Error while executing sed command - shell

Question

I am trying to execute script with commands:
sed -i "USER/c\export USER=${signumid}" .bashrc
sed -i "DEVENVHOME=$/c\export DEVENVHOME=${DEVENVHOME:-/home/${signumid}/CPM_WORKAREA/devenv.x}" .bashrc
 
I want to replace the line with string "USER" in .bashrc with export USER=${signumid} where $signumid variable is being provided through Cygwin prompt. Similarly I want to replace line with string DEVENVHOME=$ with export DEVENVHOME=${DEVENVHOME:-/home/${signumid}/CPM_WORKAREA/devenv.x} in .bashrc file, where $signumid variable is provided through Cygwin prompt.
But I am getting following errors on Cygwin termminal.:
sed: -e expression #1, char 1: unknown command: `U'
sed: -e expression #1, char 3: extra characters after command

Answer 1

The general syntax of a sed script is a sequence of address command arguments statements (separated by newline or semicolon). The most common command is the s substitution command, with an empty address, so we can perhaps assume that that is what you want to use here. You seem to be attempting to interpolate a shell variable $signumid which adds a bit of a complication to this exposition.
If your strings were simply static text, it would make sense to use single quotes; then, the shell does not change the text within the quotes at all. The general syntax of the s command is s/regex/replacement/ where the slash as the argument separator is just a placeholder, as we shall soon see.
sed -i 's/.*USER.*/export USER=you/
s% DEVENVHOME=\$%export DEVENVHOME=${DEVENVHOME:-/home/you/CPM_WORKAREA/devenv.x}%' .bashrc
This will find any line with USER and substitute the entire line with export USER=you; then find any line which contains DEVENVHOME=$ (with a space before, and a literal dollar character) and replace the matched expression with the long string. Because the substitution string uses slashes internally, we use a different regex separator % -- alternatively, we could backslash-escape the slashes which are not separators, but as we shall see, that quickly becomes untenable when we add the following twist. Because the dollar sign has significance as the "end of line" metacharacter in regular expressions, we backslash-escape it.
I have ignored the c\ in your attempt on the assumption that it is simply a misunderstanding of sed syntax. If it is significant, what do you hope to accomplish with it? c\export is not a valid Bash command, so you probably mean something else, but I cannot guess what.
Now, to interpolate the value of the shell variable signumid into the replacement, we cannot use single quotes, because those inhibit interpolation. You have correctly attempted to use double quotes instead (in your edited question), but that means we have to make some additional changes. Inside double quotes, backslashes are processed by the shell, so we need to double all backslashes, or find alternative constructs. Fortunately for us, the only backslash is in \$ which can equivalently be expressed as [$], so let's switch to that notation instead. Also, where a literal dollar sign is wanted in the replacement string, we backslash-escape it in order to prevent the shell from processing it.
sed -i "s/.*USER.*/export USER=$signumid/
s% DEVENVHOME=[$]%export DEVENVHOME=\${DEVENVHOME:-/home/$signumid/CPM_WORKAREA/devenv.x}%" .bashrc
Equivalenty, you could use single quotes around the parts of the script which are meant to be untouched by the shell, and then put an adjacent double-quoted string around the parts which need interpolation, like
'un$touched*by$(the!shell)'"$signumid"'more$[complex]!stuff'
This final script still rests on a number of lucky or perhaps rather unlucky guesses about what you actually want. On the first line, I have changed just USER to a regular expression which matches the entire line -- maybe that's not what you want? On the other hand, the second line makes the opposite assumption, just so you can see the variations -- it only replaces the actual text we matched. Probably one or the other needs to be changed.
Finally, notice how the two separate sed commands have been conflated into a single script. Many newcomers do not realize that sed is a scripting language which accepts an arbitrary number of commands in a script, and simply treat it as a "replace" program with a funny syntax.
Another common source of confusion is the evaluation order. The shell processes the double-quoted string even before sed starts to execute, so if you have mistakes in the quoting, you can easily produce syntax errors in the sed script which lead to rather uninformative error messages (because what sed tells you in the error message is based on what the script looks like after the shell's substutions). For example, if signumid contains slashes, it will produce syntax errors, because sed will see those as terminating separators for the s/// command. An easy workaround is to switch to a separator which does not occur in the value of signumid.