4 ms·

He suggests using if [ "$filename" = "foo" ]; Why not if [[ $filename == "foo" ]]; ? EDIT: aware of the fact that [[ ]] is a bash extension, but t

by veyron 15y ago

He suggests using if [ "$filename" = "foo" ]; Why not if [[ $filename == "foo" ]]; ? EDIT: aware of the fact that [[ ]] is a bash extension, but the actual title of the post is "Writing Robust Bash Shell Scripts", implying that it should be using the best stuff for BASH.

Dobbs 15y ago
First is Posix, the second is bash. The first is more portable.
- veyron 15y ago
  The title of the article is "Writing Robust Bash Shell Scripts", which differs from the title of the HN post.
ralph 15y ago
Or if [[ $filename == foo ]];
- Nick_C 15y ago
  Which is the correct idiomatic way. The article's author is wrong in enclosing foo in quotes.
waitwhat 15y ago
$ cat foo.sh #!/bin/bash echo $BASH_VERSION unset filename if [[ $filename == "foo" ]]; $ ./foo.sh 4.1.2(1)-release ./foo.sh: line 5: syntax error: unexpected end of file If $filename evaluates to an empty string, then the last line of code becomes if [[ == "foo" ]]; Similarly, if $filename evaluates to a b c, then the last line of code becomes if [[ a b c == "foo" ]]; The bash interpreter has problems with both of these.
- nescafe 15y ago
  No, that is just wrong. In double-brackets, bash will handle expansion and testing of unset variables and variables with spaces just fine, because word splitting and pathname expansion are not performed in double braces -- they are shell syntax, as opposed to single brackets, which are commands. Your syntax error happened because you don't have a fi matching that if. As a counterexample: (echo $BASH_VERSION; foo=foo; [[ $foo = foo ]] && echo $foo; foo="1 2 3"; [[ $foo = "1 2 3" ]] && echo $foo; foo=''; echo ${foo?"No foo for you"}; unset foo; echo ${foo?"No foo for you"}) 4.2.10(1)-release foo 1 2 3 bash: foo: No foo for you That last one is one of the few ways you can tell whether a variable is unset or merely set to an empty string. lrn2bash.

by veyron 15y ago

Dobbs 15y ago

First is Posix, the second is bash. The first is more portable.

veyron 15y ago
The title of the article is "Writing Robust Bash Shell Scripts", which differs from the title of the HN post.

ralph 15y ago

Or if [[ $filename == foo ]];

Nick_C 15y ago
Which is the correct idiomatic way. The article's author is wrong in enclosing foo in quotes.

waitwhat 15y ago

$ cat foo.sh #!/bin/bash echo $BASH_VERSION unset filename if [[ $filename == "foo" ]]; $ ./foo.sh 4.1.2(1)-release ./foo.sh: line 5: syntax error: unexpected end of file If $filename evaluates to an empty string, then the last line of code becomes if [[ == "foo" ]]; Similarly, if $filename evaluates to a b c, then the last line of code becomes if [[ a b c == "foo" ]]; The bash interpreter has problems with both of these.

nescafe 15y ago
No, that is just wrong. In double-brackets, bash will handle expansion and testing of unset variables and variables with spaces just fine, because word splitting and pathname expansion are not performed in double braces -- they are shell syntax, as opposed to single brackets, which are commands. Your syntax error happened because you don't have a fi matching that if. As a counterexample: (echo $BASH_VERSION; foo=foo; [[ $foo = foo ]] && echo $foo; foo="1 2 3"; [[ $foo = "1 2 3" ]] && echo $foo; foo=''; echo ${foo?"No foo for you"}; unset foo; echo ${foo?"No foo for you"}) 4.2.10(1)-release foo 1 2 3 bash: foo: No foo for you That last one is one of the few ways you can tell whether a variable is unset or merely set to an empty string. lrn2bash.