String substitution to detect empty variables?

Question

I have a script that iterates through files and does some string substitution to insert the date.

#!/bin/bash
f="/tmp/file.txt" # with .txt extension
timestamp="$(date +%H%M%S)"
echo "${f%%.*}-$timestamp.${f#*.}"

It provides the following output, which is correct.

/tmp/file-220304.txt

But if a file doesn't have an extension, the script breaks.

#!/bin/bash
f="/tmp/file" # no extension
timestamp="$(date +%H%M%S)"
echo "${f%%.*}-$timestamp.${f#*.}"

$ ./test.sh
/tmp/file-220359./tmp/file

Isn't it possible to use something like ${f:-not found} to fill in the blank if f is not defined? I can't figure out how to solve the above issue with the below method.

#!/bin/bash
f="/tmp/file" # no extension
timestamp="$(date +%H%M%S)"
echo "${f%%.*}-$timestamp.${f#*.:-not found}"

Results:

$ ./test.sh
/tmp/file-221420./tmp/file

Stéphane Chazelas · Accepted Answer · 2020-08-10 15:26:21Z

You'd also have problems with /tmp/dir.d/file files.

With zsh:

#! /bin/zsh -
f="/tmp/file.txt" # with .txt extension
timestamp=${(%):-%D{%H%M%S}}

set -o extendedglob
extension=${(M)f%.[^./]#}
print -r -- ${f%$extension}-$timestamp$extension

With standard sh syntax (would also work in bash/zsh):

#! /bin/sh -
f="/tmp/file.txt" # with .txt extension
timestamp=$(date +%H%M%S)

case ${f##*/} in
  (*.*) printf '%s\n' "${f%.*}-$timestamp.${f##*.}";;
  (*)   printf '%s\n' "$f-$timestamp";;
esac

With bash, you can replace timestamp=$(date +%H%M%S) with printf -v timestamp '%(%H%M%S)T' to avoid running that date command.

fpmurphy · Accepted Answer · 2020-08-10 15:40:31Z

1

If you want a one-liner, here is a demo of a somewhat ineligant but workable solution:

#!/bin/bash

# Test cases
FILE="/tmp/file"
# FILE="/tmp/file."
# FILE="/tmp/file.txt"
# FILE="/tmp/file.txt."

# TIMESTAMP="$(date +%H%M%S)"
printf -v TIMESTAMP '%(%H%M%S)T'

echo "${FILE%%.*}-${TIMESTAMP}.$([[ $FILE =~ \..*$ ]] && { echo "${FILE#*.}"; } || { echo "txt"; })"

The above adds .txt to the filename if missing an extension.

Use the following one-liner if you do not want a default extension added to a filename if missing an extension.

echo "${FILE%%.*}-${TIMESTAMP}$([[ $FILE =~ \..*$ ]] && { echo ".${FILE#*.}"; })"

edited Aug 10, 2020 at 15:40

answered Aug 10, 2020 at 5:33

fpmurphy

4,7463 gold badges25 silver badges26 bronze badges

@alecxs Because I got the impression that is what the OP wanted. Easily modified to something else or no extension at all.

fpmurphy
– fpmurphy

2020-08-10 13:27:22 +00:00
Commented Aug 10, 2020 at 13:27
1

@alecxs. If the filename contains a trailing dot, it is preserved.

fpmurphy
– fpmurphy

2020-08-10 15:18:29 +00:00
Commented Aug 10, 2020 at 15:18

Add a comment |

alecxs · Accepted Answer · 2020-08-10 09:13:49Z

0

to keep it simple

#!/bin/bash
f="/tmp/file"
timestamp="$(date +%H%M%S)"
out="${f}-${timestamp}";
[ "${f%.*}" != "${f##*.}" ] && out="${f%.*}-${timestamp}.${f##*.}";
echo "$out"

to make it safe against . in path

dir=${f%/*};
file=${f##*/};
name=${file%.*};
suffix=${file##*.};
[ "$dir" != "$file" ] && dir=${dir}/ || unset dir;
[ "$name" != "$suffix" ] && suffix=.${suffix} || unset suffix;
echo "${dir}${name}-${timestamp}${suffix}"

remove the newlines if you need it in one line
note: i recommend to use full timestamp (date + time) to ensure it is unique

edited Aug 10, 2020 at 9:13

answered Aug 10, 2020 at 8:56

alecxs

5944 silver badges10 bronze badges

however, that still fails for txt/txt or txt.txt

alecxs
– alecxs

2020-08-10 09:40:09 +00:00
Commented Aug 10, 2020 at 9:40

Add a comment |

ilkkachu · Accepted Answer · 2020-08-10 15:37:18Z

It's not that f isn't defined, it's that ${f#*.} is the value of f with the the part until the first dot removed. If there is no dot, nothing is removed and ${f#*.} is just the same as $f. Also, since it matches up to the first dot, you get issues with paths like ./foo.txt (it gives -181548./foo.txt).

You'd need to either test if ${f#*.} equals $f or use something like ${f#${f%.*}}, which becomes empty if there is no dot-separated suffix. The counterpart for that would be ${f%.${f##*.}}. Though you'll still have issues with ./file again.

The problem with ./file suggests splitting the filename part from the path first, and proceeding from there.

An alternative approach would use regexes. This works for the filenames I tested:

#!/bin/bash
f=$1
timestamp="$(date +%H%M%S)"
re='^((.*/)?[^/.]+)(\.[^/]+)?$'
if [[ $f =~ $re ]]; then
    echo "${BASH_REMATCH[1]}-${timestamp}${BASH_REMATCH[3]}"
fi

That's (.*/)? -- optional path ending with a slash; [^/.]+ -- initial part of filename; (\.[^/]+)? -- optional extension.

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