How do I iterate over each line in a file with Bash? - bash

Given a text file with multiple lines, I would like to iterate over each line in a Bash script. I had attempted to use cut, but cut does not accept \n (newline) as a delimiter.
This is an example of the file I am working with:
one
two
three
four
Does anyone know how I can loop through each line of this text file in Bash?

I found myself in the same problem, this works for me:
cat file.cut | cut -d$'\n' -f1
Or:
cut -d$'\n' -f1 file.cut

Use cat for concatenating or displaying. No need for it here.
file="/path/to/file"
while read line; do
echo "${line}"
done < "${file}"

Simply use:
echo -n `cut ...`
This suppresses the \n at the end

cat FILE|while read line; do # 'line' is the variable name
echo "$line" # do something here
done
or (see comment):
while read line; do # 'line' is the variable name
echo "$line" # do something here
done < FILE

So, some really good (possibly better) answers have been provided already. But looking at the phrasing of the original question, in wanting to use a BASH for-loop, it amazed me that nobody mentioned a solution with change of Field Separator IFS. It's a pure bash solution, just like the accepted read line
old_IFS=$IFS
IFS='\n'
for field in $(<filename)
do your_thing;
done
IFS=$old_IFS

If you are sure that the output will always be newline-delimited, use head -n 1 in lieu of cut -f1 (note that you mentioned a for loop in a script and your question was ultimately not script-related).
Many of the other answers, including the accepted one, have multiple lines unnecessarily. No need to do this over multiple lines or changing the default delimiter on the system.
Also, the solution provided by Ivan with -d$'\n' did not work for me either on Mac OSX or CentOS 7. Since his answer is four years old, I assume something must have changed on the logic of the $ character for this situation.

While loop with input redirection and read command.
You should not be using cut to perform a sequential iteration of each line in a file as cut was not designed to do this.
Print selected parts of lines from each FILE to standard output.
— man cut
TL;DR
You should use a while loop with the read -r command and redirect standard input to your file inside a function scope where IFS is set to \n and use -E when using echo.
processFile() { # Function scope to prevent overwriting IFS globally
file="$1" # Any file that exists
local IFS="\n" # Allows spaces and tabs
while read -r line; do # Read exits with 1 when done; -r allows \
echo -E "$line" # -E allows printing of \ instead of gibberish
done < $file # Input redirection allows us to read file from stdin
}
processFile /path/to/file
Iteration
In order to iterate over each line of a file, we can use a while loop. This will let us iterate as many times as we need to.
while <condition>; do
<body>
done
Getting our file ready to read
We can use the read command to store a single line from standard input in a variable. Before we can use that to read a line from our file, we need to redirect standard input to point to our file. We can do this with input redirection. According to the man pages for bash, the syntax for redirection is [fd]<file where fd defaults to standard input (a.k.a file descriptor 0). We can place this before or after our while loop.
while <condition>; do
<body>
done < /path/to/file
# or the non-traditional way
</path/to/file while <condition>; do
<body>
done
Reading the file and ending the loop
Now that our file can be read from standard input, we can use read. The syntax for read in our context is read [-r] var... where -r preserves the \ (backslash) character, instead of using it as an escape sequence character, and var is the name of the variable to store the input in. You can have multiple variables to store pieces of the input in but we only need one to read an entire line. Along with this, to preserve any backslashes in any output from echo you will likely need to use the -E flag to disable the interpretation of backslash escapes. If you have any indentation (spaces or tabs), you will need to temporarily change the IFS (Input Field Separators) variable to only "\n"; normally it is set to " \t\n".
main() {
local IFS="\n"
read -r line
echo -E "$line"
}
main
How do we use read to end our while loop?
There is really only one reliable way, that I know of, to determine when you've finished reading a file with read: check the exit value of read. If the exit value of read is 0 then we successfully read a line, if it is 1 or higher then we reached EOF (end of file). With that in mind, we can place the call to read in our while loop's condition section.
processFile() {
# Could be any file you want hardcoded or dynamic
file="$1"
local IFS="\n"
while read -r line; do
# Process line here
echo -E "$line"
done < $file
}
processFile /path/to/file1
processFile /path/to/file2
A visual breakdown of the above code via Explain Shell.

If I am executing a command and want to cut the output but it has multiple lines I found it helpful to do
echo $([command]) | cut [....]
This puts all the output of [command] on a single line that can be easier to process.

My opinion is that "cut" uses '\n' as its default delimiter.
If you want to use cut, I have two ways:
cut -d^M -f1 file_cut
I make ^M By click Enter After Ctrl+V. Another way is
cut -c 1- file_cut
Does that help?

Related

During a while loop file read, where is the first line of stdin lost?

Assume we have a file with the numbers 1 to 5 written down line by line.
When I open a file for reading as standard input and use 'while read,' commands which can read stdin are unable to read the first line of that file.
$ while read x; do sed ''; done<file
2
3
4
5
It makes no difference which command you use: sed, awk, cat, etc. That problem occurs if the command is able to read from stdin.There is also no difference between the shells I use. I try the same thing in sh, bash, and zsh, and the results are identical.
It's worth noting that the loop iterates five times, once for each line. For example:
$ while read x; do printf 'something\n'; done<file
something
something
something
something
something
I understand that if I want to read all lines correctly, I must specify a variable in the read command and then pass it to the command. But I'm trying to figure out what's going on here. Why does this problem occur when I do not specify input for a command directly?
Perhaps it is a side effect with no functional purpose.
I couldn't find any information about this behavior of the 'while read' statement, and neither did I find anyone who had a similar problem.
Your code only iterates once.
while read x; do sed ''; done<file
...behaves as follows:
file is opened and attached to stdin
read consumes the first line of the file from stdin and puts it into $x
sed '' consumes the entire rest of the file from stdin and prints it to stdout without changes.
read sees there's no more data (because sed consumed it all), and the loop ends.
If you want sed to operate on only the one line that read x consumed, and to safeguard against other bugs, you might instead write:
while IFS= read -r x; do printf '%s\n' "$x" | sed ''; done <file
The changes:
Using IFS= prevents leading or trailing whitespace from being deleted by read.
Using the -r argument prevents backslashes from being consumed by read.
Piping from printf '%s\n' "$x" into sed changes sed's stdin, such that instead of containing the rest of the file, it only contains the one line. Thus, this ensures that sed is processing the line that was consumed by read, instead of ignoring that line and processing the entire rest of the file. (Using printf instead of echo is a correctness concern; see Why is printf better than echo? on UNIX & Linux Stack Exchange).
the first line of stdin is not lost, but rather it is consumed by the shell when the redirection operator '<' is used to redirect the contents of the file to the while loop. The first line is used as the input to initialize the while loop, and subsequent lines are read inside the loop. This is why the first line is not processed by the commands inside the loop. To avoid this, you can redirect the file to a new file descriptor using '<&', as follows:
$ while read x; do sed ''; done <&3 3<file

Reading and writing line by line in a bash script

After searching online I was able to figure out how to read a file line by line:
while read p; do
echo $p
done < file.txt
But I would actually like to modify the line in the file.
For example:
while read p; do
if condition
then
echo $p | perl -i -pe 's/a/b/'
fi
done < file.txt
However this doesn't actually modify the file.
Update   A far better version of bash code added. Thanks to Charles Duffy for comments.
Your Perl one-liner takes a line piped into it by echo $p |, getting its standard input that way. It doesn't do anything with the file itself, so the -i flag has no effect. The -p makes it print to the standard output stream. So that whole line, echo ..., doesn't touch the file.
You can redirect the output to a new file and then move that to overwrite file.txt. Here is a simple minded example, that appends each line to a new file. For better bash code see the update below.
while read p; do
if condition
then
echo $p | perl -pe 's/a/b/' >> temp_out.txt
else
echo $p >> temp_out.txt
fi
done < file.txt
mv temp_out.txt file.txt
We have to add the else where all unmodified lines are also appended. Note that in general we cannot have just some lines replaced but the whole file has to be re-written.
If this is all that the script does you can do it with a very simple one-liner, see the end. If more work is done you can also put it all in a Perl script but I take it that there may be other good reasons for a bash script.
Update   A much better version of the above. See read and echo in Builtins in Bash manual
Appending each line opens the file anew each time without a need for that.
Just redirect at the end of the loop, much like it is done in the terminal
read uses backslash for escaping, removing it from input. Turn that off with -r
Trailing white space is removed, as a part of breaking the line into words. Suppress this by unsetting the variable that controls which characters are used for splitting, IFS=
The echo $p can do all kinds of unintended things. A formatted print is better, printf '%s\n' "$p", or at least echo "$p"
With this,
while IFS= read -r p; do
if condition
then
echo "$p" | perl -pe 's/a/b/'
else
echo "$p"
fi
done < file.txt > temp_out.txt
mv temp_out.txt file.txt
Finally, if the sole purpose of the Perl one-liner were to run a simple substitution, it is much better to simply do that in the shell itself than to have a pipeline and run a whole new process for each line.
echo "${p//a/b}"
Thanks to Charles Duffy for raising all these points in comments.
A few comments on Perl one-liners. See documentation at perlrun.
The command perl -e '...' executes any valid Perl code between ''. When we add the -n or -p switch it also reads standard input and executes that code on a line of it at the time, where -p also prints out each line after it's processed. The standard input can be supplied to it from a file,
perl -pe '...' input.txt
in which case adding -i flag will result in the file being changed in-place. Or, the input can be piped into it, for example
echo "input text" | perl -pe '...'
in which case the processed line is printed to standard output. This can be redirected to a file, as in the answer above.
To make changes to a given file a line at a time you only need this on the command line
perl -i -pe 's/a/b/' file.txt
If there is more work to do then it may well be better to put it in a script, of course. In this case the one-liner can be a command in the bash script as well, replacing all that code above (unless some bash-specific functionality is preferred for processing lines).

Read an input file in shell script and store its lines in a variable

I'm new to UNIX and have this really simple problem:
I have a text-file (input.txt) containing a string in each line. It looks like this:
House
Monkey
Car
And inside my shell script I need to read this input file line by line to get to a variable like this:
things="House,Monkey,Car"
I know this sounds easy, but I just couldnt find any simple solution for this. My closest attempt so far:
#!/bin/sh
things=""
addToString() {
things="${things},$1"
}
while read line; do addToString $line ;done <input.txt
echo $things
But this won't work. Regarding to my google research I thought the while loop would create a new sub shell, but this I was wrong there (see the comment section). Nevertheless the variable "things" was still not available in the echo later on. (I cannot just write the echo inside the while loop, because I need to work with that string later on)
Could you please help me out here? Any help will be appreciated, thank you!
What you proposed works fine! I've only made two changes here: Adding missing quotes, and handling the empty-string case.
things=""
addToString() {
if [ -n "$things" ]; then
things="${things},$1"
else
things="$1"
fi
}
while read -r line; do addToString "$line"; done <input.txt
echo "$things"
If you were piping into while read, this would create a subshell, and that would eat your variables. You aren't piping -- you're doing a <input.txt redirection. No subshell, code works without changes.
That said, there are better ways to read lists of items into shell variables. On any version of bash after 3.0:
IFS=$'\n' read -r -d '' -a things <input.txt # read into an array
printf -v things_str '%s,' "${things[#]}" # write array to a comma-separated string
echo "${things_str%,}" # print that string w/o trailing comma
...on bash 4, that first line can be:
readarray -t things <input.txt # read into an array
This is not a shell solution, but the truth is that solutions in pure shell are often excessively long and verbose. So e.g. to do string processing it is better to use special tools that are part of the “default” Unix environment.
sed ':b;N;$!bb;s/\n/,/g' < input.txt
If you want to omit empty lines, then:
sed ':b;N;$!bb;s/\n\n*/,/g' < input.txt
Speaking about your solution, it should work, but you should really always use quotes where applicable. E.g. this works for me:
things=""
while read line; do things="$things,$line"; done < input.txt
echo "$things"
(Of course, there is an issue with this code, as it outputs a leading comma. If you want to skip empty lines, just add an if check.)
This might/might not work, depending on the shell you are using. On my Ubuntu 14.04/x64, it works with both bash and dash.
To make it more reliable and independent from the shell's behavior, you can try to put the whole block into a subshell explicitly, using the (). For example:
(
things=""
addToString() {
things="${things},$1"
}
while read line; do addToString $line ;done
echo $things
) < input.txt
P.S. You can use something like this to avoid the initial comma. Without bash extensions (using short-circuit logical operators instead of the if for shortness):
test -z "$things" && things="$1" || things="${things},${1}"
Or with bash extensions:
things="${things}${things:+,}${1}"
P.P.S. How I would have done it:
tr '\n' ',' < input.txt | sed 's!,$!\n!'
You can do this too:
#!/bin/bash
while read -r i
do
[[ $things == "" ]] && things="$i" || things="$things","$i"
done < <(grep . input.txt)
echo "$things"
Output:
House,Monkey,Car
N.B:
Used grep to tackle with empty lines and the probability of not having a new line at the end of file. (Normal while read will fail to read the last line if there is no newline at the end of file.)

Read file line by line and perform action for each in bash

I have a text file, it contains a single word on each line.
I need a loop in bash to read each line, then perform a command each time it reads a line, using the input from that line as part of the command.
I am just not sure of the proper syntax to do this in bash. If anyone can help, it would be great. I need to use the line from the test file obtained as a paramter to call another function. The loop should stop when there are no more lines in the text file.
Psuedo code:
Read testfile.txt.
For each in testfile.txt
{
some_function linefromtestfile
}
How about:
while read line
do
echo $line
// or some_function "$line"
done < testfile.txt
As an alternative, using a file descriptor (#4 in this case):
file='testfile.txt'
exec 4<$file
while read -r -u4 t ; do
echo "$t"
done
Don't use cat! In a loop cat is almost always wrong, i.e.
cat testfile.txt | while read -r line
do
# do something with "$line" here
done
and people might start to throw an UUoCA at you.
while read line
do
nikto -Tuning x 1 6 -h $line -Format html -o NiktoSubdomainScans.html
done < testfile.txt
Tried this to automate nikto scan of list of domains after changing from cat approach. Still just read the first line and ignored everything else.

How to read entire line from bash

I have a file file.txt with contents like
i love this world
I hate stupid managers
I love linux
I have MS
When I do the following:
for line in `cat file.txt`; do
echo $line
done
It gives output like
I
love
this
world
I
..
..
But I need the output as entire lines like below — any thoughts ?
i love this world
I hate stupid managers
I love linux
I have MS
while read -r line; do echo "$line"; done < file.txt
As #Zac noted in the comments, the simplest solution to the question you post is simply cat file.txt so i must assume there is something more interesting going on so i have put the two options that solve the question as asked as well:
There are two things you can do here, either you can set IFS (Internal Field Separator) to a newline and use existing code, or you can use the read or line command in a while loop
IFS="
"
or
(while read line ; do
//do something
done) < file.txt
I believe the question was how to read in an entire line at a time. The simple script below will do this. If you don't specify a variable name for "read" it will stuff the entire line into the variable $REPLY.
cat file.txt|while read; do echo $REPLY; done
Dave..
You can do it by using read if the file is coming into stdin. If you need to do it in the middle of a script that already uses stdin for other purposes, you can temporarily reassign the stdin file descriptor.
#!/bin/bash
file=$1
# save stdin to usually unused file descriptor 3
exec 3<&0
# connect the file to stdin
exec 0<"$file"
# read from stdin
while read -r line
do
echo "[$line]"
done
# when done, restore stdin
exec 0<&3
Try
(while read l; do echo $l; done) < temp.txt
read: Read a line from the standard
input and split it into fields.
Reads a single line from the standard input, or from file
descriptor FD
if the -u option is supplied. The line is split into fields as with word
splitting, and the first word is assigned to the first NAME, the second
word to the second NAME, and so on, with any leftover words assigned
to
the last NAME. Only the characters found in $IFS are
recognized as word
delimiters.

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