Bash Shell: What is the differences in syntax?
I've seen two ways in tutorials to do syntax for if statements in BASH shell:
This one wouldn't work unless I put quotes around the variable and added additional [ and ]:
if [[ "$step" -eq 0 ]]
This one worked without putting quotes around the variable and the additional [ and ] weren't needed:
if [ $step -ge 1 ] && [ $step -le 52 ]
Which is correct and best practice? What are the differences? Thanks!
"When referencing a variable, it is generally advisable to enclose its name in double quotes" -- http://tldp.org/LDP/abs/html/quotingvar.html
if [ $step -ge 1 ] && [ $step -le 52 ] can be replaced as
if [ "$step" -ge 1 -a "$step" -le 52 ]
if [[ "$step" -eq 0 ]] can be replaced as if [ "$step" -eq 0 ]
Also, suppose you have the following script:
#!/bin/bash
if [ $x -eq 0 ]
then
echo "hello"
fi
You get this error when you run the script -- example.sh: line 2: [: -eq: unary operator expected
But using if [ "$x" -eq 0 ]
You get a different error when you run the script -- example.sh: line 2: [: : integer expression expected
Thus, it is always best to put variables inside quotes...
if [[ .... ]] syntax is particularly useful when you have regex in the condition statement -- http://honglus.blogspot.com/2010/03/regular-expression-in-condition.html
EDIT: When we deal with strings --
#!/bin/bash
if [ $x = "name" ]
then
echo "hello"
fi
You get this error when you run the script -- example.sh: line 2: [: =: unary operator expected
But, if you use if [ "$x" = "name" ] it runs fine (ie no errors ) and if statement is evaluated as false , as value of x is null which does not match name .
上一篇: 方括号和
下一篇: Bash Shell：语法有什么区别？