Linux Bash/sh 'if else' 语句
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Bash/sh 'if else' statement
提问by AabinGunz
I want to understand the if elsestatement in shscripting.
我想了解脚本中的if else语句sh。
So I wrote the below to find out whether JAVA_HOME is set in the environment or not. I wrote the below script
所以我写了下面来找出环境中是否设置了JAVA_HOME。我写了下面的脚本
#!/bin/sh
if [ $JAVA_HOME != "" ]
then
echo $JAVA_HOME
else
echo "NO JAVA HOME SET"
fi
This my output to env:
这是我的输出env：
sh-3.2$ env
SHELL=/bin/csh
TERM=xterm
HOST=estilor
SSH_CLIENT=10.15.16.28 4348 22
SSH_TTY=/dev/pts/18
USER=asimonraj
GROUP=ccusers
HOSTTYPE=x86_64-linux
PATH=/usr/local/bin:/bin:/home/asimonraj/java/LINUXJAVA/java/bin:/usr/bin
MAIL=/var/mail/asimonraj
PWD=/home/asimonraj/nix
HOME=/home/asimonraj
SHLVL=10
OSTYPE=linux
VENDOR=unknown
LOGNAME=asimonraj
MACHTYPE=x86_64
SSH_CONNECTION=100.65.116.248 4348 100.65.116.127 22
_=/bin/env
But I get the below output:
但我得到以下输出：
sh-3.2$ ./test.sh
./test.sh: line 3: [: !=: unary operator expected
NO JAVA HOME SET
采纳答案by duskwuff -inactive-
You're running into a stupid limitation of the way shexpands arguments. Line 3 of your script is being expanded to:
您遇到了sh扩展参数方式的愚蠢限制。脚本的第 3 行正在扩展为：
if [ != ]
Which shcan't figure out what to do with. Try this nasty hack on for size:
这sh不知道该怎么办。试试这个讨厌的大小：
if [ x$JAVA_HOME != x ]
Both arguments have to be non-empty, so we'll just throw an xinto both of them and see what happens.
两个参数都必须是非空的，所以我们只需将一个x放入它们两个中，看看会发生什么。
Alternatively, there's a separate operator for testing if a string is non-empty:
或者，有一个单独的运算符用于测试字符串是否为非空：
if [ !-z $JAVA_HOME ]
(-ztests if the following string is empty.)
（-z测试以下字符串是否为空。）
回答by Kracekumar
if [ -z $JAVA_HOME ]
then
echo $JAVA_HOME
else
echo "NO JAVA HOME SET"
fi
回答by sente
The -nand -zoptions are tests that should be used here:
在-n和-z选项是应该在这里使用的测试：
if [ -n "$JAVAHOME" ]; then
echo "$JAVAHOME";
else
echo "$JAVAHOME not set";
fi
回答by William Pursell
Note that if you want to determine if a variable is set, you probably do not want to use either if/else or test ([). It is more typical to do things like:
请注意，如果您想确定是否设置了变量，您可能不想使用 if/else 或 test ([)。更典型的做法是：
# Abort if JAVA_HOME is not set (or empty)
: ${JAVA_HOME:?JAVA_HOME is unset}
or
或者
# report the value of JAVA_HOME, or a default value
echo ${JAVA_HOME:-default value}
or
或者
# Assign JAVA_HOME if it is unset (or empty)
: ${JAVAHOME:=default value}
