I'm trying to understand a shell code which includes a line like this:
grep -n data file1.txt > file2.txt
Where data is the text i want to search for.
What does this command mean?
You can have a detailled answer here: http://explainshell.com/explain?cmd=%20grep%20-n%20data%20file1.txt%20%3E%20file2.txt
To sum it up:
grep will look for the string data in file1.txt and will output both the matching lines and their line number (because of the -n flag).
You could read the manual (man grep) to have a better understanding of what grep does.
The output will be redirected into file2.txt; that's what > is used for
Related
This question already has answers here:
How can I use a file in a command and redirect output to the same file without truncating it?
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Closed 3 years ago.
I'm testing basic bash command on Ubuntu, when I put on terminal
wc -l < file1.txt > file2.txt
all lines from file1 are read by wc and the number is saved to file2.txt.
However if I use the similar command wc -l < file1.txt > file1.txt but I try to save it in the same > file1 the output is always 0.
What is the reason of this behavior ?
When you write the command wc -l < file1.txt > file1.txt, your shell will first read the command as a whole and redirect the standard input, output from/to file1.txt.
Then when a file is open for writing using >, if a file exists it will be overwritten and an empty file is created. the process (your wc) will then start its execution and it will write in it until the end of the execution.
Your wc -l will however have to read the file and count the number of lines in it, as the file is empty (overwritten) it will just dump in it:
0 file1.txt
To prove this look at what happens in appending mode using >>
$ cat file1.txt
a
b
b
$ wc -l file1.txt
3 file1.txt
$ wc -l file1.txt >> file1.txt
$ cat file1.txt
a
b
b
3 file1.txt
Your file content is intact and wc does properly read the number of lines before appending the result to the file.
Note:
In general, it is never recommended to write and read in the same file except if you know exactly what you are doing. Some commands have an inline option to modify the file during the run of the command and it is highly recommended to use them, as the file will not be corrupted even if the command crash in the middle of the execution.
Its because redirections have higher priority than other commands.
Here the file1 will be emptied before any operations can be performed on it
Try >> on file1.txt instead of >. >> will append to file unlike > overwriting whole file. Try -
wc -l < file1.txt >> file1.txt
I need to append a file in a specific location of another file.
I got the line number so, my file is:
file1.txt:
I
am
Cookie
While the second one is
file2.txt:
a
black
dog
named
So, after the solution, file1.txt should be like
I
am
a
black
dog
named
Cookie
The solution should be compatible with the presence of characters like " and / in both files.
Any tool is ok as long as it's native (I mean, no new software installation).
Another option apart from what RavinderSingh13 suggested using sed:
To add the text of file2.txt into file1.txt after a specific line:
sed -i '2 r file2.txt' file1.txt
Output:
I
am
a
black
dog
named
Cookie
Further to add the file after a matched pattern:
sed -i '/^YourPattern/ r file2.txt' file1.txt
Could you please try following and let me know if this helps you.
awk 'FNR==3{system("cat file2.txt")} 1' file1.txt
Output will be as follows.
I
am
a
black
dog
named
Cookie
Explanation: Checking here if line number is 3 while reading Input_file named file1.txt, if yes then using system utility of awk which will help us to call shell's commands, then I am printing the file2.txt with use of cat command. Then mentioning 1 will be printing all the lines from file1.txt. Thus we could concatenate lines from file2.txt into file1.txt.
How about
head -2 file1 && cat file2 && tail -1 file1
You can count the number of lines to decide head and tail parameters in file1 using
wc -l file1
I am running a script that has been working fine. However, yesterday, I got a couple errors. These errors are after several loops of the script:
sed: cant read file3.txt: No such file or directory
grep: file3.txt: No such file or directory
grep: file3.txt: No such file or directory
sed: cant read file3.txt: No such file or directory
grep: file3.txt: No such file or directory
Keep in mind, these errors do not happen consistently. It's occurring once in a while somewhere near this part of the script. File3.txt is the file not being found:
cat file1.txt | while read LINE; do grep -m 1 $LINE file2.txt >> file3.txt; done
sed -i 's/string//g' file3.txt
grep 'string' file3.txt | cut -d '|' -f1-2 > file4.txt
grep -v 'string' file3.txt | cut -d '|' -f1-2 >> file5.txt
sed -i 's/string//' file3.txt
grep -Fvf file3.txt file1.txt > file6.txt
Now, I'm thinking that since file3.txt is being appended, or later operated on by SED, sometimes the next command starts too soon and it can't find the file? Should I put a wait command in between?
I have looked up many pages with this error, but was unable to find anything:
cat file_name | grep "something" results "cat: grep: No such file or directory" in shell scripting
Pipe multiple commands to a single command with no EOF signal wait
grep command works in command line, but not in bash script: get no such file or directory erro
https://serverfault.com/questions/169539/sed-cant-find-a-file-that-obviously-exists
"No such file or directory" but it exists
If you think that putting a wait or sleep command will help, please let me know. Or, if you think there's a better solution, that would be great too. I'm running on Cygwin terminal. Any insight is greatly appreciated.
Instead of redirecting to file3.txt inside the while loop, redirect the whole loop. Then the file will be created even if the loop never runs because the input file is empty.
while read LINE; do
grep -m 1 $LINE file2.txt
done < file1.txt > file3.txt
If file1.txt is ever empty then file3.txt won't be created.
Also do grep -m 1 $LINE file2.txt will cause problems if there are crucial characters (space is the easiest of them).
Let's assume that the $LINE variable contains more than one word separated by spaces: hello world.
Now the command looks like this: grep -m 1 hello world file2.txt - grep interpretation will look something like this: let's find all hello in file named world and file named file2.txt in current folder.
Using "$LINE" instead of $LINE will lead you to a whole different scenario.
Look at the difference between the following two:
grep -m 1 $LINE file2.txt
grep -m 1 "$LINE" file2.txt
I am looking to compare file1.txt contents with the last n file contents of file2.txt Can someone help in identifying this logic using anything in shell script.
Example if file1.txt has 10 lines, the last 10 lines of file2.txt should be compared for difference.
With bash's process substitution <() and command substitution $().
diff file1.txt <(tail -n $(wc -l < file1.txt) file2.txt)
I have practiced in using command line in CentOS6.
I tried to create file, which content would be the output of command man grep. Also I used command man with col -b option to save file as Text-Only. All of this must be in one command.
I tried to do like this:
grep man grep | col -b > output.txt
But it didn't work.
What is the proper way to save output of command man grep as Text-Only file with using option col -b?
Don't you really need this:
man grep | col -b > output.txt
Why do you need to call grep in the first place?
Other, hacky way using grep:
man grep | grep -v somephrasethatwontoccur | col -b > output.txt
But, truly, it makes no sense. grep -v looks for lines without the specified phrase.