If else with one variable


 
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# 8  
Old 09-23-2014
Quote:
Originally Posted by sea
Could be shorter..
Code:
read -p "Who are you: " line
[ -z $line ] && line=kim
echo "Hi $line"


Hope this helps
Although the test [ -z $line ] will work as long as no more than one word was typed in; it isn't doing what most people expect.

If an empty line is entered, it will expand to [ -z ] which will yield TRUE because "-z" is not an empty string (not because $line expands to an empty string).

If a single word is entered (and it does not contain any pattern matching characters that would match more than one file in the current directory), then it will expand to [ -z word ] which will evaluate to TRUE as expected.

If more than one word is entered or any pattern matching characters in a single word expand to a list of more than one filename, you will either get a syntax error or some random result depending on what the 1st word is and, if there are more than two words (after pattern matching), it will also depend on the subsequent words.

To use this form reliably, the line:
Code:
[ -z $line ] && line=kim

should be changed to:
Code:
[ -z "$line" ] && line=kim

# 9  
Old 09-23-2014
I'm a bit confused now...

Quote:
Originally Posted by Don
If an empty line is entered, it will expand to [ -z ] which will yield TRUE because "-z" is not an empty string (not because $line expands to an empty string).

If a single word is entered (and it does not contain any pattern matching characters that would match more than one file in the current directory), then it will expand to [ -z word ] which will evaluate to TRUE as expected.
Now i belived -z is testing if argument is empty.
Thus, expecting that an empty line (variable in general) returns true.

Code:
 ~ $ line=" "
+ ~ $ [ -z "$line" ] && echo empty
 ~ $ [ -z $line ] && echo empty
:( ~ $

However, i got the 'multiple expanding part'...
Code:
 ~ $ line=$(ls)
:) ~ $[ -z $line ] && echo empty
bash: [: too many arguments
2 ~ $

So in conclusion, the quotes around make it less error prone, but it doesnt matter whether or not the variable contains spacechar.
Truely a standards guy you are, me gotta get used again to do quotes around there again Smilie

Thank you Don.
# 10  
Old 09-23-2014
Thank you every one for sharing and helping , all codes which you provided to me working fine , now I'am confused with which one I go with. I like the one provided by sea because it is the shorter one. Thank you Don for the info
# 11  
Old 09-23-2014
Quote:
Originally Posted by samsan
Thank you every one for sharing and helping , all codes which you provided to me working fine , now I'am confused with which one I go with. I like the one provided by sea because it is the shorter one. Thank you Don for the info
As long as you only use bash as your shell, the code sea suggested (with the expansion of $line inside double quotes) will do what you want:
Code:
#!/bin/bash
read -p'Who are you: ' line
[ -z "$line" ] && line=kim
echo "Hi $line"

The -p option to the read shell built-in is bash specific. The read -p option in ksh means something very different (read from a co-process), and in many other shells it would just be rejected as an unknown option. The equivalent Korn shell-specific code would be:
Code:
#!/bin/ksh
read line?'Who are you: '
[ -z "$line" ] && line=kim
echo "Hi $line"

If you only want to use constructs that will work with any shell that is based on Bourne shell syntax (which will include the default shell on any Linux or UNIX system), try:
Code:
#!/bin/sh
printf 'Who are you: '
read line
[ -z "$line" ] && line=kim
echo "Hi $line"

# 12  
Old 09-23-2014
Quote:
Originally Posted by sea
Could be shorter..
Code:
read -p "Who are you: " line
[ -z "$line" ] && line=kim
echo "Hi $line"

Could be even shorter:
Code:
read -p "Who are you: " line
echo "Hi ${line:=Kim}"

Is this sort of parameter expansion a ksh speciality? I wonder why you prefer those awkward if/else constructs...?
# 13  
Old 09-23-2014
Quote:
Originally Posted by Cochise
Could be even shorter:
Code:
read -p "Who are you: " line
echo "Hi ${line:=Kim}"

Is this sort of parameter expansion a ksh speciality? I wonder why you prefer those awkward if/else constructs...?
No, ${line:=Kim} is not ksh-specific. It is supported by any shell that meets the POSIX requirements for shell parameter expansions. It will not, however, work with a pure Bourne shell (such as /bin/sh on Solaris systems).

Note, however, that as I said before, read -p is bash-specific and will not work with ksh.

Last edited by Don Cragun; 09-23-2014 at 03:06 PM.. Reason: Add note.
# 14  
Old 09-24-2014
Thanks for pointing this out.

To make this complete, the ksh variant would be:
Code:
read line?"Who are you: "
echo "Hi ${line:=Kim}"

Are programmers still paid for number of lines of code?
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