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 

Answer 1

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.

Answer 2

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#*.}"; })" 

Answer 3

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

Answer 4

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.