cat：concatenate files and print on the standard output 由第一行开始显示内容
tac：concatenate and print files in reverse 从最后一行开始显示内容
nl ：number lines of files 显示的时候输出行号
-b(--body -numbering=STYLE)：use STYLE for numbering body lines;STYLE is one of :
a:number all lines
t:number only nonempty lines
也就是 nl -b a ,不论空行或非空行，都显示行号
nl -b t,非空行，显示行号
-n(--number-format=FORMAT)：insert line numbers according to FORMAT;FORMAT is one of:
ln:left justified,no leading zeros
rn:right justified, no leading zeros
rz:right justified,leading zeros
也就是nl -n ln，行号左对齐，且不加0
nl -n rn，行号右对齐，且不加0
nl -n rz，行号右对齐，且加0
cat -n /etc/issue
cat -b /etc/issue
cat -A /etc/issue
nl /etc/issue 非空行，显示行号
nl -b t /etc/issue 非空行，显示行号
cat -b /etc/issue 非空行，显示行号
nl -b a /etc/issue 不论空行或非空行，全部显示行号
cat -n /etc/issue 不论空行或非空行，全部显示行号
nl -b a /etc/issue
1
2 Welcome to SUSE Linux Enterprise Server 11 SP3 (x86_64) - Kernel \r (\l).
3
4
nl -b a -n ln /etc/issue
1
2 Welcome to SUSE Linux Enterprise Server 11 SP3 (x86_64) - Kernel \r (\l).
3
4
nl -b a -n rn /etc/issue
1
2 Welcome to SUSE Linux Enterprise Server 11 SP3 (x86_64) - Kernel \r (\l).
3
4
nl -b a -n rz /etc/issue
000001
000002 Welcome to SUSE Linux Enterprise Server 11 SP3 (x86_64) - Kernel \r (\l).
000003
000004