我如何知道是否在Bash中设置了变量?
例如,如何检查用户是否向函数提供了第一个参数?
function a {
# if $1 is set ?
}
我如何知道是否在Bash中设置了变量?
例如,如何检查用户是否向函数提供了第一个参数?
function a {
# if $1 is set ?
}
当前回答
如果未设置,则要退出
这对我很有用。如果没有设置参数,我希望脚本退出并显示错误消息。
#!/usr/bin/env bash
set -o errexit
# Get the value and empty validation check all in one
VER="${1:?You must pass a version of the format 0.0.0 as the only argument}"
运行时返回错误
peek@peek:~$ ./setver.sh
./setver.sh: line 13: 1: You must pass a version of the format 0.0.0 as the only argument
仅检查,不退出-空和未设置无效
如果您只想检查值set=VALID或unset/empty=INVALID,请尝试此选项。
TSET="good val"
TEMPTY=""
unset TUNSET
if [ "${TSET:-}" ]; then echo "VALID"; else echo "INVALID";fi
# VALID
if [ "${TEMPTY:-}" ]; then echo "VALID"; else echo "INVALID";fi
# INVALID
if [ "${TUNSET:-}" ]; then echo "VALID"; else echo "INVALID";fi
# INVALID
或者,即使是短期测试;-)
[ "${TSET:-}" ] && echo "VALID" || echo "INVALID"
[ "${TEMPTY:-}" ] && echo "VALID" || echo "INVALID"
[ "${TUNSET:-}" ] && echo "VALID" || echo "INVALID"
仅检查,不退出-仅空为无效
这就是问题的答案。如果您只想检查值set/empty=VALID或unset=INVALID,请使用此选项。
注意,“..-1}”中的“1”无关紧要,它可以是任何东西(比如x)
TSET="good val"
TEMPTY=""
unset TUNSET
if [ "${TSET+1}" ]; then echo "VALID"; else echo "INVALID";fi
# VALID
if [ "${TEMPTY+1}" ]; then echo "VALID"; else echo "INVALID";fi
# VALID
if [ "${TUNSET+1}" ]; then echo "VALID"; else echo "INVALID";fi
# INVALID
短期测试
[ "${TSET+1}" ] && echo "VALID" || echo "INVALID"
[ "${TEMPTY+1}" ] && echo "VALID" || echo "INVALID"
[ "${TUNSET+1}" ] && echo "VALID" || echo "INVALID"
我把这个答案献给了@mklement0(comments),他要求我准确回答这个问题。
参考:2.6.2参数扩展
其他回答
case "$1" in
"") echo "blank";;
*) echo "set"
esac
如果你想检查$@中的任何内容,我找到了一个更好的代码。
if [[ $1 = "" ]] then echo '$1 is blank' else echo '$1 is filled up' fi
为什么会这样?$@中的所有内容都存在于Bash中,但默认情况下为空,因此test-z和test-n无法帮助您。
更新:您还可以计算参数中的字符数。
if [ ${#1} = 0 ] then echo '$1 is blank' else echo '$1 is filled up' fi
我的首选方式是:
$ var=10
$ if ! ${var+false};then echo "is set";else echo "NOT set";fi
is set
$ unset -v var
$ if ! ${var+false};then echo "is set";else echo "NOT set";fi
NOT set
因此,基本上,如果一个变量被设置,它就变成了“对结果false的否定”(true=“被设置”)。
并且,如果它未设置,它将变成“对结果true的否定”(因为空结果的求值结果为true)(因此将以false=“NOT set”结束)。
您可以执行以下操作:
function a {
if [ ! -z "$1" ]; then
echo '$1 is set'
fi
}
我很惊讶没有人尝试编写一个shell脚本来以编程方式生成这个臭名昭著的难以摸索的表。既然我们在这里试图学习编码技术,为什么不用代码表达答案?:)这是我的看法(应该在任何POSIX shell中都适用):
H="+-%s-+-%s----+-%s----+-%s--+\n" # table divider printf format
R="| %-10s | %-10s | %-10s | %-10s |\n" # table row printf format
S='V' # S is a variable that is set-and-not-null
N='' # N is a variable that is set-but-null (empty "")
unset U # U is a variable that is unset
printf "$H" "----------" "-------" "-------" "---------";
printf "$R" "expression" "FOO='V'" "FOO='' " "unset FOO";
printf "$H" "----------" "-------" "-------" "---------";
printf "$R" "\${FOO:-x}" "${S:-x}" "${N:-x}" "${U:-x} "; S='V';N='';unset U
printf "$R" "\${FOO-x} " "${S-x} " "${N-x} " "${U-x} "; S='V';N='';unset U
printf "$R" "\${FOO:=x}" "${S:=x}" "${N:=x}" "${U:=x} "; S='V';N='';unset U
printf "$R" "\${FOO=x} " "${S=x} " "${N=x} " "${U=x} "; S='V';N='';unset U
# "${N:?x}" "${U:?x} "
printf "$R" "\${FOO:?x}" "${S:?x}" "<error>" "<error> "; S='V';N='';unset U
# "${U?x} "
printf "$R" "\${FOO?x} " "${S?x} " "${N?x} " "<error> "; S='V';N='';unset U
printf "$R" "\${FOO:+x}" "${S:+x}" "${N:+x}" "${U:+x} "; S='V';N='';unset U
printf "$R" "\${FOO+x} " "${S+x} " "${N+x} " "${U+x} "; S='V';N='';unset U
printf "$H" "----------" "-------" "-------" "---------";
以及运行脚本的输出:
+------------+------------+------------+------------+
| expression | FOO='V' | FOO='' | unset FOO |
+------------+------------+------------+------------+
| ${FOO:-x} | V | x | x |
| ${FOO-x} | V | | x |
| ${FOO:=x} | V | x | x |
| ${FOO=x} | V | | x |
| ${FOO:?x} | V | <error> | <error> |
| ${FOO?x} | V | | <error> |
| ${FOO:+x} | x | | |
| ${FOO+x} | x | x | |
+------------+------------+------------+------------+
该脚本缺少了一些功能,比如在发生(或不发生)副作用分配时显示,但也许其他更有野心的人希望以此为出发点,并以此为出发。