Working with dates in the shell often causes problems. Aside from the usual typos, this is usually due to mismatched language and country settings. In practice, you will especially be confronted with this when working with databases.

Manual input can be relatively easily checked for formal correctness. The simplest aspect to change is the separators or the order of day, month, and year. An example of this is shown in Listing 1, and the corresponding output is shown in Figure 1. The short script supports both correct input with the dot as a separator and input with the comma in the numeric keypad for quick data entry.

#!/bin/bash
# Comma as field separator
# Output in format YYYY-MM-DD
# Input
read -p "Enter date: " input
# Replace comma with dot for uniformity
input=$(echo $input | tr \, \. )
# Output to variable with Awk, minus sign as separator
output=$(echo $input | awk -F \. '{ print $3 "-" $2 "-" $1 } ')
echo "Date converted: $output"

Figure 1: Using a script to convert a date format. Date formats are defined by standards such as DIN 5008 (for Germany) [1] and ISO 8601 (international) [2].

dialog and YAD

If you attach importance to user-friendliness as well as correct date values, why not use the attractive (semi-)graphical input options offered by dialog or YAD?

With dialog [3], simple date input takes the basic form of:

dialog --calendar 0 0

The two zeros automatically set the size to match the terminal. In the calling shell script, dialog's various exit codes can be used for control, as shown in Listing 2. In the resulting mask (Figure 2), you can use the tab and arrow keys to navigate and press the Enter key to complete the entry.

#!/bin/bash
date=$(dialog --stdout \
  --title "Calendar" \
  --calendar "Select date" 0 0)
if [ $? -eq 0 ]; then
  datum=$(echo $date | tr \/ \.)
  echo "Input date: $date"
elif [ $? -eq 1 ]; then
  echo "Input canceled"
fi

Figure 2: The dialog date mask in a terminal window.

YAD [4] lets you output a shell script's user dialog in the GUI. Because of the very extensive options, you might want to read the YAD's documentation or at least excerpts of it. The input is conveniently mouse controlled. In the example shown in Listing 3, I replaced the standard buttons with my own, resulting in different exit codes (i.e., 2 for input). You can see the output of the script in Figure 3.

#!/bin/bash
date=$(yad --title=Calendar \
  --text="Select date" \
  --calendar --show-weeks \
  --button="Cancel":1 \
  --button="Intput":2)
if [ $? -eq 2 ]; then
  date=$(echo $date | tr \/ \.)
  echo "Input date: $date"
elif [ $? -eq 1 ]; then
  echo "Cancel entry"
fi

Figure 3: A YAD date entry in the GUI.

smenu

Display redirection via SSH does not work everywhere. In addition, displaying dialog masks can cause problems with some terminal settings. However, there is another method you can use if it is important to avoid incorrect entries (such as April 31). You specify the possible values with smenu [5].

For the calendar year, the user can select the current, previous, and next year in Listing 4 (line 7). The selection for the day works in the same way (line 17). To save space in the terminal display, smenu's -n1 option is used to limit the selection to one line. You can use the arrow keys to select the day. A cal command (line 16) determines beforehand which specifications are permissible for it. The script extracts additional specifications such as the month, year, and weekday from this. To make the date suitable for further processing, single-digit day specifications are given a leading zero (lines 19 to 20). (Figure 4 shows the script output from Listing 4.)

01 #!/bin/bash
02 # Year: current, last, next
03 ayear=$(date +%Y)
04 vyear=$(echo $ayear -1 | bc)
05 fyear=$(echo $ayear + 1 | bc)
06 # Use Smenu to select months
07 year=$(echo $vyear $ayear $fyear | smenu -m "Select calendar year")
08 echo ""
09 echo "------------------------"
10 echo ""
11 monat=$(echo 01 02 03 04 05 06 07 08 09 10 11 12 | smenu -m "Select month" )
12 echo "------------------------"
13 # Create header for selection
14 menuhead=$(cal $month $year | head -1)
15 # Calendar field
16 calendar=$(cal $month $year | tail -6 | tr -d [:alpha:])
17 day=$(echo $calendar | smenu -m "Use arrow keys to select day" -t1 -n1 )
18 # Prepend zero to single-figure days
19 if [ $(echo $day | wc -L) -eq 1 ]; then
20   day=$(echo "0$day")
21 fi
22 date=$(echo $day.$month.$year)
23 echo $date

Figure 4: The script output from Listing 4.

Shell Calendars

To display a calendar, you can use cal or the spruced-up, newer ncal variant, which the popular distributions already preinstall. The options for both programs can be found in Table 1. Called with the -C parameter, ncal behaves exactly like cal. Figure 5 shows the two resulting calendars, which display three months along with the week numbers.

Figure 5: A three-month calendar output with ncal (top) and cal (bottom).