Bash: Create a file if it does not exist, otherwise check to see if it is writeable - linux

I have a bash program that will write to an output file. This file may or may not exist, but the script must check permissions and fail early. I can't find an elegant way to make this happen. Here's what I have tried.
set +e
touch $file
set -e
if [ $? -ne 0 ]; then exit;fi
I keep set -e on for this script so it fails if there is ever an error on any line. Is there an easier way to do the above script?


Why complicate things?
file=exists_and_writeable
if [ ! -e "$file" ] ; then
touch "$file"
fi
if [ ! -w "$file" ] ; then
echo cannot write to $file
exit 1
fi
Or, more concisely,
( [ -e "$file" ] || touch "$file" ) && [ ! -w "$file" ] && echo cannot write to $file && exit 1


Rather than check $? on a different line, check the return value immediately like this:
touch file || exit
As long as your umask doesn't restrict the write bit from being set, you can just rely on the return value of touch


You can use -w to check if a file is writable (search for it in the bash man page).
if [[ ! -w $file ]]; then exit; fi


Why must the script fail early? By separating the writable test and the file open() you introduce a race condition. Instead, why not try to open (truncate/append) the file for writing, and deal with the error if it occurs? Something like:
$ echo foo > output.txt
$ if [ $? -ne 0 ]; then die("Couldn't echo foo")
As others mention, the "noclobber" option might be useful if you want to avoid overwriting existing files.


Open the file for writing. In the shell, this is done with an output redirection. You can redirect the shell's standard output by putting the redirection on the exec built-in with no argument.
set -e
exec >shell.out # exit if shell.out can't be opened
echo "This will appear in shell.out"
Make sure you haven't set the noclobber option (which is useful interactively but often unusable in scripts). Use > if you want to truncate the file if it exists, and >> if you want to append instead.
If you only want to test permissions, you can run : >foo.out to create the file (or truncate it if it exists).
If you only want some commands to write to the file, open it on some other descriptor, then redirect as needed.
set -e
exec 3>foo.out
echo "This will appear on the standard output"
echo >&3 "This will appear in foo.out"
echo "This will appear both on standard output and in foo.out" | tee /dev/fd/3
(/dev/fd is not supported everywhere; it's available at least on Linux, *BSD, Solaris and Cygwin.)

Related

Automatically print command output from a separate file

Is it possible to have the output of commands separate_file.sh be visible to the caller of caller.sh automatically with the following setup?
I'm aware of adding >&2 to each of the commands in separate_file.sh, but I'm looking for a more automatic solution.
I'm also aware that the output is visible if I call separate_file.sh like so: separate_file.sh instead of $(separate_file.sh), but I'd like to preserve the -e option in caller.sh (it's used for other calls).
caller.sh:
#!/bin/bash -e
if [[ ! $(separate_file.sh) ]]; then
echo 'separate_file.sh failed'
fi
separate_file.sh:
#!/bin/bash
echo '1'
ls -la

Silent while loop in bash

I am looking to create a bash script that keeps checking a file in directory and perform certain operation on it. I am using while loop, if file does not exists I want that while loop stays quite and keeps on checking condition. Here is what i created but it keeps throwing error that file not found, if file is not there.
while [ ! -f /home/master/applications/tmp/mydata.txt ]
do
cat mydata.txt;
rm mydata.txt;
sleep 1; done

There are two issue in your implementation:
You should use the same (absolute or relative) path in your while loop test statement [ ! -f $file ] and in your cat and rm commands.
The cat command is looking for the file in the current working directory (pwd) and your while statement might be looking somewhere else and hence, your implementation is buggy and won't work as expected if your pwd isn't /home/master/applications/tmp.
You need to move your cat command and rm command after the while block. It doesn't make sense to cat a file if a file doesn't exist. I think your misplaced those commands.
Try this:
file="/home/master/applications/tmp/mydata.txt"
while [ ! -f "$file" ]
do
sleep 1
done
cat $file
rm $file
EDIT
As per suggestion from #Ivan, you could use until instead of while as it suits more to your requirements.
file="/home/master/applications/tmp/mydata.txt"
until [ -f "$file" ]; do sleep 1; done
cat $file
rm $file

Making a different assumption than abhiarora, I'll guess maybe you meant for the file to reappear, and you want it shown each time.
file=/home/master/applications/tmp/mydata.txt
while :
do if [[ -f "$file" ]]
then echo "$(<"$file")"
rm "$file"
fi
sleep 1
done
This creates an infinite loop. If that's NOT what you wanted, use abhiarora's solution.

Bash script to iterate contents of directory moving only the files not currently open by other process

I have people uploading files to a directory on my Ubuntu Server.
I need to move those files to the final location (another directory) only when I know these files are fully uploaded.
Here's my script so far:
#!/bin/bash
cd /var/uploaded_by_users
for filename in *; do
lsof $filename
if [ -z $? ]; then
# file has been closed, move it
else
echo "*** File is open. Skipping..."
fi
done
cd -
However it's not working as it says some files are open when that's not true. I supposed $? would have 0 if the file was closed and 1 if it wasn't but I think that's wrong.
I'm not linux expert so I'm looking to know how to implement this simple script that will run on a cron job every 1 minute.

[ -z $? ] checks if $? is of zero length or not. Since $? will never be a null string, your check will always fail and result in else part being executed.
You need to test for numeric zero, as below:
lsof "$filename" >/dev/null; lsof_status=$?
if [ "$lsof_status" -eq 0 ]; then
# file is open, skipping
else
# move it
fi
Or more simply (as Benjamin pointed out):
if lsof "$filename" >/dev/null; then
# file is open, skip
else
# move it
fi
Using negation, we can shorten the if statement (as dimo414 pointed out):
if ! lsof "$filename" >/dev/null; then
# move it
fi
You can shorten it even further, using &&:
for filename in *; do
lsof "$filename" >/dev/null && continue # skip if the file is open
# move the file
done

You may not need to worry about when the write is complete, if you are moving the file to a different location in the same file system. As long as the client is using the same file descriptor to write to the file, you can simply create a new hard link for the upload file, then remove the original link. The client's file descriptor won't be affected by one of the links being removed.
cd /var/uploaded_by_users
for f in *; do
ln "$f" /somewhere/else/"$f"
rm "$f"
done

Looping through files in different directory given command line argument

I'm trying to extend a script that implements something like a recycling bin for files on Linux. I have the code that I'm extending at the bottom.
In my extension, when the script is presented with the command line argument -cleanup I want to loop through files that are in the /home/7/bearm/.garbage directory, and have the user decide whether they want to delete the file or not.
However, I don't know how to detect when the command line argument is there. The command line can have other parameters, I just want to loop through the files when -cleanup is used.
I also do not know how to loop through files that are in a different directory (/home/7/bearm/.garbage).
How would I go around doing these things?
set directory = '/home/7/bearm/.garbage/'
if(! -d "$directory") then
mkdir .garbage
mv .garbage /home/7/bearm/
endif
set n = 1
while ($n <= $#argv)
set file = $argv[$n]
if(-d $file) then
#do nothing
echo "Cannot trash directory $file"
else
mv $file /home/7/bearm/.garbage
echo "Trashed $file"
endif
# n++
end
du -h /home/7/bearm/.garbage

To test if arguments contains -cleanup, you can do that (tested with ash on Minix3):
if echo "$#" | grep -- "-cleanup" >/dev/null 2>&1; then
echo "-cleanup is present..."
fi
Moreover, if you want a proper solution to use long GNU style options, see http://www.sputnick-area.net/scripts/getopts_long_example.sh and http://www.sputnick-area.net/scripts/getopts_long.sh
A bash version of your pseudo script :
#!/bin/bash
directory='/home/7/bearm/.garbage/'
mkdir -p "$directory"
for arg; do
if [[ -d $arg ]]; then
#do nothing
echo "Cannot trash directory $arg" >&2
else
mv "$arg" "$directory"
echo "Trashed $arg"
fi
done
du -sh "$directory"
Feel free to improve it with -cleanup switch.

Bash - Update terminal title by running a second command

On my terminal in Ubuntu, I often run programs which keep running for a long time. And since there are a lot of these programs, I keep forgetting which terminal is for which program, unless I tab through all of those. So I wanted to find a way to update my terminal title to the program name, whenever I run a command. I don't want to do it manually.
I use gnome-terminal, but answer shouldn't really depend on that. Basically, If I'm able to run a second command, then I can simply use gconftool command to update the title. So I was hoping to find a way to capture the command in bash and update the title after every command. How do I do that?

I have some answers for you :) You're right that it shouldn't matter that you're using gnome-terminal, but it does matter what command shell you're using. This is a lot easier in zsh, but in what follows I'm going to assume you're using bash, and that it's a fairly recent version (> 3.1).
First of all:
Which environment variable would
contain the current 'command'?
There is an environment variable which has more-or-less what you want - $BASH_COMMAND. There's only one small hitch, which is that it will only show you the last command in a pipe. I'm not 100% sure what it will do with combinations of subshells, either :)
So I was hoping to find a way to
capture the command in bash and update
the title after every command.
I've been thinking about this, and now that I understand what you want to do, I realized the real problem is that you need to update the title before every command. This means that the $PROMPT_COMMAND and $PS1 environment variables are out as possible solutions, since they're only executed after the command returns.
In bash, the only way I can think of to achieve what you want is to (ab)use the DEBUG SIGNAL. So here's a solution -- stick this at the end of your .bashrc:
trap 'printf "\033]0;%s\007" "${BASH_COMMAND//[^[:print:]]/}"' DEBUG
To get around the problem with pipes, I've been messing around with this:
function settitle () {
export PREV_COMMAND=${PREV_COMMAND}${#}
printf "\033]0;%s\007" "${BASH_COMMAND//[^[:print:]]/}"
export PREV_COMMAND=${PREV_COMMAND}' | '
}
export PROMPT_COMMAND=${PROMPT_COMMAND}';export PREV_COMMAND=""'
trap 'settitle "$BASH_COMMAND"' DEBUG
but I don't promise it's perfect!

Try this:
trap 'echo -ne "\033]2;$(history 1 | sed "s/^[ ]*[0-9]*[ ]*//g")\007"' DEBUG
Thanks to the history 1 it works even with complicated expressions like:
true && (false); echo $? | cat
For which approaches relying on $BASH_COMMAND or $# fail. For example simon's displays:
true | echo $? | cat
Thanks to Gilles and simon for providing inspiration.

I see what stoutie is trying to do, except it's a lot more work than needed. And doesn't cause all sorts of other potentially bad things that can occur as a result of redefining 'cd' and putting in all of that testing just to change directories. Bash has built in support for most of this.
You can put this in your .bashrc anywhere after you set your current PS1 prompt (this way it just prepends it)
# If this is an xterm set the titlebar to user#host:dir
case "$TERM" in
xterm*|rxvt*)
PS1="\[\e]0;\u#\h: \w\a\]$PS1"
;;
*)
;;
esac

The OP asked for bash, but others might be interested to learn that (as mentioned above) this is indeed a lot easier using the zsh shell. Example:
# Set window title to command just before running it.
preexec() { printf "\x1b]0;%s\x07" "$1"; }
# Set window title to current working directory after returning from a command.
precmd() { printf "\x1b]0;%s\x07" "$PWD" }
In preexec, $1 contains the command as typed (requires shell history to be enabled, which seems to be a fair assumption), $2 the expanded command (shell aliases etc.) and $3 the "very expanded" command (shell function bodies). (more)

I'm doing something like this, to show my pwd in the title, which could be modified to do whatever you want to do with the title:
function title { echo -en "\033]2;$1\007"; }
function cd { dir=$1; if [ -z "$dir" ]; then dir=~; fi; builtin cd "$dir" && title `pwd`; }
I just threw this in my ~/.bash_aliases.
Update
I ran into strange bugs with my original answer. I ended up picking apart the default Ubuntu PS1 and breaking it into parts only to realize one of the parts was the title:
# simple prompt
COLOR_YELLOW_BOLD="\[\033[1;33m\]"
COLOR_DEFAULT="\[\033[0m\]"
TITLE="\[\e]0;\u#\h:\w\a\]"
PROMPT="\w\n$ "
HUH="${debian_chroot:+($debian_chroot)}"
PS1="${COLOR_YELLOW_BOLD}${TITLE}${HUH}${PROMPT}${COLOR_DEFAULT}"
Without breaking into variables, it would look like this:
PS1="\[\033[1;33m\]\[\e]0;\u#\h:\w\a\]${debian_chroot:+($debian_chroot)}\w\n$ \[\033[0m\]"

I have tested three method, all is OK, use any one for your pleasure.
export PROMPT_COMMAND='echo -ne "\033]2;$(history 1 | sed "s/^[ ]*[0-9]*[ ]*//g")\007"'
trap 'echo -ne "\033]2;$(history 1 | sed "s/^[ ]*[0-9]*[ ]*//g")\007"' DEBUG
trap 'echo -ne "\e]0;"; echo -n $BASH_COMMAND; echo -ne "\a"' DEBUG
please note if use $BASH_COMMAND, it don't recognize bash alias, and use PROMPT_COMMAND show finished command, but use trap show running command.

Based on the the need to auto position putty windows I have modified my /etc/bash.bashrc file on a Debian/Ubuntu system. I have posted the full contents for completeness but the relevant bit to starts on the # Display command ... comment line.
# System-wide .bashrc file for interactive bash(1) shells.
# To enable the settings / commands in this file for login shells as well,
# this file has to be sourced in /etc/profile.
# If not running interactively, don't do anything
[ -z "$PS1" ] && return
# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize
# set variable identifying the chroot you work in (used in the prompt below)
if [ -z "${debian_chroot:-}" ] && [ -r /etc/debian_chroot ]; then
debian_chroot=$(cat /etc/debian_chroot)
fi
# set a fancy prompt (non-color, overwrite the one in /etc/profile)
PS1='${debian_chroot:+($debian_chroot)}\u#\h:\w\$ '
# Display command run in title which allows us to distinguish Kitty/Putty
# windows and re-position easily using AutoSizer window utility. Based on a
# post here: http://mg.pov.lt/blog/bash-prompt.html
case "$TERM" in
xterm*|rxvt*)
# Show the currently running command in the terminal title:
# http://www.davidpashley.com/articles/xterm-titles-with-bash.html
show_command_in_title_bar()
{
case "$BASH_COMMAND" in
*\033]0*)
# The command is trying to set the title bar as well;
# this is most likely the execution of $PROMPT_COMMAND.
# In any case nested escapes confuse the terminal, so don't
# output them.
;;
*)
echo -ne "\033]0;${USER}#${HOSTNAME}: ${BASH_COMMAND}\007"
;;
esac
}
trap show_command_in_title_bar DEBUG
;;
*)
;;
esac
# Commented out, don't overwrite xterm -T "title" -n "icontitle" by default.
# If this is an xterm set the title to user#host:dir
#case "$TERM" in
#xterm*|rxvt*)
# PROMPT_COMMAND='echo -ne "\033]0;${USER}#${HOSTNAME}: ${PWD}\007"'
# ;;
#*)
# ;;
#esac
# enable bash completion in interactive shells
if ! shopt -oq posix; then
if [ -f /usr/share/bash-completion/bash_completion ]; then
. /usr/share/bash-completion/bash_completion
elif [ -f /etc/bash_completion ]; then
. /etc/bash_completion
fi
fi
# if the command-not-found package is installed, use it
if [ -x /usr/lib/command-not-found -o -x /usr/share/command-not-found/command-not-found ]; then
function command_not_found_handle {
# check because c-n-f could've been removed in the meantime
if [ -x /usr/lib/command-not-found ]; then
/usr/bin/python /usr/lib/command-not-found -- "$1"
return $?
elif [ -x /usr/share/command-not-found/command-not-found ]; then
/usr/bin/python /usr/share/command-not-found/command-not-found -- "$1"
return $?
else
printf "%s: command not found\n" "$1" >&2
return 127
fi
}
fi

You can set up bash such that it sends a certain escape sequence to the terminal every time it starts an external program. If you use the escape sequence that terminals use to update their titles, your problem should be solved.
I have used that before, so I know it is possible. but I cannot remember it off the top of my head and do not have time to research the details right now, though.

Some of the old methods were removed from gnome-terminal 3.14 due to these two bugs (724110 and 740188).
In Ubuntu 20.04
PS1=$PS1"\[\e]0;New_Terminal_Name\a\]"
\[ begin a sequence of non-printing characters
\e]0; is the char sequence for setting the terminal title. Bash identifies this sequence and set the tile with the following characters. Number 0 turns out to be the value to reference the title property.
New_Terminal_Name is the tile we gave
\a is the ASCII bell character, also in this case, it marks the end of the tile to read from Bash.
\] end a sequence of non-printing characters
We can create a function for future use
function set_title(){
if [ -z "$PS1_BACK" ]; # set backup if it is empty
then
PS1_BACK="$PS1"
fi
TITLE="\[\e]0;$*\a\]"
PS1="${PS1_BACK}${TITLE}"
}
Open the ~/.bashrc file in your home directory with a text editor and append the above function at the end of it. Save and close.
To use it immediately source it to the current terminal.
source ~/.bashrc
We can use it then like this
set_title <New terminal tab title>

My terminal window titler script
This dynamic backgrounded script show all running command with pid number and elapsed time in seconds, like if I run du -h | less, this will build title looking like:
204640 6 du -h | 204641 6 less
Then when no command (other than himself) are running, don't change the terminal title, so standard behaviours works normaly.
First run start backgroud task. Second run in same terminal ask for kill previous backgrounded task.
Save this into a file, set execute flag then run it without argument:
cat <<"EOF" >titleWin.sh
#!/bin/bash
## Ask for kill process if already started
mapfile -t pids < <(ps -C ${0##*/} ho pid)
for pid in ${pids[#]} ;do
if [[ $pid != $$ ]] && [ -d /proc/$pid ]; then
echo -n "STARTED: [$pid]: ${0##*/}. Kill them (Y/n)? "
read -rsn 1 act
case $act in
n|N ) echo No;;
* ) echo Yes;kill $pid ;;
esac
exit
fi
done
## Title win for xterm or screen (or tmux).
case $TERM in
xterm*|rxvt* ) titleFmt='\e];%s\a';;
screen* ) titleFmt='\ek%s\e\\';;
* ) echo "Unable to title window.";exit 1;;
esac
tty=$(tty)
## Date to epochseconds converter
exec {dateout}<> <(:)
exec {datein}> >(exec stdbuf -o0 date -f - +%s >&$dateout)
DPID=$!
trap "echo TRAP;kill $DPID" 1 2 3 6 9 15
# Main loop
while :;do
string=""
while read -r pid wday mon day time year cmd; do
if [[ $pid != $$ ]] && [[ $pid != $PPID ]] && [[ $pid != $BASHPID ]] &&
[[ $pid != $DPID ]] && [ "${cmd#*pid,lstart,cmd}" ] &&
[ -d /proc/$pid ] ;then
echo >&${datein} $wday $mon $day $time $year
read -ru $dateout date
string+="$pid $((EPOCHSECONDS-date)) $cmd | "
fi
done < <(exec ps --tty ${tty#*/dev/} ho pid,lstart,cmd)
[[ "$string" ]] && printf "$titleFmt" "${string% | }"
sleep .333
done &
EOF
chmod +x titleWin.sh
./titleWin.sh

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