What's the proper way to do a for loop over a file mask?
E.g. if the mask doesn't expand to any file, then the loop should not run; else, it should run over all the files that it expands to.
The problem with naive approach is that if the * doesn't expand to anything, then the whole loop will run once as if the * part is an actual part of a filename (of a file that doesn't actually exist).
One way to do this is:
for f in abc*; do if [[ -f "$f" ]]; then
# Do something with "$f"
fi; done
That will also filter directories out of the list, which might or might not be what you want.
If you want to keep directories, use -e instead of -f.
Another way to do it is to set the shell option nullglob (may not be available on all bash versions). With nullglob set, patterns which don't match any filename will result in nothing instead of being unaltered.
shopt -s nullglob
for f in abc*; do
# do something with $f
done
shopt -u nullglob
(That leaves nullglob set for the duration of the for loop. You can unset it inside the loop instead of waiting for the end, at the smallish cost of executing the shopt -u on every loop.)
Use the nullglob shell option to make a wildcard that doesn't match anything expand into nothing instead of returning the wildcard itself:
shopt -s nullglob
for file in abc*
do
...
done
Related
I am very new to writing bash scripts. This question is likely very basic, but I have not been able to find a clear answer yet. I'm working on an Ubuntu subsystem installation on Windows 10.
I'm running a script that contains the following conditional:
if [ -z "$date1" ]; then
date1=$(head -n 1 "$dir"/*.txt | sed "s/^[^0-9]*//g" | date +%Y%m%d -f - 2>/dev/null)
fi
It runs into issues when it encounters a directory (the dir variable) that has no .txt file, but I don't quite understand the nature of the problem. I do know the issue is in the head command, at least partially. I don't get an error, the script just stalls when it reaches a directory without a .txt file. I want the script to simply move on. If I run the line on its own (without the conditional) in the terminal, I get a No such file or directory error, which makes sense. What really confuses me is that if I place quotes (single or double) around the wildcard portion (i.e. '*.txt'), then the script spits out the head error and moves on. My limited and perhaps incorrect understanding is that the quotes in this case mean the program no longer treats the * as a wildcard and simply looks for a file by the literal name *.txt. But I thought that when the * was interpreted by bash that it first looks for any possible expansion and then tries the literal interpretation if it finds none. So why does the script stall in one case and not the other. Shouldn't both simply give me the same No such file or directory error, as they do when run outside the script?
I'll also mention that the script includes preceding conditionals that first look for .docx files and only moves on to .txt files when there are no .docx files. It handles the cases where there are no .docx files perfectly well, although the first command in that pipe is unzip rather than head. This question seems relevant, but since the script is able to move on when there are quotes around the wildcard, and since it moves on in the similar scenario where there are no .docx files, I wanted to understand what the issue is here and the best way to fix it.
I appreciate your help.
In quotes, * will not expand and will be a literal * character.
On the other hand, when * tries to expand and fails, one of three things happens:
it is interpreted literally as the string *.txt (plus whatever $dir/ expands to)
You can enforce this behavior with shopt -u nullglob, which should be the default.
it expands to nothing, making the string $dir/*.txt equal to the empty string
You can enforce this behavior with shopt -s nullglob.
it raises an error
You can enforce this behavior with shopt -s failglob (or turn it off with shopt -u failglob).
Examples:
bash-5.0# shopt -s | grep glob
globasciiranges on
bash-5.0# echo *.asdf
*.asdf
bash-5.0# shopt -s nullglob
bash-5.0# echo *.asdf
bash-5.0# shopt -u nullglob
bash-5.0# echo *.asdf
*.asdf
bash-5.0# shopt -s failglob
bash-5.0# echo *.asdf
bash: no match: *.asdf
bash-5.0# shopt -s | grep glob
failglob on
globasciiranges on
When the glob expands to the empty string, head will hang forever unless you enter stdin (head $(echo '') | cat will never complete unless you type)
I have extglob set and dotglob unset.
.* also yields . and .., very evil in conjunction with mv or cp.
I played around a bit and found that *(?(.)+([^.])) and $(ls -A) give the desired result, but I think there should be an easier way...
EDIT: Sorry, I should have mentioned that I am looking for an expression to be used at the prompt, not within a script.
unset GLOBIGNORE # empty-by-default, but let's make sure
shopt -s dotglob # disable special handling for "hidden" files
# ...and with the above items both done:
files=( * ) # just an example use of a glob
...sets the array files to contain all objects in the current directory except . and ..; any other use of * would behave similarly.
When I pass a flag containing [...] to my bash script, getops gives me an empty string when I try to grab the value with $OPTARG.
shopt -s nullglob
while getopts ":f:" opt; do
case $opt in
f)
str=$OPTARG
;;
esac
done
echo ${str}
Running the script:
$ script.sh -f [0.0.0.0]
<blank line>
How can I get the original value back inside the script?
Short summary: Double-quote your variable references. And use shellcheck.net.
Long explanation: When you use a variable without double-quotes around it (e.g. echo ${str}), the shell tries to split its value into words, and expand anything that looks like a wildcard expression into a list of matching files. In the case of [0.0.0.0], the brackets make it a wildcard expression that'll match either the character "0" or "." (equivalent to [0.]). If you had a file named "0", it would expand to that string. With no matching file(s), it's normally left unexpanded, but with the nullglob set it expands to ... null.
Turning off nullglob solves the problem if there are no matching files, but isn't really the right way do it. I remember (but can't find right now) a question we had about a script that failed on one particular computer, and it turned out the reason was that one computer happened to have a file that matched a bracket expression in an unquoted variable's value.
The right solution is to put double-quotes around the variable reference. This tells the shell to skip word splitting and wildcard expansion. Here's an interactive example:
$ str='[0.0.0.0]' # Quotes aren't actually needed here, but they don't hurt
$ echo $str # This works without nullglob or a matching file
[0.0.0.0]
$ shopt -s nullglob
$ echo $str # This fails because of nullglob
$ shopt -u nullglob
$ touch 0
$ echo $str # This fails because of a matching file
0
$ echo "$str" # This just works, no matter whether file(s) match and/or nullglob is set
[0.0.0.0]
So in your script, simply change the last line to:
echo "${str}"
Note that double-quotes are not required in either case $opt in or str=$OPTARG because variables in those specific contexts aren't subject to word splitting or wildcard expansion. But IMO keeping track of which contexts it's safe to leave the double-quotes off is more hassle than it's worth, and you should just double-quote 'em all.
BTW, shellcheck.net is good at spotting common mistakes like this; I recommend feeding your scripts through it, since this is probably not the only place you have this problem.
Assuming that shopt -s nullglob is needed in the bigger script.
You can temporary disable shopt -s nullglob using shopt -u nullglob
shopt -s nullglob
shopt -u nullglob
while getopts ":f:" opt; do
case $opt in
f)
str=$OPTARG
;;
esac
done
echo ${str}
shopt -s nullglob
If I have a file name with spaces and a random set of numbers that looks like this:
file name1234.csv
I want to rename it to this (assuming date is previously specified):
file_name_${date}.csv
I am able to do it like this:
mv 'file name'*'.csv file_name_${date}.csv
However, in a situation that 'file name*.csv' can actually match multiple files, I want to specify that it's 'file name[random numbers].csv'
I've searched around and can't find any relevant answers.
You need what is called a "pathname expansion", to match one or more digits:
+([0-9])
A functional script could be like this one:
date=$(date +'%Y-%m-%d')
shopt -s extglob nullglob
for f in 'file name'+([[:digit:]]).csv; do
file="${f%%[0-9]*}"
echo mv "$f" "${file// /_}_${date}.csv"
done
Warning: all files found will be renamed to just one name, make sure that that is what you want before removing the echo.
To activate the extended version of "Pathname Expansion" we use shopt -s extglob.
To avoid the case where no file is matched, we also need the nullglob set.
We can set the positional arguments to the result of the above expansion.
Then we loop over all files found to change each of their names.
The ${f%%[0-9]*} removes all from the digits to the end.
The ${file// /_} replaces spaces with underscores.
The mv is not actually done with the script presented because of the echo.
If after running a test, you want the change(s) performed, remove the echo.
Use Extended Globs and Parameter Expansion
You can do what you want with Bash extended globs and a few parameter expansions, without resorting to external or non-standard utilities.
date="2016-11-21"
shopt -s extglob
for file in 'file name'+([[:digit:]]).csv; do
newfile="${file%%[0-9]*}"
newfile="${newfile// /_}"
mv "$file" "${newfile}_${date}.csv"
done
I am making a shell script that allows you to select a file from a directory using YAD. I am doing this:
list='';
exc='!'
for f in "$SHOTS_NOT_CONVERTED_DIR"/*;do
f=`basename $f`
list="${list}${exc}${f}"
done
The problem is that if there are no files in that directory, I end up with a selection with *.
What's the easiest, most elegant way to make this work in Bash?
The goal is to have an empty list if there are no files there.
* expansion is called a glob expressions. The bash manual calls it filename expansion.
You need to set the nullglob option. Doing so gives you an empty result if the glob expression does not find files:
shopt -s nullglob
list='';
exc='!'
for f in "$SHOTS_NOT_CONVERTED_DIR"/*;do
# Btw, use $() instead of ``
f=$(basename "$f")
list="${list}${exc}${f}"
done