如何从固定宽度字符串中提取文本？
例如这是docker history命令的输出...
# docker history mysql
IMAGE CREATED CREATED BY SIZE COMMENT
f008d8ff927d 3 weeks ago /bin/sh -c #(nop) CMD ["mysqld"] 0B
<missing> 3 weeks ago /bin/sh -c #(nop) EXPOSE 3306/tcp 0B
<missing> 3 weeks ago /bin/sh -c #(nop) ENTRYPOINT ["docker-ent... 0B
<missing> 3 weeks ago /bin/sh -c ln -s usr/local/bin/docker-entr... 34B
<missing> 3 weeks ago /bin/sh -c #(nop) COPY file:52f06a5715711e... 6.04kB
<missing> 3 weeks ago /bin/sh -c #(nop) VOLUME [/var/lib/mysql] 0B
<missing> 3 weeks ago /bin/sh -c { echo mysql-community-server... 242MB
<missing> 3 weeks ago /bin/sh -c echo "deb http://repo.mysql.com... 55B
<missing> 3 weeks ago /bin/sh -c #(nop) ENV MYSQL_VERSION=5.7.2... 0B
<missing> 2 months ago /bin/sh -c #(nop) ENV MYSQL_MAJOR=5.7 0B
<missing> 2 months ago /bin/sh -c set -ex; key='A4A9406876FCBD3C... 21.8kB
<missing> 2 months ago /bin/sh -c apt-get update && apt-get insta... 38.6MB
<missing> 2 months ago /bin/sh -c mkdir /docker-entrypoint-initdb.d 0B
<missing> 2 months ago /bin/sh -c set -x && apt-get update && ap... 4.52MB
<missing> 2 months ago /bin/sh -c #(nop) ENV GOSU_VERSION=1.7 0B
<missing> 2 months ago /bin/sh -c groupadd -r mysql && useradd -r... 330kB
<missing> 2 months ago /bin/sh -c #(nop) CMD ["bash"] 0B
<missing> 2 months ago /bin/sh -c #(nop) ADD file:1dd78a123212328... 123MB
我只需要选择第三列＆＃34;由＆＃34;创建。这就是我试过的......
# docker history mysql | awk '{printf "%-20s %-20s %-48s %-2s \n", $1,$2,$3,$4}'
IMAGE CREATED CREATED BY
f008d8ff927d 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 3 weeks ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
<missing> 2 months ago
我猜这里的空间不是字段分隔符。我需要从第40位到第88位的角色。
答案 0 :(得分：2)
sed "s:^.\{40\}\(.\{48\}\).*:\1:" file
1) ^.\{40\} - 字符串开头的40个字符（. - char，{number of repeats}）
2) \(.\{48\}\) - 另外48个字符（40到88个字符）。
3) .* - 字符串的其余部分
4) \(\)，\1 - 用48个字符替换字符串（从40到88）
答案 1 :(得分：1)
如果您的历史记录总是相同，则以下内容可能对您有所帮助：
解决方案第一： 由于OP提到的输出宽度是固定的，所以在这里使用该策略：
awk 'substr($0,41,48){print substr($0,41,48)}' Input_file
解决方案第二： 在此使用match awk实用程序：
Your_command | awk '{match($0,/\/bin[^[:alnum:]].*/);num=split(substr($0,RSTART,RLENGTH),array," ");for(i=1;i<=(num-1);i++){printf("%s%s",array[i],i==(num-1)?ORS:OFS)}}'
现在添加一种非单线形式的解决方案：
your_command | awk '
{
match($0,/\/bin[^[:alnum:]].*/);
num=split(substr($0,RSTART,RLENGTH),array," ");
for(i=1;i<=(num-1);i++){
printf("%s%s",array[i],i==(num-1)?ORS:OFS)}
}
' Input_file
输出如下：
/bin/sh -c #(nop) CMD ["mysqld"]
/bin/sh -c #(nop) EXPOSE 3306/tcp
/bin/sh -c #(nop) ENTRYPOINT ["docker-ent...
/bin/sh -c ln -s usr/local/bin/docker-entr...
/bin/sh -c #(nop) COPY file:52f06a5715711e...
/bin/sh -c #(nop) VOLUME [/var/lib/mysql]
/bin/sh -c { echo mysql-community-server...
/bin/sh -c echo "deb http://repo.mysql.com...
/bin/sh -c #(nop) ENV MYSQL_VERSION=5.7.2...
/bin/sh -c #(nop) ENV MYSQL_MAJOR=5.7
/bin/sh -c set -ex; key='A4A9406876FCBD3C...
/bin/sh -c apt-get update && apt-get insta...
/bin/sh -c mkdir /docker-entrypoint-initdb.d
/bin/sh -c set -x && apt-get update && ap...
/bin/sh -c #(nop) ENV GOSU_VERSION=1.7
/bin/sh -c groupadd -r mysql && useradd -r...
/bin/sh -c #(nop) CMD ["bash"]
/bin/sh -c #(nop) ADD file:1dd78a123212328...