, ${VAR2} ${VAR3}"
sh: VAR1: Parameter not set.

...to:

# . /tmp/env
echo "${VAR1}, ${VAR2} ${VAR3}"
jrf, hein peter

Regards!

...JRF...

Re: Parameter checking in scripts

Bill Hassell — Tue, 02 May 2006 08:19:19 GMT

As James points out, 'sourcing' a script means that the current shell remembers any variables that are previously set. So to allow for maximum flexibility, you should always test for the quantity of parameters passed to the script and ignore anything in the variables if the quantity is not correct (or in your case, preset the default values):

#!/usr/bin/sh
set -u

[ $# -eq 1 ] && VAR1=$1 || VAR1=4

echo "VAR1 is $VAR1"

In this case, the test is explicitly for one parameter so anything else (ie, no parameters, two parameters, etc) will leave VAR1=4, otherwise, VAR1 is assigned the first parameter on the command line. As with all interactive scripts, you should assume that monkeys are typing on the keyboard and the value for VAR1 should be tested for validity (ie, not a number, number too large or small, etc)

Re: Parameter checking in scripts

PJJ — Tue, 02 May 2006 08:35:55 GMT

I'm lost now, seeing no diff in your explanation.
Using /usr/bin/sh :
$ more /tmp/env
#!/usr/bin/sh
echo "${VAR1}, ${VAR2} ${VAR3}"

$ export VAR1=jrf
$ export VAR2=hein
$ export VAR3=peter

$ /tmp/env
jrf, hein peter
$ . /tmp/env
jrf, hein peter

Re: Parameter checking in scripts

Peter Nikitka — Tue, 02 May 2006 08:52:23 GMT

Hi,

JRF's solutions was (sorry) a bit misleading, because the variables VARi should NOT exported in the calling shell.

Let us try my way:
cat myscript
#!/usr/bin/sh
v1=${1:-jrf}
v2=${2:-hein}
v3=${3:-nik}
echo v1=$v1 v2=$v2 v3=$v3

Now try:
myscript
v1=jrf v2=hein v3=nik

myscript set your params
v1=set v2=your v3=params

. myscript set your params
v1=set v2=your v3=params

. myscript
v1=set v2=your v3=params

myscript
v1=jrf v2=hein v3=nik

set try this
. myscript
v1=try v2=this v3=nik

But you always get your desired result via
myscript Jong
v1=Jong v2=hein v3=nik

mfG Peter

Re: Parameter checking in scripts

James R. Ferguson — Tue, 02 May 2006 09:02:33 GMT

Hi (again):

THe 'echo' is run after either running the script or sourcing it. It wasn't part of the script I meant you to run. Eliminating the superflous (yes, misleading) 'export', what I showed was:

#!/usr/bin/sh
VAR1=jrf
VAR2=hein
VAR3=peter
exit

Now, compare:

# /tmp/env #...run the script
# echo "${VAR1}, ${VAR2} ${VAR3}"
sh: VAR1: Parameter not set.
# set #...to see the environmental variables
...to:

# . /tmp/env #...source the script
# echo "${VAR1}, ${VAR2} ${VAR3}"
jrf, hein peter
# set #...now shows VAR1, VAR2 and VAR3 too!

Regards!

...JRF...

Re: Parameter checking in scripts

PJJ — Wed, 03 May 2006 01:46:20 GMT

With all this help the 'global-' or 'localness' is clear now.

And for the parameterquestion this is elegant: v1=${1:-jrf}

Thanks all!
Pete

Re: Parameter checking in scripts

PJJ — Wed, 03 May 2006 01:50:25 GMT

Closed, thank to all.