Test command line argument

English Programming/Scripting
1

I want to write an easy script which should include a test: if the command line argument is introduced by the user, then the argument is used; if it’s not, then ignore it. I can’t do it because the stdout of echo is interpreted as stdin for ls. So how can I first test if $1 is typed in by the user without using a second argument $2?


#!/bin/bash
echo "Directory:"
if test -z "$1"
then ls $1
2

Why not try this and see if it works.

if  "$1" == "" ] ; then
  echo "there was no command line entry"
else
  echo "You entered the command line entry of: $1"
fi

OR

if  "$1" != "" ] ; then
  echo "You entered the command line entry of: $1"
else
  echo "there was no command line entry"
fi

Thank You,

3

Thank you, James.

4
  • To begin with, your construct is not complete: an if
    construct should end in a corresponding fi. - I never use test
    (or tthe alias ** ), but the internal bash notation* (a matter of taste). But I would guess that -z means: " if the length of the string equals 0 (zero)". Thus that means that you execute the ls $1* when $1 is empty. This will result in an ls of the working directory. - I do not understand what you mean by " because the stdout of echo is interpreted as stdin for ls.". Explain please.

My solution of this would be:

#!/bin/bash

echo "Directory:"
 -n ${1} ]] && ls ${1}

or written longer

#!/bin/bash

echo "Directory:"
if  -n ${1} ]] 
then      ls ${1}
fi

Where you may of course write $1 for* ${1}* (again personal whim).

5

You are very welcome. I love to work with bash scripts, so please let me know if you have any other questions.

Here are some other interesting values:

$@ or $* = all command line values
$0 = The name of the running script
shift = null <– $1 <– $2 <– $3 … <– $x and so forth where x is the last value entered.

Thank You,

6

@hcvv You’re right, I wrote it in a hurry :frowning: But thank you for the comments.

7

Actually I had one more question related to shift. I wrote a bash script for sending e-mails from command line with mutt, and it looks a bit too complicated:

#!/bin/bash
(echo -e $3; echo "--";  makesig.sh) |
(if test -z "$4"
then 
        mutt -s "${2}" ${1} 
        echo "Message successfully sent."
else
        dashes='--'
        mutt -s "${2}" -a ${4} $dashes ${1} 
                echo "Message successfully sent."
        fi)

I use the dashes variable to separate the attachment from the e-mail addresses, that’s a mutt requirement. Do you think I could use “shift” to simplify it a bit? Or, at least, if you could give me an example of how it works.

8

And thank you for doing:

… " because the stdout of echo is interpreted as stdin for ls.". Explain please.

9

I am no expert on mutt, but here is an example of using shift. Here is the bash script:

#!/bin/bash

#: Title       : testline
#: Date Created: Sun Jan 8 13:54:50 CST 2012
#: Last Edit   : Sun Jan 8 13:54:50 CST 2012
#: Author      : James D. McDaniel
#: Version     : 1.00
#: Description : 
#: Options     : 

while  "$1" != "" ] ; do
  echo $1
  shift
done

exit 0

# End Of Script

I save the script as testline in ~/bin and use the** chmod +x ~/bin/testline** to make it executable. If I run it as follows, here is what I get.

> testline one two three four five
one
two
three
four
five

The terminal command echo $1, never changes, but the shift command keeps moving over the values, until all $values are equal to null. The beauty is the ability to use the same while command, over and over, for all command line values. Again, refer to how I displayed what it does:

shift = null ← $1 ← $2 ← $3 … ← $x and so forth where x is the last value entered.

Thank You,

10

You can eliminate the whole if by using parameter substitution:

mutt -s "$2" ${4:+-a} $4 ${4:+$dashes} "$1"

See Parameter Substitution in the bash man page for how ${parameter:+word} works.

11

@hcvv I couldn’t explain, because the command I wrote was wrong.

@jdmcdaniel3 Ok, I get it now :slight_smile:

@ken_yap That saves a lot of writing, thanks!