Why "[ 1 > 2 ]" evaluates to True? - linux

Question

I have two files:
f1.txt:
1
dest/f1.txt:
1
2
When I run wc -l on both of those files in linux terminal - I get my expected results:
$ wc -l < f1.txt
$ 1
$ wc -l < dest/f1.txt
$ 2
But when I run the following .sh file:
#!/bin/bash
if [ $(wc -l < f1.txt) > $(wc -l < dest/f1.txt) ]; then
echo -e "f1 has more lines"
else
echo -e "f1 doesn't have more lines"
fi
The output is:
f1 has more lines
Can you explian how could this be possible?

Answer 1

You should use the -gt for integer comparison in a if clause.
If you use > or < you will end up doing ASCII alphabetic order comparison.
integer comparison
-eq
is equal to
if [ "$a" -eq "$b" ]
-ne
is not equal to
if [ "$a" -ne "$b" ]
-gt
is greater than
if [ "$a" -gt "$b" ]
-ge
is greater than or equal to
if [ "$a" -ge "$b" ]
-lt
is less than
if [ "$a" -lt "$b" ]
-le
is less than or equal to
if [ "$a" -le "$b" ]
<
is less than (within double parentheses)
(("$a" < "$b"))
<=
is less than or equal to (within double parentheses)
(("$a" <= "$b"))
>
is greater than (within double parentheses)
(("$a" > "$b"))
>=
is greater than or equal to (within double parentheses)
(("$a" >= "$b"))
string comparison
=
is equal to
if [ "$a" = "$b" ]
Caution
Note the whitespace framing the =.
if [ "$a"="$b" ] is not equivalent to the above.
==
is equal to
if [ "$a" == "$b" ]
This is a synonym for =.
Note
The == comparison operator behaves differently within a double-brackets test than within single brackets.
[[ $a == z* ]] # True if $a starts with an "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).
[ $a == z* ] # File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).
!=
is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]] construct.
<
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]
Note that the "<" needs to be escaped within a [ ] construct.
>
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]]
if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
-z
string is null, that is, has zero length
String='' # Zero-length ("null") string variable.
if [ -z "$String" ]
then
echo "\$String is null."
else
echo "\$String is NOT null."
fi # $String is null.
-n
string is not null.
Source: http://tldp.org/LDP/abs/html/comparison-ops.html

Answer 2

[ is also a command in Bash so [ 1 > 2 ] is the same as [ 1 ] > 2 which would succeed and create a file named 2.
As others pointed out you need to use the following syntax to compare integers:
[ 1 -gt 2 ]
[[ 1 -gt 2 ]]
(( 1 > 2 ))

Answer 3

Try with this code:
#!/bin/bash
if [ $(wc -l < f1.txt) -gt $(wc -l < dest/f1.txt) ]; then
echo -e "f1 has more lines"
else
echo -e "f1 doesn't have more lines"
fi
The -gt will proceed with a numeric compare and not ASCII.