I'm trying to run a program with each of my script's arguments passed as a different line on stdin. My current attempt involves using a for loop, as follows:
#!/bin/bash
[My Program] <<EOF
for i in "$#"
do
$i
done
EOF
This doesn't work -- the program actually receives for i in as part of its input, instead of being given the list of arguments themselves. How would I change it to function?
To feed your program's stdin a newline-separated list of the command-line arguments with which your script was called:
#!/usr/bin/env bash
./your-program <<<"$(printf '%s\n' "$#")"
...or, with POSIX sh-compatible heredoc syntax:
#!/bin/sh
./your-program <<EOF
$(printf '%s\n' "$#")
EOF
If for some reason you really want to use a for loop, you can do that:
#!/bin/sh
./your-program <<EOF
$(for i; do
echo "$i"
done)
EOF
...though note that printf would be a preferable replacement to echo even here; to understand why, see the APPLICATION USAGE section of the POSIX spec for echo.
I have this bash script to prepend timestamp to each entry of a log file but I am stuck at converting it into korn shell syntax. I read korn shell doesn't like subshell. I tried to utilize function but that didn't work. In particular, I tried to convert the entire exec line into korn syntax. Could someone please take a look and help me?
#!/usr/bin/bash
exec > >(
while read line ; do
echo "$(date '+%Y%m%d %H:%M:%S') ${line}"
done > n.log
) 2>&1
echo 'first line; should have an initial timestamp'
sleep 2
echo 'printed two seconds later, should have a timestamp with a comparable offset'
The portable thing -- which will work in any POSIX-compliant shell -- is to use a named FIFO.
Here, a ksh extension (printf %()T) also available in recent bash is used to avoid needing to launch date inside the subshell.
mkfifo log.fifo
(while IFS= read -r line; do
printf '%(%Y%m%d %H:%M:%S)T '
printf '%s\n' "$line"
done >n.log <log.fifo) &
exec >log.fifo
A little expansion on Charles' excellent answer: Make it more explicit
by creating separate functions for the output modification and for the main functional code; without named pipes, there's nothing to clean up after you're done.
print_with_timestamps() {
(while IFS= read -r line; do
printf '%(%Y%m%d %H:%M:%S)T '
printf '%s\n' "$line"
done
) > n.log 2>&1
}
main() {
echo 'first line; should have an initial timestamp'
sleep 2
echo 'printed two seconds later, should have a timestamp with a comparable offset'
}
main | print_with_timestamps
Is the output of a Bash command stored in any register? E.g. something similar to $? capturing the output instead of the exit status.
I could assign the output to a variable with:
output=$(command)
but that's more typing...
You can use $(!!)
to recompute (not re-use) the output of the last command.
The !! on its own executes the last command.
$ echo pierre
pierre
$ echo my name is $(!!)
echo my name is $(echo pierre)
my name is pierre
The answer is no. Bash doesn't allocate any output to any parameter or any block on its memory. Also, you are only allowed to access Bash by its allowed interface operations. Bash's private data is not accessible unless you hack it.
Very Simple Solution
One that I've used for years.
Script (add to your .bashrc or .bash_profile)
# capture the output of a command so it can be retrieved with ret
cap () { tee /tmp/capture.out; }
# return the output of the most recent command that was captured by cap
ret () { cat /tmp/capture.out; }
Usage
$ find . -name 'filename' | cap
/path/to/filename
$ ret
/path/to/filename
I tend to add | cap to the end of all of my commands. This way when I find I want to do text processing on the output of a slow running command I can always retrieve it with ret.
If you are on mac, and don't mind storing your output in the clipboard instead of writing to a variable, you can use pbcopy and pbpaste as a workaround.
For example, instead of doing this to find a file and diff its contents with another file:
$ find app -name 'one.php'
/var/bar/app/one.php
$ diff /var/bar/app/one.php /var/bar/two.php
You could do this:
$ find app -name 'one.php' | pbcopy
$ diff $(pbpaste) /var/bar/two.php
The string /var/bar/app/one.php is in the clipboard when you run the first command.
By the way, pb in pbcopy and pbpaste stand for pasteboard, a synonym for clipboard.
One way of doing that is by using trap DEBUG:
f() { bash -c "$BASH_COMMAND" >& /tmp/out.log; }
trap 'f' DEBUG
Now most recently executed command's stdout and stderr will be available in /tmp/out.log
Only downside is that it will execute a command twice: once to redirect output and error to /tmp/out.log and once normally. Probably there is some way to prevent this behavior as well.
Inspired by anubhava's answer, which I think is not actually acceptable as it runs each command twice.
save_output() {
exec 1>&3
{ [ -f /tmp/current ] && mv /tmp/current /tmp/last; }
exec > >(tee /tmp/current)
}
exec 3>&1
trap save_output DEBUG
This way the output of last command is in /tmp/last and the command is not called twice.
Yeah, why type extra lines each time; agreed.
You can redirect the returned from a command to input by pipeline, but redirecting printed output to input (1>&0) is nope, at least not for multiple line outputs.
Also you won't want to write a function again and again in each file for the same. So let's try something else.
A simple workaround would be to use printf function to store values in a variable.
printf -v myoutput "`cmd`"
such as
printf -v var "`echo ok;
echo fine;
echo thankyou`"
echo "$var" # don't forget the backquotes and quotes in either command.
Another customizable general solution (I myself use) for running the desired command only once and getting multi-line printed output of the command in an array variable line-by-line.
If you are not exporting the files anywhere and intend to use it locally only, you can have Terminal set-up the function declaration. You have to add the function in ~/.bashrc file or in ~/.profile file. In second case, you need to enable Run command as login shell from Edit>Preferences>yourProfile>Command.
Make a simple function, say:
get_prev() # preferably pass the commands in quotes. Single commands might still work without.
{
# option 1: create an executable with the command(s) and run it
#echo $* > /tmp/exe
#bash /tmp/exe > /tmp/out
# option 2: if your command is single command (no-pipe, no semi-colons), still it may not run correct in some exceptions.
#echo `"$*"` > /tmp/out
# option 3: (I actually used below)
eval "$*" > /tmp/out # or simply "$*" > /tmp/out
# return the command(s) outputs line by line
IFS=$(echo -en "\n\b")
arr=()
exec 3</tmp/out
while read -u 3 -r line
do
arr+=($line)
echo $line
done
exec 3<&-
}
So what we did in option 1 was print the whole command to a temporary file /tmp/exe and run it and save the output to another file /tmp/out and then read the contents of the /tmp/out file line-by-line to an array.
Similar in options 2 and 3, except that the commands were exectuted as such, without writing to an executable to be run.
In main script:
#run your command:
cmd="echo hey ya; echo hey hi; printf `expr 10 + 10`'\n' ; printf $((10 + 20))'\n'"
get_prev $cmd
#or simply
get_prev "echo hey ya; echo hey hi; printf `expr 10 + 10`'\n' ; printf $((10 + 20))'\n'"
Now, bash saves the variable even outside previous scope, so the arr variable created in get_prev function is accessible even outside the function in the main script:
#get previous command outputs in arr
for((i=0; i<${#arr[#]}; i++))
do
echo ${arr[i]}
done
#if you're sure that your output won't have escape sequences you bother about, you may simply print the array
printf "${arr[*]}\n"
Edit:
I use the following code in my implementation:
get_prev()
{
usage()
{
echo "Usage: alphabet [ -h | --help ]
[ -s | --sep SEP ]
[ -v | --var VAR ] \"command\""
}
ARGS=$(getopt -a -n alphabet -o hs:v: --long help,sep:,var: -- "$#")
if [ $? -ne 0 ]; then usage; return 2; fi
eval set -- $ARGS
local var="arr"
IFS=$(echo -en '\n\b')
for arg in $*
do
case $arg in
-h|--help)
usage
echo " -h, --help : opens this help"
echo " -s, --sep : specify the separator, newline by default"
echo " -v, --var : variable name to put result into, arr by default"
echo " command : command to execute. Enclose in quotes if multiple lines or pipelines are used."
shift
return 0
;;
-s|--sep)
shift
IFS=$(echo -en $1)
shift
;;
-v|--var)
shift
var=$1
shift
;;
-|--)
shift
;;
*)
cmd=$option
;;
esac
done
if [ ${#} -eq 0 ]; then usage; return 1; fi
ERROR=$( { eval "$*" > /tmp/out; } 2>&1 )
if [ $ERROR ]; then echo $ERROR; return 1; fi
local a=()
exec 3</tmp/out
while read -u 3 -r line
do
a+=($line)
done
exec 3<&-
eval $var=\(\${a[#]}\)
print_arr $var # comment this to suppress output
}
print()
{
eval echo \${$1[#]}
}
print_arr()
{
eval printf "%s\\\n" "\${$1[#]}"
}
Ive been using this to print space-separated outputs of multiple/pipelined/both commands as line separated:
get_prev -s " " -v myarr "cmd1 | cmd2; cmd3 | cmd4"
For example:
get_prev -s ' ' -v myarr whereis python # or "whereis python"
# can also be achieved (in this case) by
whereis python | tr ' ' '\n'
Now tr command is useful at other places as well, such as
echo $PATH | tr ':' '\n'
But for multiple/piped commands... you know now. :)
-Himanshu
Like konsolebox said, you'd have to hack into bash itself. Here is a quite good example on how one might achieve this. The stderred repository (actually meant for coloring stdout) gives instructions on how to build it.
I gave it a try: Defining some new file descriptor inside .bashrc like
exec 41>/tmp/my_console_log
(number is arbitrary) and modify stderred.c accordingly so that content also gets written to fd 41. It kind of worked, but contains loads of NUL bytes, weird formattings and is basically binary data, not readable. Maybe someone with good understandings of C could try that out.
If so, everything needed to get the last printed line is tail -n 1 [logfile].
Not sure exactly what you're needing this for, so this answer may not be relevant. You can always save the output of a command: netstat >> output.txt, but I don't think that's what you're looking for.
There are of course programming options though; you could simply get a program to read the text file above after that command is run and associate it with a variable, and in Ruby, my language of choice, you can create a variable out of command output using 'backticks':
output = `ls` #(this is a comment) create variable out of command
if output.include? "Downloads" #if statement to see if command includes 'Downloads' folder
print "there appears to be a folder named downloads in this directory."
else
print "there is no directory called downloads in this file."
end
Stick this in a .rb file and run it: ruby file.rb and it will create a variable out of the command and allow you to manipulate it.
If you don't want to recompute the previous command you can create a macro that scans the current terminal buffer, tries to guess the -supposed- output of the last command, copies it to the clipboard and finally types it to the terminal.
It can be used for simple commands that return a single line of output (tested on Ubuntu 18.04 with gnome-terminal).
Install the following tools: xdootool, xclip , ruby
In gnome-terminal go to Preferences -> Shortcuts -> Select all and set it to Ctrl+shift+a.
Create the following ruby script:
cat >${HOME}/parse.rb <<EOF
#!/usr/bin/ruby
stdin = STDIN.read
d = stdin.split(/\n/)
e = d.reverse
f = e.drop_while { |item| item == "" }
g = f.drop_while { |item| item.start_with? "${USER}#" }
h = g[0]
print h
EOF
In the keyboard settings add the following keyboard shortcut:
bash -c '/bin/sleep 0.3 ; xdotool key ctrl+shift+a ; xdotool key ctrl+shift+c ; ( (xclip -out | ${HOME}/parse.rb ) > /tmp/clipboard ) ; (cat /tmp/clipboard | xclip -sel clip ) ; xdotool key ctrl+shift+v '
The above shortcut:
copies the current terminal buffer to the clipboard
extracts the output of the last command (only one line)
types it into the current terminal
I have an idea that I don't have time to try to implement immediately.
But what if you do something like the following:
$ MY_HISTORY_FILE = `get_temp_filename`
$ MY_HISTORY_FILE=$MY_HISTORY_FILE bash -i 2>&1 | tee $MY_HISTORY_FILE
$ some_command
$ cat $MY_HISTORY_FILE
$ # ^You'll want to filter that down in practice!
There might be issues with IO buffering. Also the file might get too huge. One would have to come up with a solution to these problems.
I think using script command might help. Something like,
script -c bash -qf fifo_pid
Using bash features to set after parsing.
Demo for non-interactive commands only: http://asciinema.org/a/395092
For also supporting interactive commands, you'd have to hack the script binary from util-linux to ignore any screen-redrawing console codes, and run it from bashrc to save your login session's output to a file.
You can use -exec to run a command on the output of a command. So it will be a reuse of the output as an example given with a find command below:
find . -name anything.out -exec rm {} \;
you are saying here -> find a file called anything.out in the current folder, if found, remove it. If it is not found, the remaining after -exec will be skipped.
I'm getting this error
Syntax error: redirection unexpected
in the line:
if grep -q "^127.0.0." <<< "$RESULT"
How I can run this in Ubuntu?
<<< is a bash-specific redirection operator (so it's not specific to Ubuntu). The documentation refers to it as a "Here String", a variant of the "Here Document".
3.6.7 Here Strings
A variant of here documents, the format is:
<<< word
The word is expanded and supplied to the command on its
standard input.
A simple example:
$ cat <<< hello
hello
If you're getting an error, it's likely that you're executing the command using a shell other than bash. If you have #!/bin/sh at the top of your script, try changing it to #!/bin/bash.
If you try to use it with /bin/sh, it probably assumes the << refers to a "here document", and then sees an unexpected < after that, resulting in the "Syntax error: redirection unexpected" message that you're seeing.
zsh and ksh also support the <<< syntax.
if grep -q "^127.0.0." <<< "$RESULT"
then
echo IF-THEN
fi
is a Bash-specific thing. If you are using a different bourne-compatable shell, try:
if echo "$RESULT" | grep -q "^127.0.0."
then
echo IF-THEN
fi
It works for me on Ubuntu, if I complete you IF block:
if grep -q "^127.0.0." <<< "$RESULT"; then echo ""; fi
When I try to use the read command in Bash like this:
echo hello | read str
echo $str
Nothing echoed, while I think str should contain the string hello. Can anybody please help me understand this behavior?
The read in your script command is fine. However, you execute it in the pipeline, which means it is in a subshell, therefore, the variables it reads to are not visible in the parent shell. You can either
move the rest of the script in the subshell, too:
echo hello | { read str
echo $str
}
or use command substitution to get the value of the variable out of the subshell
str=$(echo hello)
echo $str
or a slightly more complicated example (Grabbing the 2nd element of ls)
str=$(ls | { read a; read a; echo $a; })
echo $str
Other bash alternatives that do not involve a subshell:
read str <<END # here-doc
hello
END
read str <<< "hello" # here-string
read str < <(echo hello) # process substitution
Typical usage might look like:
i=0
echo -e "hello1\nhello2\nhello3" | while read str ; do
echo "$((++i)): $str"
done
and output
1: hello1
2: hello2
3: hello3
The value disappears since the read command is run in a separate subshell: Bash FAQ 24
To put my two cents here: on KSH, reading as is to a variable will work, because according to the IBM AIX documentation, KSH's read does affects the current shell environment:
The setting of shell variables by the read command affects the current shell execution environment.
This just resulted in me spending a good few minutes figuring out why a one-liner ending with read that I've used a zillion times before on AIX didn't work on Linux... it's because KSH does saves to the current environment and BASH doesn't!
I really only use read with "while" and a do loop:
echo "This is NOT a test." | while read -r a b c theRest; do
echo "$a" "$b" "$theRest"; done
This is a test.
For what it's worth, I have seen the recommendation to always use -r with the read command in bash.
You don't need echo to use read
read -p "Guess a Number" NUMBER
Another alternative altogether is to use the printf function.
printf -v str 'hello'
Moreover, this construct, combined with the use of single quotes where appropriate, helps to avoid the multi-escape problems of subshells and other forms of interpolative quoting.
Do you need the pipe?
echo -ne "$MENU"
read NUMBER