Suppose there's a text file with the following line:

export MYSQL_ADMIN=''

I want to insert text inside that single quote using the sed command, so that it changes to something like this for example:

export MYSQL_ADMIN='abc1'

What is the appropriate sed command for that in Linux? I tried

sed -i -e ''/MYSQL_ADMIN/s/''/'abc1'/g"

but it didn't work.

CodePudding user response：

Something like sed -i "s;export MYSQL_ADMIN=.*;export MYSQL_ADMIN='abc1';" /path/to/file.ext

-i modify file in place s means substitute,

First block is what you are matching as an regular expression - the .* matches everything to the end of the line, this ensures you don't keep any text on that line after the substitue - and second block is what you are replacing with that match.

Always check the file after each run of sed if there is no error and check what changed.

To get the single quotes to print you may have to do ""'"" like ""'""abc1""'""

CodePudding user response：

It is important to understand that although

I want to insert text inside that single quote using the sed command

is a perfectly good characterization of the effect you want to achieve, it does not map directly onto operations from sed's repertoire. With sed, the appropriate tool for most line modifications is the s command, which substitutes specified text for one or more matches to a specified regular expression. That would be the most natural thing to use for your case.

Additionally, it is important with sed to understand how and when to bind commands to specific lines. If you don't do that for a given command then it is applied to all lines. Sometimes that's fine, but other times it will produce unwanted results.

I tried

sed -i -e ''/MYSQL_ADMIN/s/''/'abc1'/g"

but it didn't work.

The two leading single quotes in that sed expression match each other, leaving the trailing double quote unmatched. Also, you do not specify the name of the file to modify. This variation would at least be valid shell syntax, and it would have the desired effect on the specified line appearing in file my_script:

sed -i -e "/MYSQL_ADMIN/s/''/'abc1'/g" my_script

That might also make other, unwanted changes, however.

You need to make some assumptions about the content of the file in order to do such a thing at all. The above depends on the text MYSQL_ADMIN and '' to appear on the same line only in the line(s) you want to modify. That may turn out to hold, but it seems unnecessarily risky. An assumption more likely to hold in general would be that there will be only one assignment to variable MYSQL_ADMIN, or that it is acceptable to modify all such assignments that assign a single-quote-delimited empty value.

Going with the latter, one might end up with this:

sed -i -e "s/\<MYSQL_ADMIN=''\(\s\|$\)/MYSQL_ADMIN='abc1'\1/g" my_script

The pattern \<MYSQL_ADMIN=''\(\s\|$\) improves on your plain MYSQL_ADMIN in these significant ways:

• the \< causes it to match only immediately after a word boundary -- start of line, whitesepace, or punctuation. This prevents substitutions for other variables whose names happen to end with MYSQL_ADMIN. If you prefer, it would be even stronger to instead anchor the match to the beginning of the line with ^.
• including the ='' in the pattern distinguishes between MYSQL_ADMIN and variables whose names contain that as an initial substring. It also ensures that the '' that gets replaced, if any, goes with the variable and does not merely appear somewhere else on the line.
• the \(\s\|$\) both matches and captures either a whitespace character or the empty string at the end of a line. This distinguishes between assignments of an empty value and assignments of values that are merely prefixed by '' (which is valid if the file is a shell script). Having included it in the match, the capture allows the matched text, if any, to be preserved in the output (via the \1 in the replacement).

Because that matches the whole assignment, a complete assignment must appear in the replacement, too. On the other hand, this means that (probably) you can apply the command to every line, as shown, with no particular loss of efficiency relative to the previous command.

Even that might produce changes you didn't want, however, such as in comment lines or quoted text.