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Old 31st December 2015
J65nko J65nko is offline
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Default Comparing strings with the shell

From test(1):
Code:
 s1 = s2
    True if the strings s1 and s2 are identical.
s1 != s2
    True if the strings s1 and s2 are not identical.
s1 < s2
    True if string s1 comes before s2 based on the ASCII value of their characters.
s1 > s2
    True if string s1 comes after s2 based on the ASCII value of their characters.
So here we go .....
Code:
$ if [ 'aaa' < 'aab' ] ; then echo stringwise smaller ; fi
ksh: cannot open aab: No such file or directory
Ok the '<' in the shell means to take the contents of file 'aab' and use it as input. So we escape the '<' ...
Code:
$ if [ 'aaa' \< 'aab' ] ; then echo stringwise smaller ; fi
ksh: [: <: unexpected operator/operand
Yes, this is OpenBSD and the standard shell is ksh(1) .... Obviously the escaping does not work here.

Then I remember that shells have built-in's commands that sometimes behave differently than the ones residing in /bin/ for example. Checking the shell test command From ksh(1):
Code:
test expression
[ expression ]
        test evaluates the expression and returns zero status if true, 1
        if false, or greater than 1 if there was an error.  It is
        normally used as the condition command of if and while
        statements. 
[snip]
        -n string          string is not empty.

        -z string          string is empty.

        string = string    Strings are equal.

        string == string   Strings are equal.

        string != string   Strings are not equal

        number -eq number  Numbers compare equal.
Notice that the string comparison operators < and > are missing.
The /bin/test executable instead of the ksh one works with escaping the '<':
Code:
$ if  /bin/test "aap" \< "aap2" ; then echo Smaller! ; fi
Smaller!
If you forget to escape the shell output redirector ">" then ....
Code:
$ if  /bin/test "aap2" > "aap" ; then echo Larger! ; fi   
Larger!
$ if  /bin/test "aap" > "aap" ; then echo Larger! ; fi   
Larger!
$ ls -l aap
-rw-r--r--  1 adriaan  adriaan  0 Dec 31 02:05 aap
With an escape it works:
Code:
$  if  /bin/test "aap" \> "aap" ; then echo Larger! ; fi
$
Yes, this has cost me some headache to figure out .....
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Old 31st December 2015
J65nko J65nko is offline
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Default

Two demo shell scripts that show the problem as well as the solution. The problematic one stringwise_less.sh:
Code:
 #!/bin/sh

#set -o posix

if [ "X$1" = "X" ]; then
   prev_date="20151229"
else
   prev_date="$1"
fi

cur_date=$(date "+%Y-%m-%d")
cur_date=$(date "+%Y%m%d")

show() {
   echo
   echo "Previous date : [$prev_date]"
   echo "Current date  : [$cur_date]"
   cat <<END
-----file list-------------------------------------
$(ls -ltr)
---------------------------------------------------
END
}

if [ "$prev_date" < "$cur_date" ]; then
   show
   echo "Previous date [$prev_date] < current date [$cur_date]"
fi
 
if [ "$prev_date" = "$cur_date" ]; then
   show
   echo "Previous date [$prev_date] = current date [$cur_date]"
fi

 
if [ "$prev_date" > "$cur_date" ]; then
   show
   echo "Previous date [$prev_date] > current date [$cur_date]"
fi
Running the script:
Code:
$ stringwise_less.sh 20141231                              
./stringwise_less.sh[28]: cannot open 20151231: No such file or directory

Previous date : [20141231]
Current date  : [20151231]
-----file list-------------------------------------
total 8
-rwxr--r--  1 adriaan  adriaan  739 Dec 30 23:59 stringwise_less.sh
-rwxr--r--  1 adriaan  adriaan  764 Dec 31 01:24 stringwise_less2.sh
-rw-r--r--  1 adriaan  adriaan    0 Dec 31 02:27 20151231
---------------------------------------------------
Previous date [20141231] > current date [20151231]
$ rm 2015*
$ stringwise_less.sh 20191231 
./stringwise_less.sh[28]: cannot open 20151231: No such file or directory

Previous date : [20191231]
Current date  : [20151231]
-----file list-------------------------------------
total 8
-rwxr--r--  1 adriaan  adriaan  739 Dec 30 23:59 stringwise_less.sh
-rwxr--r--  1 adriaan  adriaan  764 Dec 31 01:24 stringwise_less2.sh
-rw-r--r--  1 adriaan  adriaan    0 Dec 31 02:29 20151231
---------------------------------------------------
Previous date [20191231] > current date [20151231]
$ date           
Thu Dec 31 02:31:58 CET 2015
$ stringwise_less.sh 20151231                              
./stringwise_less.sh[28]: cannot open 20151231: No such file or directory

Previous date : [20151231]
Current date  : [20151231]
-----file list-------------------------------------
total 8
-rwxr--r--  1 adriaan  adriaan  739 Dec 30 23:59 stringwise_less.sh
-rwxr--r--  1 adriaan  adriaan  764 Dec 31 01:24 stringwise_less2.sh
---------------------------------------------------
Previous date [20151231] = current date [20151231]

Previous date : [20151231]
Current date  : [20151231]
-----file list-------------------------------------
total 8
-rwxr--r--  1 adriaan  adriaan  739 Dec 30 23:59 stringwise_less.sh
-rwxr--r--  1 adriaan  adriaan  764 Dec 31 01:24 stringwise_less2.sh
-rw-r--r--  1 adriaan  adriaan    0 Dec 31 02:32 20151231
---------------------------------------------------
Previous date [20151231] > current date [20151231]
$
The second one, named stringwise_less2.sh solves the issue by using /bin/test. A grep(1) of the changes:
Code:
$ grep test stringwise_less2.sh  
if /bin/test "$prev_date" \< "$cur_date" ; then
if /bin/test  "$prev_date" = "$cur_date" ; then
if /bin/test  "$prev_date" \> "$cur_date" ; then
PS: A warning note lifted from the OpenBSD ksh man page:
Code:
Note: A common mistake is to use ``if [ $foo = bar ]'' which
fails if parameter ``foo'' is NULL or unset, if it has embedded
spaces (i.e. IFS characters), or if it is a unary operator like
`!' or `-n'.  Use tests like ``if [ "X$foo" = Xbar ]'' instead.
Attached Files
File Type: sh stringwise_less.sh (739 Bytes, 10 views)
File Type: sh stringwise_less2.sh (764 Bytes, 11 views)
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Last edited by J65nko; 31st December 2015 at 01:54 AM.
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Old 1st January 2016
drl's Avatar
drl drl is offline
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Default

Hi.

The ksh Version AJM 93u+ 2012-08-01 allows the use of [[ ... ]] which allows <,> in conditionals.

Sorry, I don't have OpenBSD accessible to me right now.

Best wishes ... cheers, drl
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Old 1st January 2016
J65nko J65nko is offline
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Yes, I know that the Korn shell [[ .... ]] constructs allows it. On OpenBSD it also does and you don't need to quote it.
Code:
$ if [[ 'zzz' > 'aaa' ]]; then echo Greater ; fi   
Greater
$ ls -l aaa
ls: aaa: No such file or directory
$ if [[ 'zzz' \> 'aaa' ]]; then echo Greater ; fi
ksh: syntax error: `"zzz"' missing expression operator
$
But that is not portable for /bin/sh of for example FreeBSD:
Code:
$ echo $SHELL
/bin/sh
$ uname
FreeBSD
$ if [[ 'zzz' > 'aaa' ]]; then echo Greater ; fi 
[[: not found
$ ls -l aaa
-rw-r--r--  1 j65nko  j65nko  0 Jan  1 16:39 aaa
$ if [ 'zzz' > 'aaa' ]; then echo Greater ; fi
Greater
$ ls -l aaa
-rw-r--r--  1 j65nko  j65nko  0 Jan  1 16:42 aaa
$ rm aaa
$ if [ 'zzz' \> 'aaa' ]; then echo Greater ; fi
Greater
$ ls -l aaa
ls: aaa: No such file or directory
$
But as you see, there the single [ ...... ] works. As stated in the FreeBSD sh(1) man page under the heading Built-in Commands:

Code:
[	     A built-in	equivalent of test(1).
OpenBSD's ksh which can be run as sh lacks the '<' and '>' string comparison of OpenBSD's /bin/[. This difference is documented in the manual pages of that shell, but I just did not realize that
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Old 2nd January 2016
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FYI, the < and > operators are not in POSIX
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Old 2nd January 2016
J65nko J65nko is offline
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I tried to enable POSIX mode but disabled that with #set -o posix (see the second post) when I found the POSIX documents.
While missing from POSIX test(1), these < and > operators are in POSIX expr(1).

With Linux expr works:
Code:
$ if expr 'zzz' \> 'aaa' ; then echo Greater ; fi
Greater
But OpenBSD as well as FreeBSD, besides setting the error level, write a "1" or "0" followed by a <newline> to standard output, as mandated by POSIX :
Quote:
The expr utility shall evaluate the expression and write the result, followed by a <newline>, to standard output.
So in those BSD's you get:
Code:
$ if expr 'zzz' \> 'aaa' ; then echo Greater ; fi
1
Greater
That would mess up the output of the shell script that I was writing, unless you redirect that to /dev/null. But I found that too messy, so I sticked with /bin/test for now
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Last edited by J65nko; 2nd January 2016 at 06:30 PM.
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Old 3rd January 2016
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IdOp IdOp is offline
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Quote:
Originally Posted by J65nko View Post
With Linux expr works:
Code:
$ if expr 'zzz' \> 'aaa' ; then echo Greater ; fi
Greater
Hmm, what version of expr did you use? I tried three different Linux versions

expr (GNU coreutils) 8.19
expr (GNU coreutils) 8.21
expr (GNU coreutils) 8.4

They all print out the result
Code:
expr aaa \> zzz
0
The Gnu/Linux man page says they print out the result.
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Old 3rd January 2016
J65nko J65nko is offline
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Yes, you are right. The GNU coreutils 8.21 expr on a Linux Mint live USB stick also does print out the result:
Code:
mint@mint ~ $ if expr 'zzz' \> 'aaa' ; then echo Greater ; fi
1
Greater
Probably forgot to escape the '>'
Code:
mint@mint ~ $ if expr 'zzz' > 'aaa' ; then echo Greater ; fi
Greater
mint@mint ~ $ ls -l aaa
-rw-r--r-- 1 mint mint 4 Jan  3 21:33 aaa
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Old 4th January 2016
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Thanks for clarifying. These things happen very easily when experimenting with different variants of programs, OS, example categories, shell syntaxes and so on.
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