这个命令在if条件句中用得很多。test命令后都会跟一个表达式,作为它的参数。它有两种写法:
test EXPRESSION [ EXPRESSION ]
test的执行过程就是拿一个元素与另一个元素进行比较。在网络上找了一个很有意思的例子,用它来说明一下test命令的使用:
test 1 -eq 2 && echo "true" || echo "false"
如果比较的结果是true,打印true,否则打印false
我们可以通过$?拿到test的结果。如果表达式的值是false,则$?的值是1,否则就是0。
上面的语句与下同的表达是一样的:
[ 1 -eq 2 ] && echo "true" || echo "false"
整型相关的表达式用到的两个数据的比较方法如下:
#!/usr/bin/env bash test 1 -eq 2 && echo "true" || echo "false" test 1 -ne 2 && echo "true" || echo "false" test 1 -gt 2 && echo "true" || echo "false" test 1 -ge 2 && echo "true" || echo "false" test 1 -lt 2 && echo "true" || echo "false" test 1 -le 2 && echo "true" || echo "false" [ 1 -eq 2 ] && echo "true" || echo "false" [ 1 -ne 2 ] && echo "true" || echo "false" [ 1 -gt 2 ] && echo "true" || echo "false" [ 1 -ge 2 ] && echo "true" || echo "false" [ 1 -lt 2 ] && echo "true" || echo "false" [ 1 -le 2 ] && echo "true" || echo "false"
shell提供了字符串比较相关的表达式:
针对字符串,shell提供了这些方便使用的表达式。比如说:-n <string>这个表达式就是将字符串长度与0作比较。其他依次类推。
test -n string1 && echo "true" || echo "false" test -z string1 && echo "true" || echo "false" test string1 && echo "true" || echo "false" test string1=string2 && echo "true" || echo "false" test string1!=string2 && echo "true" || echo "false" [ -n string1 ] && echo "true" || echo "false" [ -z string1 ] && echo "true" || echo "false" [ string1 ] && echo "true" || echo "false" [ string1=string2 ] && echo "true" || echo "false" [ string1!=string2 ] && echo "true" || echo "false"
shell也提供了与文件相关的比较表达式:
#!/usr/bin/env bash test -e /bin/bash && echo $? || echo $? test -f /bin/bash && echo $? || echo $? test -d /bin/bash && echo $? || echo $? test -r /bin/bash && echo $? || echo $? test -w /bin/bash && echo $? || echo $? test -x /bin/bash && echo $? || echo $? test -s /bin/bash && echo $? || echo $? test -S /bin/bash && echo $? || echo $? test -O /bin/bash && echo $? || echo $? test -G /bin/bash && echo $? || echo $? test -h /bin/bash && echo $? || echo $? test -L /bin/bash && echo $? || echo $? test -b /bin/bash && echo $? || echo $? test -c /bin/bash && echo $? || echo $? #!/usr/bin/env bash [ -e /bin/bash ] && echo $? || echo $? [ -f /bin/bash ] && echo $? || echo $? [ -d /bin/bash ] && echo $? || echo $? [ -r /bin/bash ] && echo $? || echo $? [ -w /bin/bash ] && echo $? || echo $? [ -x /bin/bash ] && echo $? || echo $? [ -s /bin/bash ] && echo $? || echo $? [ -S /bin/bash ] && echo $? || echo $? [ -O /bin/bash ] && echo $? || echo $? [ -G /bin/bash ] && echo $? || echo $? [ -h /bin/bash ] && echo $? || echo $? [ -L /bin/bash ] && echo $? || echo $? [ -b /bin/bash ] && echo $? || echo $? [ -c /bin/bash ] && echo $? || echo $?
shell提供了上面这些方便的表达式,我们就少做了很多功夫。
所以,现在看来test很简单,但是很有用。因为shell脚本里会出现很多条件语句,test会用到很多。