In several scripts that process files matched by name pattern I needed to add a check for file existence. Just to illustrate let's say I need to process all N??? files:
Code:
/tmp$ touch N100 N101
/tmp$ l ?10[01]
-rw-rw-r-- 1 moss group 0 Apr 19 11:22 N100
-rw-rw-r-- 1 moss group 0 Apr 19 11:22 N101
/tmp$ for fn in N???; do echo $fn; ls -l $fn; done
N100
-rw-rw-r-- 1 moss group 0 Apr 19 11:22 N100
N101
-rw-rw-r-- 1 moss group 0 Apr 19 11:22 N101
If there is no N files, this for loop would try to execute ls with non-existent pattern:
Code:
/tmp$ rm N???
/tmp$ for fn in N???; do echo $fn; ls -l $fn; done
N???
ls: N??? not found: No such file or directory (error 2)
To avoid running a process against non-existent file I add if file exists check:
Code:
/tmp$ for fn in N???; do if [ -f $fn ]; then ls -l $fn; fi; done
/tmp$
Is there any way in shell to not use those file exists checks and still run process only for an exisant file?
Could you please try following and let me know this helps you.
Code:
for fn in N???; do if [[ -f $fn ]]; then echo $fn; ls -l $fn; fi; done
Above will not print anything errors on standard output. Although you could simply use following too.
Code:
for fn in N???; do echo $fn; ls -l $fn; done 2> /dev/null
But only thing above will print value N??? even it is not present but no error will be printed on standard output, so in case you don't want to print filename then you could directly use as follows.
I added the echo in the loop to show that when nothing matches the N??? pattern the fn variable assumes value of N??? and that is the root of the question - when using
Code:
for fname in some_pattern
construct I thought the pattern evaluation happens before do and shell would simply not enter the do loop part when nothing matches.
I added the echo in the loop to show that when nothing matches the N??? pattern the fn variable assumes value of N??? and that is the root of the question - when using
Code:
for fname in some_pattern
construct I thought the pattern evaluation happens before do and shell would simply not enter the do loop part when nothing matches.
Some, shells have an option (as an extension to the standards) that does remove globbing patterns that do not match any existing files. For instance, if you are using a recent bash, the code:
Code:
shopt -s nullglob
for i in N???
do printf '%s\n' "$i"
done
would not run the loop at all if N??? does not match any files in the current working directory, while the other options work with any shell based on Bourne shell syntax:
Code:
for i in N???
do if [ "$i" = "N???" ]
then break
fi
printf '%s\n' "$i"
done
which works fine as long as you know that there will never be a file actually named N???. (Note that the break could be replaced by a continue and get the same results.) Or, you can use:
Code:
for i in N???
do if [ -e "$i" ]
then continue
fi
printf '%s\n' "$i"
done
(Note that you need to use continue in this form (not break) to protect yourself in case a file is removed while the loop is running (you just want to skip processing the file that was removed; not skip processing other remaining files with names that sort after the file that was removed).
These 2 Users Gave Thanks to Don Cragun For This Post:
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04-19-2016
