I have a while loop that reads from the below text file:
hashicorp/aws/4.27.0
hashicorp/aws/4.26.0
hashicorp/aws/4.11.0
...
The while loop that reads from it is as follows:
while read -r owner provider version; do \
IFS=/
echo $owner
done < providers.txt
output:
hashicorp aws 4.27.0
hashicorp
hashicorp
It for some reason seems to not use the set IFS on the first line.
However, I can get the correct result by moving the IFS:
while IFS=/ read -r owner provider version; do \
echo $owner
done < providers.txt
output:
hashicorp
hashicorp
hashicorp
But I don't understand why in the first instance it doesn't use the IFS correctly as IFS=/ is still being set to before the echo statement. Can someone please explain?
It's because the first version doesn't set IFS to "/" until after the first line has been read and split. When and where you set a variable determines what it affects, and it's important to understand the differences.
Walking through what gets executed might help. When the script reaches the while loop, it first executes the while clause:
while read -r owner provider version
...which reads the first line from the file. Since IFS still has its default value (space + tab + newline) at this point, the line is split based on that (i.e. not split at all), so the entire line goes in the owner variable.
Next, since the while clause succeeded, the body of the while loop executes:
IFS=/
echo $owner
...this sets IFS and then echoes $owner. Since $owner is not double-quoted, and by the time that runs IFS has been set to "/", the value gets word-split based on "/", so what's effectively executed is echo "hashicorp" "aws" "4.27.0", which sort-of has the effect of turning the "/" characters in the variable into spaces. This sort of weird parsing effect is why you should almost always double-quote variable references.
Next, the loop runs again. This time IFS has been set to "/", so the read command will split the line based on that, and from this point on it'll behave as you expect.
Now, compare that to the second version:
while IFS=/ read -r owner provider version; do
Here, the assignment to IFS isn't a separate command, it's a prefix to the read command. This means it always applies to the read command, and also makes it apply only to the read command. So it applies every time read runs, and also it doesn't affect other things later, like any unquoted variable references. Setting IFS to a nonstandard value can cause trouble, and restricting it to a single command like this is a good way to keep it from causing trouble later.
Lessons: 1) Double-quote your variables, and 2) set IFS as a prefix to specific commands when possible (when that's not possible, set it before the command it needs to affect, and then set it back to normal as soon as possible afterward).
The IFS variable in bash controls how words are split by the shell. By default this is space, tab, and newline. The reason that you set IFS is so when wordspliting happens on your read call, the words are split by / instead of spaces. This variable is not relevant to the echo call because the owner variable already contains just the word hashicorp.
Related
I'm writing a shell script intended to edit audio files using the sox command. I've been running into a strange problem I never encountered in bash scripting before: When defining space separated effects in sox, the command will work when that effect is written directly, but not when it's stored in a variable. This means the following works fine and without any issues:
sox ./test.in.wav ./test.out.wav delay 5
Yet for some reason the following will not work:
IFS=' ' # set IFS to only have a tab character because file is tab-separated
while read -r file effects text; do
sox $file.in.wav $file.out.wav $effects
done <in.txt
...when its in.txt is created with:
printf '%s\t%s\t%s\n' "test" "delay 5" "other text here" >in.txt
The error indicates this is causing it to see the output file as another input.
sox FAIL formats: can't open input file `./output.wav': No such file or directory
I tried everything I could think of: Using quotation marks (sox "$file.in.wav" "$file.out.wav" "$effects"), echoing the variable in-line (sox $file.in.wav $file.out.wav $(echo $effects)), even escaping the space inside the variable (effects="delay\ 5"). Nothing seems to work, everything produces the error. Why does one command work but not the other, what am I missing and how do I solve it?
IFS does not only change the behavior of read; it also changes the behavior of unquoted expansions.
In particular, unquoted expansions' content are split on characters found in IFS, before each element resulting from that split is expanded as a glob.
Thus, if you want the space between delay and 5 to be used for word splitting, you need to have a regular space, not just a tab, in IFS. If you move your IFS assignment to be part of the same simple command as the read, as in IFS=$'\t' read -r file effects text; do, that will stop it from changing behavior in the rest of the script.
However, it's not good practice to use unquoted expansions for word-splitting at all. Use an array instead. You can split your effects string into an array with:
IFS=' ' read -r -a effects_arr <<<"$effects"
...and then run sox "$file.in.wav" "$file.out.wav" "${effects_arr[#]}" to expand each item in the array as a separate word.
By contrast, if you need quotes/escapes/etc to be allowed in effects, see Reading quoted/escaped arguments correctly from a string
I need to rename the files inside the folder that has a space in it eg(Deco/main library/file1.txt )
code:
while IFS="," read orig new pat
do
mv -v $pat$new $pat$orig
done < new.csv
csv file:
newname,file1.txt,Deco/main\\\ library/
error:
mv: invalid option -- '\'
Welcome to Stackoverflow!
First: Use quotes around the use of variables. That means except in very rare occasions, you always should use "$foo" instead of $foo because if you are using the latter, the shell is supposed (and will) interpret spaces in the variables as word delimiters which you rarely want. Especially in your case you do not want it.
Second: Your CSV file seems to contain backslashes to quote the spaces. And some additional step seems to have added another level of quotation so than now you end up with three backslashes and a space for each original space. If this really is the case (please double check if what you wrote in your question is correct, otherwise my answer doesn't fit), you need to unquote this before you can use it.
There are security issues involved in using eval, so do not use it lightly (this disclaimer is necessary whenever proposing to use eval), but if you have trust in the input you are handling to not contain any nastinesses, then you can do this using this code:
while IFS="," read orig new pat
do
eval eval mv -v "$pat$new" "$pat$orig"
done < new.csv
Using this, two levels of quotation are evaluated (that's what eval does) before the mv command is executed.
I strongly suggest to do a dry run by adding echo before the mv first. Then instead of executing your commands they are merely printed first.
I've written a bunch of environment variables in Docker format, but now I want to use them outside of that context. How can I source them with one line of bash?
Details
Docker run and compose have a convenient facility for importing a set of environment variables from a file. That file has a very literal format.
The value is used as is and not modified at all. For example if the value is surrounded by quotes (as is often the case of shell variables), the quotes are included in the value passed
Lines beginning with # are treated as comments and are ignored
Blank lines are also ignored.
"If no = is provided and that variable is…exported in your local environment," docker "passes it to the container"
Thankfully, whitespace before the = will cause the run to fail
so, for example, this env-file:
# This is a comment, with an = sign, just to mess with us
VAR1=value1
VAR2=value2
USER
VAR3=is going to = trouble
VAR4=this $sign will mess with things
VAR5=var # with what looks like a comment
#VAR7 =would fail
VAR8= but what about this?
VAR9="and this?"
results in these env variables in the container:
user=ubuntu
VAR1=value1
VAR2=value2
VAR3=is going to = trouble
VAR4=this $sign will mess with things
VAR5=var # with what looks like a comment
VAR8= but what about this?
VAR9="and this?"
The bright side is that once I know what I'm working with, it's pretty easy to predict the effect. What I see is what I get. But I don't think bash would be able to interpret this in the same way without a lot of changes. How can I put this square Docker peg into a round Bash hole?
tl;dr:
source <(sed -E -e "s/^([^#])/export \1/" -e "s/=/='/" -e "s/(=.*)$/\1'/" env.list)
You're probably going to want to source a file, whose contents
are executed as if they were printed at the command line.
But what file? The raw docker env-file is inappropriate, because it won't export the assigned variables such that they can be used by child processes, and any of the input lines with spaces, quotes, and other special characters will have undesirable results.
Since you don't want to hand edit the file, you can use a stream editor to transform the lines to something more bash-friendly. I started out trying to solve this with one or two complex Perl 5 regular expressions, or some combination of tools, but I eventually settled on one sed command with one simple and two extended regular expressions:
sed -E -e "s/^([^#])/export \1/" -e "s/=/='/" -e "s/(=.*)$/\1'/" env.list
This does a lot.
The first expression prepends export to any line whose first character is anything but #.
As discussed, this makes the variables available to anything else you run in this session, your whole point of being here.
The second expression simply inserts a single-quote after the first = in a line, if applicable.
This will always enclose the whole value, whereas a greedy match could lop off some of (e.g.) VAR3, for example
The third expression appends a second quote to any line that has at least one =.
it's important here to match on the = again so we don't create an unmatched quotation mark
Results:
# This is a comment, with an =' sign, just to mess with us'
export VAR1='value1'
export VAR2='value2'
export USER
export VAR3='is going to = trouble'
export VAR4='this $sign will mess with things'
export VAR5='var # with what looks like a comment'
#VAR7 ='would fail'
export VAR8=' but what about this?'
export VAR9='"and this?"'
Some more details:
By wrapping the values in single-quotes, you've
prevented bash from assuming that the words after the space are a command
appropriately brought the # and all succeeding characters into the VAR5
prevented the evaluation of $sign, which, if wrapped in double-quotes, bash would have interpreted as a variable
Finally, we'll take advantage of process substitution to pass this stream as a file to source, bring all of this down to one line of bash.
source <(sed -E -e "s/^([^#])/export \1/" -e "s/=/='/" -e "s/(=.*)$/\1'/" env.list)
Et voilà!
This question already has answers here:
Why a variable assignment replaces tabs with spaces
(2 answers)
Closed 7 years ago.
I'm having some troubles with the printf function in bash.
I wrote a little script on which I pass a name and two letters (such as "sh", "py", "ht") and it creates a file in the current working directory named "name.extension".
For instance, if I execute seed test py a file named test.py is created in the current working dir with the shebang #!/usr/bin/python3.
So far, so good, nothing fancy: I'm learning shell scripting and I thought this could be a simple exercise to test the knowledge gained so far.
The problem is when I want to create an HTML file. This is the function that I use:
creaHtml(){
head='<!--DOCTYPE html-->\n<html>\n\t<head>\n\t\t<meta charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>'
percorso=$CARTELLA_CORRENTE/$NOME_FILE.html
printf $head>>$percorso
chmod 755 $percorso
}
If I run, for instance, seed test ht the correct function (creaHtml) is called, test.html is created but if I try to look into it I only see:
<!--DOCTYPE
And nothing else.
This is the trace for that function:
[sviluppo:~/bin]$ seed test ht
+ creaHtml
+ head='<!--DOCTYPE html-->\n<html>\n\t<head>\n\t\t<meta charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>'
+ percorso=/home/sviluppo/bin/test.html
+ printf '<!--DOCTYPE' 'html-->\n<html>\n\t<head>\n\t\t<meta' 'charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>'
+ chmod 755 /home/sviluppo/bin/test.html
+ set +x
However, if I try to run printf '<!--DOCTYPE html-->\n<html>\n\t<head>\n\t\t<meta charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>' from the terminal, I see the correct output: the "skeleton" of an HTML file neatly displayed with indentation and everything. What am I missing here?
Try echo -e instead of printf. printf is for printing formatted strings. Since you didn't protect $head with quotes, bash splits the string to form the command. The first word (before first white space) forms the format string. The rest are just arguments for things you didn't specify to print.
echo -e "$head" > "$percorso"
The -e evaluates your \n into newlines. I changed your >> to > since it looks like you want this to be the whole file, rather than append to any existing file you might have.
You have to be careful with quotes in bash. One thing can become many things. This actually makes it more powerful, but it can be confusing for people learning. Notice that I also put the file name "$percorso" in double quotes too. This evaluates the variable and makes sure that it ends up as one thing. If you use single quotes, it will be one word, but not evaluated. Unlike Python, there is a big difference between single and double quotes.
If you want to use printf for compatibility as #chepner pointed out, just be sure to quote it:
printf "$head" > "$percorso"
Actually that is much simpler anyway.
A config file that the last line contains data that I want to assign everything to the RIGHT of the = sign into a variable that I can display and call later in the script.
Example: /path/to/magic.conf:
foo
bar
ThisOption=foo.bar.address:location.555
What would be the best method in a bash shell script to read the last line of the file and assign everything to the right of the equal sign? In this case, foo.bar.address:location.555.
The last line always has what I want to target and there will only ever be a single = sign in the file that happens to be the last line.
Google and searching here yielded many close but non-relative results with using sed/awk but I couldn't come up with exactly what I'm looking for.
Use sed:
variable=$(sed -n 's/^ThisOption=//p' /path/to/magic.conf)
echo "The option is: $variable")
This works by finding and removing the ThisOption= marker at the start of the line, and printing the result.
IMPORTANT: This method absolutely requires that the file be trusted 100%. As mentioned in the comments, anytime you "eval" code without any sanitization there are grave risks (a la "rm -rf /" magnitude - don't run that...)
Pure, simple bash. (well...using the tail utility :-) )
The advantage of this method, is that it only requires you to know that it will be the last line of the file, it does not require you to know any information about that line (such as what the variable to the left of the = sign will be - information that you'd need in order to use the sed option)
assignment_line=$(tail -n 1 /path/to/magic.conf)
eval ${assignment_line}
var_name=${assignment_line%%=*}
var_to_give_that_value=${!var_name}
Of course, if the var that you want to have the value is the one that is listed on the left side of the "=" in the file then you can skip the last assignment and just use "${!var_name}" wherever you need it.