Shell脚本中sed命令匹配段内容类型的技术疑问

Hey there! Let's break down exactly what's going on with that sed command you're looking at—great question about distinguishing between shell commands and sed's pattern matching.

First, let's recap the command for clarity:

sed -n "/set -A $REC/,/#${REC}_END/p"

The Short Answer #

The text inside the /../ delimiters are sed pattern matches (not executing shell commands). The set -A part is just plain text that sed uses to locate the start of the section you want to print—sed never runs the set -A shell command here.

The Detailed Breakdown #

Let's unpack each part step by step:

1. Shell Variable Expansion First: Before sed even runs, your shell replaces $REC and ${REC} with the actual value stored in the REC variable. For example, if REC=MY_CONFIG, the command becomes:

   sed -n "/set -A MY_CONFIG/,/#MY_CONFIG_END/p"
   

2. Sed's Address Range Matching: The two patterns (/set -A .../ and /#..._END/) define a range of lines for sed to target:
   • /set -A MY_CONFIG/: Sed searches for the first line that contains the exact string set -A MY_CONFIG—this is the start of the range.
   • /#MY_CONFIG_END/: Sed then looks for the first line after that start line which contains the exact string #MY_CONFIG_END—this is the end of the range.
3. The -n and p Flags: As you already noted, -n stops sed from automatically printing every line, and p tells sed to print only the lines that fall within the defined range (from the start pattern line to the end pattern line, inclusive).

Example in Action #

Suppose your file has this content:

# Random setup lines
echo "Starting config"
set -A MY_CONFIG foo bar baz
echo "Config values set"
# This is part of the config section
#MY_CONFIG_END
echo "Done with config"

Running the expanded sed command would print these lines:

set -A MY_CONFIG foo bar baz
echo "Config values set"
# This is part of the config section
#MY_CONFIG_END

To sum up: The patterns inside /../ are just text strings (with shell variables expanded first) that sed uses to find the right lines—no shell commands are executed as part of those patterns.

内容的提问来源于stack exchange，提问作者Sindhu Choudary