{"id":685,"date":"2019-11-21T08:37:13","date_gmt":"2019-11-21T08:37:13","guid":{"rendered":"http:\/\/www.liutianfeng.com\/?p=685"},"modified":"2019-11-23T02:33:00","modified_gmt":"2019-11-23T02:33:00","slug":"shell%e4%b8%ad%e7%9a%84%e5%8f%98%e9%87%8f%e6%93%8d%e4%bd%9c%ef%bc%9a","status":"publish","type":"post","link":"https:\/\/www.liutianfeng.com\/?p=685","title":{"rendered":"shell\u4e2d\u7684\u53d8\u91cf\u64cd\u4f5c\uff1a-\/=\/?"},"content":{"rendered":"

Shell\u4e2d\uff0c\u53ef\u4ee5\u901a\u8fc7\u5192\u53f7\u540e\u9762\u8ddf\u5b57\u7b26\u7684\u5f62\u5f0f\uff0c\u5b8c\u6210\u4e00\u5b9a\u7684\u529f\u80fd\uff0c\u4e0b\u9762\u505a\u5177\u4f53\u4ecb\u7ecd\uff1a<\/p>\n

\u4e00:\u00a0\u53d8\u91cf\u64cd\u4f5c<\/h3>\n

\u683c\u5f0f\uff1a<\/strong><\/p>\n

${VAR_NAME:-}   ${VAR_NAME:?}   ${VAR_NAME:=}<\/code><\/pre>\n
\u529f\u80fd\uff1a<\/p>\n
-\uff1a\u5728VAR_NAME\u4e3a\u7a7a\uff0c\u5c06-\u540e\u9762\u7684\u5185\u5bb9\u8d4b\u503c\u7ed9\u5176\u4ed6\u53d8\u91cf\uff0cVAR_NAME\u4e0d\u4f1a<\/span>\u88ab\u8d4b\u503c\uff1bVAR_NAME\u4e0d\u4e3a\u7a7a\uff0c\u5c06VAR_NAME\u7684\u503c\u8d4b\u7ed9\u5176\u4ed6\u53d8\u91cf\uff1b\u89c1Exp1\u3002<\/p>\n
\uff1f:\u00a0\u5728VAR_NAME\u4e3a\u7a7a\uff0c\u4e0d\u4f1a<\/span>\u5c06\uff1f\u540e\u7684\u503c\u4f20\u7ed9\u5176\u4ed6\u53d8\u91cf\uff0c\u800c\u662f\u8fd4\u56de\u6253\u5370\u5230\u5c4f\u5e55\uff0c\u7c7b\u4f3c\u63d0\u793a\u4fe1\u606f\uff1bVAR_NAME\u4e0d\u4e3a\u7a7a\uff0c\u5c06\u5176\u503c\u8d4b\u503c\u7ed9\u5176\u4ed6\u53d8\u91cf\uff1b\u89c1Exp2\u3002<\/p>\n
=\uff1a\u65e0\u8bbaVAR_NAME\u662f\u5426\u4e3a\u7a7a\uff0c\u5c06=\u540e\u7684\u5185\u5bb9\u5148\u8d4b\u503c\u7ed9VAR_NAME\uff0c\u5728\u5c06\u5176\u8d4b\u503c\u7ed9\u5176\u4ed6\u53d8\u91cf\uff1b\u5982\u679cVAR_NAME\u4e0d\u4e3a\u7a7a\uff0cVAR_NAME\u4e0d\u53d8\uff0c\u5e76\u5c06VAR_NAME\u8d4b\u503c\u7ed9\u5176\u4ed6\u53d8\u91cf\uff1b\u89c1Exp3\u3002<\/p>\n
Exp1\uff1a<\/p>\n
[lius@VM_0_9_centos ~]$ unset A B C\r\n[lius@VM_0_9_centos ~]$ echo $A $B $C\r\n\r\n[lius@VM_0_9_centos ~]$ A=Hello\r\n[lius@VM_0_9_centos ~]$ B=${A:-UNSET}\r\n[lius@VM_0_9_centos ~]$ echo $B\r\nHello\r\n[lius@VM_0_9_centos ~]$ echo $A\r\nHello\r\n[lius@VM_0_9_centos ~]$ unset A B C\r\n[lius@VM_0_9_centos ~]$ A=\r\n[lius@VM_0_9_centos ~]$ B=${A:-UNSET}\r\n[lius@VM_0_9_centos ~]$ echo $B\r\nUNSET\r\n[lius@VM_0_9_centos ~]$ echo $A\r\n\r\n[lius@VM_0_9_centos ~]$<\/code><\/pre>\n
Exp2\uff1a<\/p>\n
[lius@VM_0_9_centos ~]$ unset A B C\r\n[lius@VM_0_9_centos ~]$ A=Hello\r\n[lius@VM_0_9_centos ~]$ B=${A:?\"Not defined...\"}\r\n[lius@VM_0_9_centos ~]$ echo $A\r\nHello\r\n[lius@VM_0_9_centos ~]$ echo $B\r\nHello\r\n[lius@VM_0_9_centos ~]$ unset A B C\r\n[lius@VM_0_9_centos ~]$ A=\r\n[lius@VM_0_9_centos ~]$ B=${A:?\"Not defined...\"}\r\n-bash: A: Not defined...\r\n[lius@VM_0_9_centos ~]$ echo $A\r\n\r\n[lius@VM_0_9_centos ~]$ echo $B\r\n\r\n[lius@VM_0_9_centos ~]$<\/code><\/pre>\n
Exp3:<\/p>\n
[lius@VM_0_9_centos ~]$ unset A B C\r\n[lius@VM_0_9_centos ~]$ A=Hello\r\n[lius@VM_0_9_centos ~]$ B=${A:=UNSET}\r\n[lius@VM_0_9_centos ~]$ echo $A\r\nHello\r\n[lius@VM_0_9_centos ~]$ echo $B\r\nHello\r\n[lius@VM_0_9_centos ~]$ unset A B C\r\n[lius@VM_0_9_centos ~]$ A=\r\n[lius@VM_0_9_centos ~]$ B=${A:=UNSET}\r\n[lius@VM_0_9_centos ~]$ echo $A\r\nUNSET\r\n[lius@VM_0_9_centos ~]$ echo $B\r\nUNSET\r\n[lius@VM_0_9_centos ~]$<\/code><\/pre>\n
 <\/p>\n
\u4e8c:\u00a0\u5b57\u7b26\u4e32\u64cd\u4f5c<\/h3>\n
\u5b57\u7b26\u4e32\u622a\u53d61-8:<\/p>\n
expr substr \"$a\" 1 8\r\necho $a |awk '{print substr(,1,8)}'\r\necho $a |cut -c1-8\r\necho $a |dd bs=1 count=8 2>\/dev\/null<\/code><\/pre>\n
\u5b57\u7b26\u4e32\u6bd4\u8f83\/\u5339\u914d:<\/p>\n
[[ \"a.txt\" == a* ]]        # \u903b\u8f91\u771f (pattern matching)  \r\n[[ \"a.txt\" =~ .*\\.txt ]]   # \u903b\u8f91\u771f (regex matching)  \r\n[[ \"abc\" == \"abc\" ]]       # \u903b\u8f91\u771f (string comparision)   \r\n[[ \"11\" < \"2\" ]]           # \u903b\u8f91\u771f (string comparision), \u6309ascii\u503c\u6bd4\u8f83  <\/code><\/pre>\n
 <\/p>\n
\u8f6c\u8f7d\u8bf7\u6ce8\u660e\uff1aliutianfeng.com<\/a> » shell\u4e2d\u7684\u53d8\u91cf\u64cd\u4f5c\uff1a-\/=\/?<\/a><\/p>","protected":false},"excerpt":{"rendered":"
Shell\u4e2d\uff0c\u53ef\u4ee5\u901a\u8fc7\u5192\u53f7\u540e\u9762\u8ddf\u5b57\u7b26\u7684\u5f62\u5f0f\uff0c\u5b8c\u6210\u4e00\u5b9a\u7684\u529f\u80fd\uff0c\u4e0b\u9762\u505a\u5177\u4f53\u4ecb\u7ecd\uff1a \u4e00:\u00a0\u53d8\u91cf\u64cd\u4f5c \u683c\u5f0f\uff1a ${VA […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=\/wp\/v2\/posts\/685"}],"collection":[{"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=685"}],"version-history":[{"count":4,"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=\/wp\/v2\/posts\/685\/revisions"}],"predecessor-version":[{"id":696,"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=\/wp\/v2\/posts\/685\/revisions\/696"}],"wp:attachment":[{"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=685"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=685"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.liutianfeng.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=685"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}