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Comment: a few little clarifications
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Comment: Separated string and integer comparisons (e.g. [[ 10 -gt 5 ]] but [[ 10 < 5 ]], and [[ 5 -eq 05 ]] but [[ 5 != 05 ]])
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| ||<rowspan=4>string comparison||>||\>||-|| ||<||\<||-|| ||= (or ==)||=||-|| ||!=||!=||-|| |
||<rowspan=4>string comparison||>||\>||{{{[[ a > b ]] || echo "a does not come before b"}}}|| ||<||\<||{{{[[ az < za ]] && echo "az comes before za"}}}|| ||= (or ==)||=||{{{[[ a == a ]] && echo "a equals a"}}}|| ||!=||!=||{{{[[ a != b ]] && echo "a is not equal to b"}}}|| ||<rowspan=6>integer comparison||-gt||-gt||{{{[[ 5 -gt 10 ]] || echo "5 is not bigger than 10"}}}|| ||-lt||-lt||{{{[[ 8 -lt 9 ]] && echo "8 is less than 9"}}}|| ||-ge||-ge||{{{[[ 3 -ge 3 ]] && echo "3 is greater than or equal to 3"}}}|| ||-le||-le||{{{[[ 3 -le 8 ]] && echo "3 is less than or equal to 8"}}}|| ||-eq||-eq||{{{[[ 5 -eq 05 ]] && echo "5 equals 05"}}}|| ||-ne||-ne||{{{[[ 6 -ne 20 ]] && echo "6 is not equal to 20"}}}|| |
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| * As of bash 4.1, string comparisons using `<` or `>` respect the current [[locale]] when done in `[[`, but '''not''' in `[` or `test`. In fact, `[` and `test` have ''never'' used locale collating order even though past man pages ''said'' they did. Bash versions prior to 4.1 do not use locale collating order for `[[` either. |
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| See the [[BashGuide/Practices/BashTests|Bash Tests]] chapter in the BashGuide. | See the [[BashGuide/TestsAndConditionals|Tests and Conditionals]] chapter in the BashGuide. ---- CategoryShell |
What is the difference between test, [ and [[ ?
[ ("test" command) and [[ ("new test" command) are used to evaluate expressions. Some examples:
if [ -z "$variable" ]
then
echo "variable is empty!"
fi
if [ ! -f "$filename" ]
then
echo "not a valid, existing file name: $filename"
fiand
if [[ ! -e $file ]]
then
echo "directory entry does not exist: $file"
fi
if [[ $file0 -nt $file1 ]]
then
echo "file $file0 is newer than $file1"
fiTo cut a long story short: test implements the old, portable syntax of the command. In almost all shells (the oldest Bourne shells are the exception), [ is a synonym for test (but requires a final argument of ]). Although all modern shells have built-in implementations of [, there usually still is an external executable of that name, e.g. /bin/[.
[[ is a new improved version of it, which is a keyword, not a program. This has beneficial effects on the ease of use, as shown below. [[ is understood by KornShell and BASH (e.g. 2.03), but not by the older POSIX or BourneShell.
Although [ and [[ have much in common, and share many expression operators like "-f", "-s", "-n", "-z", there are some notable differences. Here is a comparison list:
Feature |
new test [[ |
old test [ |
Example |
string comparison |
> |
\> |
[[ a > b ]] || echo "a does not come before b" |
< |
\< |
[[ az < za ]] && echo "az comes before za" |
|
= (or ==) |
= |
[[ a == a ]] && echo "a equals a" |
|
!= |
!= |
[[ a != b ]] && echo "a is not equal to b" |
|
integer comparison |
-gt |
-gt |
[[ 5 -gt 10 ]] || echo "5 is not bigger than 10" |
-lt |
-lt |
[[ 8 -lt 9 ]] && echo "8 is less than 9" |
|
-ge |
-ge |
[[ 3 -ge 3 ]] && echo "3 is greater than or equal to 3" |
|
-le |
-le |
[[ 3 -le 8 ]] && echo "3 is less than or equal to 8" |
|
-eq |
-eq |
[[ 5 -eq 05 ]] && echo "5 equals 05" |
|
-ne |
-ne |
[[ 6 -ne 20 ]] && echo "6 is not equal to 20" |
|
expression grouping |
&& |
-a |
[[ -n $var && -f $var ]] && echo "$var is a file" |
|| |
-o |
- |
|
Pattern matching |
= (or ==) |
(not available) |
[[ $name = a* ]] || echo "name does not start with an 'a': $name" |
RegularExpression matching |
=~ |
(not available) |
[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!" |
Special primitives that [[ is defined to have, but [ may be lacking (depending on the implementation):
Description |
Primitive |
Example |
entry (file or directory) exists |
-e |
[[ -e $config ]] && echo "config file exists: $config" |
file is newer/older than other file |
-nt / -ot |
[[ $file0 -nt $file1 ]] && echo "$file0 is newer than $file1" |
two files are the same |
-ef |
[[ $input -ef $output ]] && { echo "will not overwrite input file: $input"; exit 1; } |
negation |
! |
- |
But there are more subtle differences.
No WordSplitting or glob expansion will be done for [[ (and therefore many arguments need not be quoted):
file="file name" [[ -f $file ]] && echo "$file is a file"
will work even though $file is not quoted and contains whitespace. With [ the variable needs to be quoted:
file="file name" [ -f "$file" ] && echo "$file is a file"
This makes [[ easier to use and less error-prone.
Parentheses in [[ do not need to be escaped:
[[ -f $file1 && ( -d $dir1 || -d $dir2) ]] [ -f "$file1" -a \( -d "$dir1" -o -d "$dir2" \) ]
As of bash 4.1, string comparisons using < or > respect the current locale when done in [[, but not in [ or test. In fact, [ and test have never used locale collating order even though past man pages said they did. Bash versions prior to 4.1 do not use locale collating order for [[ either.
As a rule of thumb, [[ is used for strings and files. If you want to compare numbers, use an ArithmeticExpression, e.g.
# Bash i=0 while ((i<10)); do ...
When should the new test command [[ be used, and when the old one [? If portability to the BourneShell is a concern, the old syntax should be used. If on the other hand the script requires BASH or KornShell, the new syntax is much more flexible.
See the Tests and Conditionals chapter in the BashGuide.