Shell Files Output – How to Output Text to Both Screen and File Inside a Shell Script

filesgnu-screenoutputshell

Currently I have a shell script which logs messages to a log file like this:

log_file="/some/dir/log_file.log"
echo "some text" >> $log_file
do_some_command
echo "more text" >> $log_file
do_other_command

When executing this script, there is no output to screen, and, since I'm connecting to the server via putty, I have to open another connection and do "tail -f log_file_path.log", because I can't terminate the running script and I want to see the output in real time.

Obviously, what I want is that the text messages are printed on screen and into file, but I'd like to do it in one line, not two lines, one of which has no redirection to file.

How to achieve this?

Best Answer

This works:

command | tee -a "$log_file"

tee saves input to a file (use -a to append rather than overwrite), and copies the input to standard output as well.

Because the command can detect that it's now being run in a non-interactive fashion this may change its behaviour. The most common side effect is that it disables colour output. If this happens (and you want ANSI colour coded output) you have to check the command documentation to see if it has a way to force it to revert to the interactive behaviour, such as grep --color=always. Beware that this means the log file will also include these escape codes, and you'll need to use less --RAW-CONTROL-CHARS "$log_file" to read it without distracting escape code literals. Also beware that there is no way to make the log file contents different from what is printed to screen when running the above command, so you can't have colour coded output to screen and non-coloured output in the log file.

Related Solutions

Piping output to text file within a for loop

There really is no need for reproducing find's output ina shell loop. If you want to pack the list of filenames, the generic formula is:

find ... | gzip > logfile.gz

If you want to gzip the files themselves, it changes to:

find ... | tar -czvf archive.tar.gz -T -

which tells tar to read list of file names to work on from a file and the single - stands for standard input. (The -T AKA --files-from= option is present in GNU tar, I'm not sure about other flavours.) Of course this breaks if you manage to work on files which contain \n in their names.

Shell – bash / sh script to replace text between some tags/strings in a text file

Use

sed '/#start/,/#end/replace_command'

For example, if the file is called myconfig, and you want to replace "allow" with "deny" in that section, you could say

sed '/#start/,/#end/s/allow/deny/' myconfig

That would leave the file untouched, and display on the standard output what the file would look like after the modification. You should probably do that first, to verify that you've got the command right. If you want to actually change the file, add the -i option:

sed -i '/#start/,/#end/s/allow/deny/' myconfig

If you want to replace the whole text (all the text) between those two lines, you can do something slightly simpler than Lucas's answer:

sed '/#start/,/#end/c\
New text line 1\
New text line 2\
        ︙      \
New text line n-1\
New text line n (last)'                                   ← Close quote; no backslash here

c is the change command in sed (and ed); it means "replace entire line(s)". You cannot simply leave the #start and #end lines untouched. If you want to keep them, you must re-insert them:

sed -i '/#start/,/#end/c\
#start\
FirewallRuleSet global {\
    FirewallRule allow tcp to google.com\
    FirewallRule deny tcp to facebook.com\
                      ︙                 \
\
#more rules\
}\
#end' myconfig

/#start/,/#end/ specifies a range — the lines from the first line that contains #start through the first line after that that contains #end. If you need to find lines that contain those strings and nothing else, use /^#start$/,/^#end$/.

Best Answer

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Piping output to text file within a for loop

Shell – bash / sh script to replace text between some tags/strings in a text file

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