Ubuntu – How to use sed to delete timestamp pattern from a zero-terminated list of filenames

The shell is responsible for expanding variables. When you use single quotes for strings, its contents will be treated literally, so sed now tries to replace every occurrence of the literal $var1 by ZZ.

Using double quotes

Use double quotes to make the shell expand variables while preserving whitespace:

sed -i "s/$var1/ZZ/g" "$file"

When you require the quote character in the replacement string, you have to precede it with a backslash which will be interpreted by the shell. In the following example, the string quote me will be replaced by "quote me" (the character & is interpreted by sed):

sed -i "s/quote me/\"&\"/" "$file"

Using single quotes

If you've a lot shell meta-characters, consider using single quotes for the pattern, and double quotes for the variable:

sed -i 's,'"$pattern"',Say hurrah to &: \0/,' "$file"

Notice how I use s,pattern,replacement, instead of s/pattern/replacement/, I did it to avoid interference with the / in \0/.

Example

The shell then runs the above command sed with the next arguments (assuming pattern=bert and file=text.txt):

-i
s,bert,Say hurrah to &: \0/,
text.txt

If file.txt contains bert, the output will be:

Say hurrah to bert: \0/

grep does that job well enough.

$ echo "::=BEGIN" > testfile1.txt

$ grep "::=BEGIN" -q testfile1.txt && echo "FOUND"
FOUND

$ grep "::=BEGIN" -q testfile1.txt && echo "FOUND" || echo "NOTFOUND"
FOUND

$ grep "whever" -q testfile1.txt && echo "FOUND" || echo "NOTFOUND"
NOTFOUND

What the code does is simply running a quiet search within subshell. && , or logical AND operand, checks if the first command succeeded, and if it did, then it runs echo "FOUND" . || checks if that echo has been run, i.e., if we found anything. Second echo runs only if the first command failed.

Here's an awk version:

$ awk '{i="NOTFOUND";if($0~/::=BEGIN/) i="FOUNDIT";exit } END { print i}' testfile1.txt
FOUNDIT

$ awk '{i="NOTFOUND";if($0~/whatevs/) i="FOUNDIT";exit } END { print i}' testfile1.txt
NOTFOUND

Basic idea here is to set i to NOTFOUND, and if we find the string - change it to FOUNDIT . At the end after the first set of has finished processing file, we will print i, and it will tell us if we found something or not.

Edit: In the comment you mentioned that you want to place the result into a variable. Shell already provides a variable that reports exit status of the previous command, $0. Exit status 0 means success, everything else - fail. For instance,

$ grep "::=BEGIN" -q testfile1.txt  
$ echo $?
0

If you want to use exit status in a variable, you can do it like so : MYVAR=$(echo $?). Remember , $? report exit status of previous command. So you must use this right after the grep command.

If you want to still use my earlier awk and grep one-liners, you can use same trick MYVAR=$() and place whatever command you want between the braces. The output will get assigned to variable MYVAR.