[root@test-1 ~]# file1=/etc/services;file2=/etc/rc.local

[root@test-1 ~]# echo $file1 $file2

/etc/services /etc/rc.local

[root@test-1 ~]# [ -n "$file" ] && echo 1 ||echo 0

0

#若串长度不为0则真。因$file未定义长度为0，所以为假(0)

[root@test-1 ~]# [ -z "$file" ] && echo 1 ||echo 0

1

#若串长度为0则真。因$file未定义长度为0，所以为真(1)

[root@test-1 ~]# [ -n "$file1" ] && echo 1 ||echo 0

1

#若串长度不为0则真。因$file已定义变量长度不为0，所以为真(1)

[root@test-1 ~]# [ -z "$file1" ] && echo 1 ||echo 0

0

#若串长度为0则真。因$file已定义变量长度不为0，所以为假(0)

提示：去掉双引号看看

[root@test-1 ~]# [ -n $file ] && echo 1 ||echo 0

1

[root@test-1 ~]# [ -z $file1 ] && echo 1 ||echo 0

0

提示：字符串比较一定要加双引号

# Remote quota server

[ -z "$RQUOTAD" ] && RQUOTAD=`type -path rpc.rquotad`

[ -z "$MOUNTD_NFS_V2" ] && MOUNTD_NFS_V2=default

[ -z "$MOUNTD_NFS_V3" ] && MOUNTD_NFS_V3=default

# Number of servers to be started by default

[ -z "$RPCNFSDCOUNT" ] && RPCNFSDCOUNT=8

[ -n "$NLM_GRACE_PERIOD" ] && {

	/sbin/sysctl -w fs.nfs.nlm_grace_period=$NLM_GRACE_PERIOD >/dev/null 2>&1

}

[root@test-1 ~]# file1=/etc/services;file2=/etc/rc.local

[root@test-1 ~]# echo $file1 $file2

/etc/services /etc/rc.local

[root@test-1 ~]# [ -n "$file" ] && echo 1 ||echo 0

0

#若串长度不为0则真。因$file未定义长度为0，所以为假(0)

[root@test-1 ~]# [ -z "$file" ] && echo 1 ||echo 0

1

#若串长度为0则真。因$file未定义长度为0，所以为真(1)

[root@test-1 ~]# [ -z "$file1" ] && echo 1 ||echo 0

0

[root@test-1 ~]# [ -z "$file1" -a -z "file2" ] && echo 1 ||echo 0

0

[root@test-1 ~]# [[ -z "$file1" && -z "file2" ]] && echo 1 ||echo 0

0

[root@test-1 ~]# [[ -z "$file1" || -n "file2" ]] && echo 1 ||echo 0

1

[root@test-1 ~]# [ -z "$file1" -o -z "file2" ] && echo 1 ||echo 0

0

[root@test-1 ~]# [ -n "$file1" -o -z "file2" ] && echo 1 ||echo 0

1

[root@test-1 ~]# [ "$file1" == "file2" ] && echo 1 ||echo 0

0

[root@test-1 ~]# [ "$file1" = "file2" ] && echo 1 ||echo 0

0

[root@test-1 ~]# [ "$file1" !== "file2" ] && echo 1 ||echo 0

-bash: [: !==: binary operator expected

0

[root@test-1 ~]# [ "$file1" != "file2" ] && echo 1 ||echo 0

1

[root@test-1 ~]# [ ! "$file1" == "file2" ] && echo 1 ||echo 0

1

[root@test-1 ~]# [ ! "$file1" \> "file2" ] && echo 1 ||echo 0

1

[root@test-1 ~]# [ ! "$file1" \< "file2" ] && echo 1 ||echo 0

0

[root@test-1 ~]# [ ! "${#file1}" \< "${#file2}" ] && echo 1 ||echo 0

1