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Shell Programming and Scripting
Tried many options but unable to delete blank lines from text file
Post 303000961 by durden_tyler on Monday 24th of July 2017 11:17:02 AM
07-24-2017
Quote:
You are essentially trying to read from a file, make some changes and write back to it. It doesn't work the way it looks. (*)
A few options are:
1) If your sed version allows it, try the "in-place edit" modifier; something like "sed -i". Check your sed manual "man sed".
2) Use a temporary file as you did for the "tr" command.
======
(*)
As for why it doesn't work the way it looks - you have two different processes (a) a read process and (b) a write process, both sharing the same resource - your file. When they are executed by the OS, a race condition occurs between them which renders the final output unpredictable.
If the writing is done before the reading, you get a zero-byte file. If the writing is done mid-way during the reading process, you may get a "half-baked" output file. The safest way, therefore, is to either use a temporary file explicitly or let sed do it via the "-i" modifier - which is not available in all sed versions.
You could also use other tools - Bash, awk, Perl etc. but the temporary file will be involved, either explicitly or implicitly.
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anacrontab
ANACRONTAB(5) Anacron Users' Manual ANACRONTAB(5) NAME
/etc/anacrontab - configuration file for anacron DESCRIPTION
The file /etc/anacrontab describes the jobs controlled by anacron(8). Its lines can be of three kinds: job-description lines, environment assignments, or empty lines. Job-description lines are of one of these two forms: period delay job-identifier command @period_name delay job-identify command The period is specified in days, the delay in minutes. The job-identifier can contain any non-blank character, except slashes. It is used to identify the job in Anacron messages, and as the name for the job's timestamp file. The command can be any shell command. The fields can be separated by blank spaces or tabs. The period_name can only be set to monthly at the present time. This will ensure jobs are only run once a month, no matter the number of days in this month, or the previous month. Environment assignment lines are of the form: VAR = VALUE Spaces around VAR are removed. No spaces around VALUE are allowed (unless you want them to be part of the value). The assignment takes effect from the next line to the end of the file, or to the next assignment of the same variable. Empty lines are either blank lines, line containing white-space only, or lines with white-space followed by a '#' followed by an arbitrary comment. You can continue a line onto the next line by ending it with a ''. SEE ALSO
anacron(8) The Anacron README file. AUTHOR
Itai Tzur <itzur@actcom.co.il> Currently maintained by Pascal Hakim <pasc@(debian.org|redellipse.net)>. Pascal Hakim 2004-07-11 ANACRONTAB(5)