I a new to bash but I am trying to write a bash script which does the following:
write_to_file()
{
#check if file exists
# if not create the file
# else open the file to edit
# go in a while loop
# ask input from user
# write to the end of the file
# until user types ":q"
}
If anyone can point out the literature, I would be very thankful Thanks
Update: As it's a bash question, you should try this first. ;)
cat <<':q' >> test.file
To understand what is going on, read about bash's IO redirection, heredoc syntax and the cat command
As you see above, there are many ways to do it. To explain some more bash commands I've prepared the function also in the way you've requested it:
#!/bin/bash
write_to_file()
{
# initialize a local var
local file="test.file"
# check if file exists. this is not required as echo >> would
# would create it any way. but for this example I've added it for you
# -f checks if a file exists. The ! operator negates the result
if [ ! -f "$file" ] ; then
# if not create the file
touch "$file"
fi
# "open the file to edit" ... not required. echo will do
# go in a while loop
while true ; do
# ask input from user. read will store the
# line buffered user input in the var $user_input
# line buffered means that read returns if the user
# presses return
read user_input
# until user types ":q" ... using the == operator
if [ "$user_input" = ":q" ] ; then
return # return from function
fi
# write to the end of the file. if the file
# not already exists it will be created
echo "$user_input" >> "$file"
done
}
# execute it
write_to_file
while loop. >> already obeys the correct semantics.
[ "$foo" == "bar" ] isn't POSIX-compliant; POSIX sh only allows = inside of [ ]... and if you're not going to try to be compatible with POSIX, you should be using [[ ]] (which is both more capable and less error-prone).Example with basic argument checks:
write_to_file()
{
while [ "$line" != ":q" ]; do
read line
if [ "$line" != ":q" ]; then
printf "%s\n" "$line" >> "$1"
fi
done
}
if [ "$#" -eq 1 ]; then
write_to_file "$1"
else
echo "Usage: $0 FILENAME"
exit 2
fi
Or using the probably lesser known until construct, the function can be written a bit more terse:
# append to file ($1) user supplied lines until `:q` is entered
write_to_file()
{
until read line && [ "$line" = ":q" ]; do
printf "%s\n" "$line" >> "$1"
done
}
== inside of [ ]; it is not POSIX-compatible (the compatible syntax is only a single =), and doesn't have the extra features and safety conferred by using the non-POSIX [[ ]] extension.touch "$1" and write_to_file "$1"; the quotes are necessary to avoid string-splitting and glob expansion.echo $line >> $1 won't work right with a a line such as -e foo (the -e will be treated as an echo argument), and will collapse whitespace ("foo bar" will be changed to "foo bar").
This quick example should get you started:
while true
do
read INPUT
if [[ "${INPUT}" == :q ]]
then
return
fi
echo "${INPUT}" >> file
done
file only once, when beginning the loop, instead of re-opening it every time one is ready to write another line.[[ ]] (unlike [ ]) implicitly prevents string-splitting or glob expansion.[[ ]] is special syntax. Quotes are genuinely optional there. (If you have more questions on the subject, btw, feel free to pop in in freenode's #bash channel).There are several solutions here that are working way too hard. Just do:
write_to_file() { sed '/^:q$/q' | sed '$d' >>"$1"; }
where the first argument is the name of the file. That is, invoke it as:
write_to_file test.file
man bash.man bashis more of a reference manual, and a bit challenging for beginners. I think Fraz needs a tutorial. Try tldp.org/LDP/abs/html.!absfactoid in Freenode's#bashchannel: The infamous "Advanced" Bash Scripting Guide should be avoided unless you know how to filter out the junk. It will teach you to write bugs, not scripts. In that light, the BashGuide was written: mywiki.wooledge.org/BashGuide