School of Computing. Dublin City University.
Online coding site: Ancient Brain
Remember pipes and redirection on command line.
This example uses sed to do a "search and replace" operation on text.
cat file | grep -i "http:.*dcu.ie" | sed -e "s|COMPAPP|computing|g" | sed -e "s|compapp|computing|g" | sort -u
UNIX separates "ordinary output" (standard output, stdout, 1>, >) from "error output" (standard error, stderr, 2>) prog > output 2> errors
> /dev/null to get rid of some unwanted output e.g. error/warning messages from compilation/search (redirects it into a non-existent file)The null device exists on Windows too.
script > file
exec > fileThen run:
ls (gives multi-column output) ls > file (gives single-column output) ls | prog (gives single-column output)You can detect if output is going to terminal, file or pipe, and adjust output accordingly.
if [ -t 1 ] then echo stdout else echo pipe or file fi
prog prog | cat prog > file
$1 1st argument $* all arguments $0 name of prog $# no. of args shift shift args leftwards this is useful if you want to remove some of the first args, then "shift" a couple of times, and then do "for i in $*" with the remaining args e.g. grep (switches) (string) file1 ... filen exit 20 exit with a return code $? return code of last prog executed e.g. quiet grep: grep > /dev/null and then check $? though grep may have -q (quiet) option anyway
Like global vars for all programs.
Note any environment vars that are declared within a program are local to that program only.
env printenv set may display shell functions too var=value set environment variable N.B. no spaces! echo var print the string "var" echo $var print value of environment variable echo $HOME get into the habit of using these instead of the actual hard-coded values, - makes scripts more portable echo path is $PATH echo $USER
echo `hostname` recall backquotes uname show hardware, OS, etc. arch same as "uname -m" echo `arch` recall backquotes
Example of using arch in config files:
set path = ( $home/bin/`arch` ... )
echo print something on screen, followed by new line echo -n print with no new line printf print with no new line printf "\n" print with new line On some platforms, echo -e exists (interpret special backslash chars) On DCU Linux: echo -e "\n text \n\n" print multiple new lines echo "string" echo 'string' It is useful to have 2 choices for string - single quote and double quote. If using one for something else, surround with the other. e.g. To search for single quote in file: grep ' file syntax error (why?) grep "'" file surround with quotes and it works grep '"' file to search for the double quote itself The 2 forms of string are not equal: echo "--$HOME--" --/users/group/humphrys-- echo '--$HOME--' --$HOME-- echo '--'$HOME'--' --/users/group/humphrys--
* all normal (non-hidden) files .* all hidden files "Hidden" in the sense that tools like ls won't list them by default. You can write your own tools of course to always list them by default. Not actually hidden in the security sense. Done for convenience not strict security, like write-protecting your own files (which is itself a kind of security since it stops some accidents). Doing things to * like rm * won't affect the .* files.
Q. Even on a single-user system, why separate OS files from user files?
echo * echo all files echo f* all files beginning with f echo */* files in next layer */*/* etc.Important to realise it is the shell that interprets "*" and passes the result to echo or ls or your program. It is not actually echo or ls itself that parses it.
To see that it is the shell that expands it, assign it to an environment variable. Try these:grep string * # grep does not understand * # but that's fine because grep does not actually RECEIVE * # what happens is: # the shell EXPANDS * to a list of files and passes these to grep # so grep actually receives: grep string f1 f2 .. fn
echo * echo "*" x=* echo $x echo "$x" x="*" echo $x echo "$x" x=`echo *` echo $x echo "$x"
Q. Why have the shell interpret "*"? Why not just pass "*" as argument to progs?