TIL 1:
Incrementing times with the date command
I have been working on an HPR project which is restoring externalfiles to shows which lost them when we migrated to the current static
site.
As I make changes I want to be able to check that they are correct,but to make this check I need to wait for the next update of the static
site.
Ken was away recently, and set up a cron job to refreshthe site every three hours. Each show page shows the refresh time in UTC
form in the header.
Being a bit numerically challenged I wanted a way of computing thenext refresh time in my timezone from the previous refresh time.
The GNU date command accepts a date and time expressionafter the -d option (or using the
alternative--date=STRING option). The contents of the
STRING here are very flexible but quite complex since you
can include time zone data, offsets, day and month names, etc. See the
links below for links to the GNU manual.
My first attempt used the date command like this andgot the wrong answer (using the output format +%T which
writes the time in a default form):
$ date -d '16:27:16 + 3 hours' +%T
15:27:16
It is not clear why this fails, but the GNU function which parsesthese date parameters is obviously confused. The second try included the
time zone after the time, and worked better, but is a little
confusing:
$ date -d '16:27:16 UTC + 3 hours' +%T
20:27:16
The time returned is local time for me. The datecommand has added three hours to the UTC date to get 19:27:16, but since
I am in the UK, which is in DST (called BST - British Summer Time - UTC
plus 1 hour), an hour is added.
The final try used the -u option which writes UTCtime:
$ date -u -d '16:27:16 UTC + 3 hours' +%T
19:27:16
I actually ended up using and re-using these commands (though ascript would have been better):
$ current='06:27:55'
$ next=$(date -u -d "${current}UTC + 3 hours 3 minutes" +%T); echo "$next UTC / $(date -d "${next} UTC" +'%T %Z')"
09:30:55 UTC / 10:30:55 BST
$ current=$next
$ next=$(date -u -d "${current}UTC + 3 hours 3 minutes" +%T); echo "$next UTC / $(date -d "${next} UTC" +'%T %Z')"
12:33:55 UTC / 13:33:55 BST
$ current=$next
$ next=$(date -u -d "${current}UTC + 3 hours 3 minutes" +%T); echo "$next UTC / $(date -d "${next} UTC" +'%T %Z')"
15:36:55 UTC / 16:36:55 BST
TIL 1: Links
GNUCoreutils manual
Systemcontext
date:Print or set system date and time
Dateinput formats (The quotation at the start of this section of the
manual is quite interesting!)
Dateexamples
TIL 2: Merging lines of
files with paste
While processing and "repairing" shows I came across the need togenerate a list of show numbers separated by commas. In the past I have
loaded these into a Bash array and turned them into a comma-delimited
string, using the parameter substitution capabilities of Bash which can
add a comma to each element. The trouble with this is that it leaves a
trailing comma which has to be removed. I stumbled upon
paste as an alternative way of doing this.
The GNU paste command is another from the GNUCoreutils group. This one merges lines of files. Its synopsis
is:
paste [OPTION]... [FILE]...
It merges lines consisting of the corresponding lines from each fileprovided as an argument, by default separated by TABs, and writes them
to standard output. This means it produces lines consisting of the first
line from each of the files, separated by tabs, then the second lines,
and so on.
Any of the files can be linked to standard in by using a- (hyphen) as the file name.
The delimiters can be changed with the -d LIST or--delimiters=LIST option. The use of a list of delimiters
causes the characters in the list to be used sequentially for each
delimiter.
The merged lines can be visualised as rows in a matrix, where eachfile provides a column.
The "matrix" is rotated by using the -s or--serial option. Here the lines from one file at a time are
merged - the files are processed serially rather than in
parallel.
The paste command can be used to generate thecomma-delimited list I wanted by using the options -s and
-d ',':
$ printf '%s\n' {1..10} | paste -s -d, -
1,2,3,4,5,6,7,8,9,10
Note that you can't get the same result withecho {1..10} because all the numbers will be written to one
line rather than being the separate lines that paste
requires.
The file arguments to paste may also be Bashprocess substitution expressions:
$ paste -s -d, <(printf '%s\n' {1..10})
1,2,3,4,5,6,7,8,9,10
This means you can generate more complex output by using multipleprocess substitution expressions where each is seen as a
file:
$ paste -d'|' <(printf '%d\n' {1..7}) <(printf '%s\n' {A..G}) <(printf '%d\n' {100..106})
1|A|100
2|B|101
3|C|102
4|D|103
5|E|104
6|F|105
7|G|106
Note how using the -s option "rotates" this: $ paste -s -d'|' <(printf '%d\n' {1..7}) <(printf '%s\n' {A..G}) <(printf '%d\n' {100..106})
1|2|3|4|5|6|7
A|B|C|D|E|F|G
100|101|102|103|104|105|106
This is what I was actually trying to do, so I could feed shownumbers to another script which will accept a CSV list as an
option:
$ echo "select episode_id from repairs where repair_date is null order by episode_id desc limit 5" |\
sqlite3 -list ~/HPR/InternetArchive/ia.db | paste -s -d',' -
1959,1952,1951,1946,1941
TIL 2: Links
GNUCoreutils manual
Operatingon fields
paste:Merge lines of files
