一个例子明白shell变量替换问题

脚本名为value.sh

 #!/bin/bash

 

var()

{

    echo '${file-my.file.txt}:' ${file-my.file.txt}

    echo '${file:-my.file.txt}:' ${file:-my.file.txt}

    echo '${file+my.file.txt}:' ${file+my.file.txt}

    echo '${file:+my.file.txt}:' ${file:+my.file.txt}

    echo '${file=my.file.txt}:' ${file=my.file.txt}

    echo '${file:=my.file.txt}:' ${file:=my.file.txt}

    echo '${file?my.file.txt}:' ${file?my.file.txt}

    echo '${file:?my.file.txt}:' ${file:?my.file.txt}

}

 

file=$1

echo "variable is not null"

var

 

echo -------------------------------------------------------------

echo "varable is null"

file=$2

var

 

echo -----------------------------------------------------------------------------

echo "variable is unset not variable"

unset file

var

 

运行sh value.sh a 得到如下结果，这样就可以明白各自的意思了

variable is not null

${file-my.file.txt}: a

${file:-my.file.txt}: a

${file+my.file.txt}: my.file.txt

${file:+my.file.txt}: my.file.txt

${file=my.file.txt}: a

${file:=my.file.txt}: a

${file?my.file.txt}: a

${file:?my.file.txt}: a

-------------------------------------------------------------

varable is null

${file-my.file.txt}:

${file:-my.file.txt}: my.file.txt

${file+my.file.txt}: my.file.txt

${file:+my.file.txt}:

${file=my.file.txt}:

${file:=my.file.txt}: my.file.txt

${file?my.file.txt}: my.file.txt

${file:?my.file.txt}: my.file.txt

-----------------------------------------------------------------------------

variable is unset not variables

${file-my.file.txt}: my.file.txt

${file:-my.file.txt}: my.file.txt

${file+my.file.txt}:

${file:+my.file.txt}:

${file=my.file.txt}: my.file.txt

${file:=my.file.txt}: my.file.txt

${file?my.file.txt}: my.file.txt

${file:?my.file.txt}: my.file.txt