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How to match the last line in regex SED?
The first command appends an a and the second deletes the last line. From the sed (1) man page: $ Match the last line. d Delete pattern space. Start next cycle. a text Append text, which has each embedded newline preceded by a backslash. When a line doesn’t match a, it gets appended to the variable all.
Why does SED not execute on Line 1?
If your file doesn’t contain the line you’re addressing, the sed command won’t get executed. That’s why you can’t insert/append on line 1, if your file is empty. attempts to write to insert at the line 1 of test, but that line doesn’t exist at this point.
How to insert text to the first line of a file?
Scripts 1 & 2 use 1i\\ to “at line 1, insert”. Scripts 5 & 6 use $a\\ to “at line , append”. I vertically aligned the text outputs to make the code represent what is expected in the result. Scripts 3 & 4 do substitutions (the latter applying only to “line “). The result is converting command output to valid JSON.
What does the number 1 mean in SED?
Note the number 1 in those sed expressions – that’s called the address in sed terminology. It tells you on which line the command that follows is to operate. If your file doesn’t contain the line you’re addressing, the sed command won’t get executed.
How to print out hold space in regex SED?
At end of file, print out the hold space ( x retrieve contents of hold space; p print). In a sed script, commands are optionally prefixed by an “address”. Most commonly this is a regex, but it can also be a line number.
How to remove a regex pattern in Bash?
There were no such brackets in your input text, so it didn’t match. Your regular expression would have worked if you had used GNU’s sed -r or BSD’s sed -E, the flag switching to POSIX’s extended regular expressions where ( and ) are used to group and \\ ( \\) match the literal brackets. Remove the brackets. Both will work.
Why does SED not work with POSIX regex?
It didn’t work with sed because it will by default use POSIX’s basic regular expressions, where the characters for grouping are \\ ( and \\), while ( and ) will match literal characters. There were no such brackets in your input text, so it didn’t match.