Changing characters

Changing a set of characters to another set of characters is generally a task for the tr command but since you want to do it only on certain lines, it will be best done by sed indeed:

sed -e "/^KEYWORD_1/ s/\"/'/g" \
    -e "/^KEYWORD_2/ s/'/\"/g" file

Each sed command here starts with a line selector /KEYWORD/ which instructs sed to only operate on the line matching the pattern between /. Here the patterns start with the character ^ to indicate it is to be found at the beginning of the line.

Following the line selector is sed's substitution command s/xxx/yyy/g which replaces every occurrence of pattern xxx with string yyy.

Swapping characters

Now, if on the same line you want to replace each " with ' and at the same time each ' with ", you must use the y sed command instead of s:

sed -e "/^KEYWORD_1\|^KEYWORD_2/ y/\"'/'\"/" file

Changing characters

Changing a set of characters to another set of characters is generally a task for the tr command but since you want to do it on certain lines only, it will be best done by sed which has a y command similar to tr:

sed -e "/^KEYWORD_1/  y/\"/'/" \
    -e "/^KEYWORD_2/  y/'/\"/" \
    file

Each sed command here starts with a line selector /^KEYWORD/ which instructs sed to only operate on the line matching the pattern between /. Here the patterns start with the character ^ to indicate they are to be found at the beginning of the line.

Following the line selector is sed's substitution command y/set1/set2/g which replaces every occurrence of a character in set1 with the character which has the same position in set2.

Swapping characters

Now, if on the same line you want to replace each " with ' and at the same time each ' with ", you can use only one command:

sed -e "/^KEYWORD_1\|^KEYWORD_2/  y/\"'/'\"/" file

Changing a set of characters to another set of characters is generally a task for the tr command but since you want to do it only on certain lines, it will be best done by sed indeed:

sed -e "/^KEYWORD_1/ s/\"/'/g" \
    -e "/^KEYWORD_2/ s/'/\"/g" file

Each sed command here starts with a line selector /KEYWORD/ which instructs sed to only operate on the line matching the pattern between /. Here the patterns start with the character ^ to indicate it is to be found at the beginning of the line.

Following the line selector is sed substitution command s/xxx/yyy/g which replaces every occurrence of pattern xxx with string yyy.

Now, if on the same line you want to replace each " with ' and at the same time each ' with ", you must use the y sed command instead of s:

sed -e "/^KEYWORD_1\|^KEYWORD_2/ y/\"'/'\"/" file

Changing characters

Changing a set of characters to another set of characters is generally a task for the tr command but since you want to do it only on certain lines, it will be best done by sed indeed:

sed -e "/^KEYWORD_1/ s/\"/'/g" \
    -e "/^KEYWORD_2/ s/'/\"/g" file

Each sed command here starts with a line selector /KEYWORD/ which instructs sed to only operate on the line matching the pattern between /. Here the patterns start with the character ^ to indicate it is to be found at the beginning of the line.

Following the line selector is sed's substitution command s/xxx/yyy/g which replaces every occurrence of pattern xxx with string yyy.

Swapping characters

Now, if on the same line you want to replace each " with ' and at the same time each ' with ", you must use the y sed command instead of s:

sed -e "/^KEYWORD_1\|^KEYWORD_2/ y/\"'/'\"/" file

Changing a set of characters to another set of characters is generally a task for the tr command but since you want to do it only on certain lines, it will be best done by sed indeed:

sed -e '/^KEYWORD_1/ s/"/\x27/g' \
    -e '/^KEYWORD_2/ s/\x27/"/g' file

Each sed command here starts with a line selector /KEYWORD/ which instructs sed to only operate on the line matching the pattern between /. Here the patterns start with the character ^ to indicate it is to be found at the beginning of the line.

Following the line selector is sed substitution command s/xxx/yyy/g which replaces every occurrence of pattern xxx with string yyy. Here, \x27 represents the single quote (I used this alternate notation because I couldn't use a ' in a string already enclosed in ').

Now, if on the same line you want to replace each " with ' and at the same time each ' with ", you must use the y sed command instead of s:

sed -e '/^KEYWORD_1|^KEYWORD_2/ y/"\x27|\x27"/' file

Changing a set of characters to another set of characters is generally a task for the tr command but since you want to do it only on certain lines, it will be best done by sed indeed:

sed -e "/^KEYWORD_1/ s/\"/'/g" \
    -e "/^KEYWORD_2/ s/'/\"/g" file

Each sed command here starts with a line selector /KEYWORD/ which instructs sed to only operate on the line matching the pattern between /. Here the patterns start with the character ^ to indicate it is to be found at the beginning of the line.

Following the line selector is sed substitution command s/xxx/yyy/g which replaces every occurrence of pattern xxx with string yyy.

Now, if on the same line you want to replace each " with ' and at the same time each ' with ", you must use the y sed command instead of s:

sed -e "/^KEYWORD_1\|^KEYWORD_2/ y/\"'/'\"/" file