I'm working with Mac OS X's pbpaste command, which returns the clipboard's contents. I'd like to create a shell script that executes each line returned by pbpaste as a separate bash command. For example, let's say that the clipboard's contents consists of the following lines of text:
echo 1234 >~/a.txt
echo 5678 >~/b.txt
I would like a shell script that executes each of those lines, creating the two files a.txt and b.txt in my home folder. After a fair amount of searching and trial and error, I've gotten to the point where I'm able to assign individual lines of text to a variable in a while loop with the following construct:
pbpaste | egrep -o [^$]+ | while read l; do echo $l; done
which sends the following to standard out, as expected:
echo 1234 >~/a.txt
echo 5678 >~/b.txt
Instead of simply echoing each line of text, I then try to execute them with the following construct:
pbpaste | egrep -o [^$]+ | while read l; do $l; done
I thought that this would execute each line (thus creating two text files a.txt and b.txt in my home folder). Instead, the first term (echo) seems to be interpreted as the command, and the remaining terms (nnnn >~/...) seem to get lumped together as if they were a single parameter, resulting in the following being sent to standard out without any files being created:
1234 >~/a.txt
5678 >~/b.txt
I would be grateful for any help in understanding why my construct isn't working and what changes might get it to work.
[…] the remaining terms (nnnn >~/...) seem to get lumped together as if they were a single parameter, […]
Not exactly. The line actually gets split on whitespace (or whatever $IFS specifies), but the problem is that the redirection operator > cannot be taken from a shell variable. For example, this snippet:
gt='>'
echo $gt foo.txt
will print > foo.txt, rather than printing a newline to foo.txt.
And you'll have similar problems with various other shell metacharacters, such as quotation marks.
What you need is the eval builtin, which takes a string, parses it as a shell command, and runs it:
pbpaste | egrep -o [^$]+ | while IFS= read -r LINE; do eval "$LINE"; done
(The IFS= and -r and the double-quotes around $LINE are all to prevent any other processing besides the processing performed by eval, so that e.g. whitespace inside quotation marks will be preserved.)
Another possibility, depending on the details of what you need, is simply to pipe the commands into a new instance of Bash:
pbpaste | egrep -o [^$]+ | bash
Edited to add: For that matter, it occurs to me that you can pass everything to eval in a single batch; just as you can (per your comment) write pbpaste | bash, you can also write eval "$(pbpaste)". That will support multiline while-loops and so on, while still running in the current shell (useful if you want it to be able to reference shell parameters, to set environment variables, etc., etc.).
Related
Each line in a given file 'a.txt' contains the directory/path to another unique file. Suppose we want to parse 'a.txt' line-by-line, extract the path in string format, and then use a tool such as vim to process the file at this path, and so on.
After going through this thread - Read a file line by line assigning the value to a variable, I wrote the following script, say 'open-file.sh' on bash (I'm new to it)
#!/bin/bash
while IFS='' read -r line || [[ -n "$line" ]]; do
vim -c ":q" -cq $line # Just open the file and close it using :q
done < "$1"
We would then run the above script as -
./open-file.sh a.txt
The problem is that although the path to a new file is correctly specified by $line, when vim opens the file, vim continues to receive the text contained in 'a.txt' as a command. How can I write a script where I can correctly obtain the path from 'a.txt', open it using vim, and then continue parsing the remaining lines in 'a.txt' ?
Replace:
vim -c ":q" -cq $line
With:
vim -c ":q" -cq "$line" </dev/tty
The redirection </dev/tty tells vim to take its standard input from the terminal. Without that, the standard input for vim is "$1".
Also, it is good practice to put $line in double-quotes to protect it from word splitting, etc.
Lastly, while vim is excellent for interactive work, if your end-goal is fully automated processing of each file, you might want to consider tools such as sed or awk.
Although I'm not sure of your ultimate goal, this shell command will execute vim once per line in a.txt:
xargs -o -n1 vim -c ':q' < a.txt
As explained in the comments to Read a file line by line assigning the value to a variable, the issue you're encountering is due to the fact that vim is an interactive program and thus continues to read input from $line.
The problem was already mentioned in a comment under the answer you based your script on.
vim is consuming stdin which is given to the loop by done < $1. We can observe the same behavior in the following example:
$ while read i; do cat; done < <(seq 3)
2
3
<(seq 3) simulates a file with the three lines 1, 2, and 3. Instead of three silent iterations we get only one iteration and the output 2 and 3.
stdin is not only passed to read in the head of the loop, but also to cat in the body of the loop. Therefore read reads one line, the loop is entered, cat reads all remaining lines, stdin is empty, read has nothing to read anymore, the loop exits.
You could circumvent the problem by redirecting something to vim, however there is an even better way. You don't need the loop at all:
< "$1" xargs -d\\n -n1 vim -c :q -cq
xargs will execute vim once for every line in the file given by $1.
In my program I need to know the maximum number of process I can run. So I write a script. It works when I run it in shell but but when in program using system("./limit.sh"). I work in bash.
Here is my code:
#/bin/bash
LIMIT=\`ulimit -u\`
ACTIVE=\`ps -u | wc -l \`
echo $LIMIT > limit.txt
echo $ACTIVE >> limit.txt
Anyone can help?
Why The Original Fails
Command substitution syntax doesn't work if escaped. When you run:
LIMIT=\`ulimit -u\`
...what you're doing is running a command named
-u`
...with the environment variable named LIMIT containing the value
`ulimit
...and unless you actually have a command that starts with -u and contains a backtick in its name, this can be expected to fail.
This is because using backticks makes characters which would otherwise be syntax into literals, and running a command with one or more var=value pairs preceding it treats those pairs as variables to export in the environment for the duration of that single command.
Doing It Better
#!/bin/bash
limit=$(ulimit -u)
active=$(ps -u | wc -l)
printf '%s\n' "$limit" "$active" >limit.txt
Leave off the backticks.
Use modern $() command substitution syntax.
Avoid multiple redirections.
Avoid all-caps names for your own variables (these names are used for variables with meaning to the OS or system; lowercase names are reserved for application use).
Doing It Right
#!/bin/bash
exec >limit.txt # open limit.txt as output for the rest of the script
ulimit -u # run ulimit -u, inheriting that FD for output
ps -u | wc -l # run your pipeline, likewise with output to the existing FD
You have a typo on the very first line: #/bin/bash should be #!/bin/bash - this is often known as a "shebang" line, for "hash" (#) + "bang" (!)
Without that syntax written correctly, the script is run through the system's default shell, which will see that line as just a comment.
As pointed out in comments, that also means only the standardised options available to the builtin ulimit command, which doesn't include -u.
I have a file with below commands
cat /some/dir/with/files/file1_name.tsv|awk -F "\\t" '{print $21$19$23$15}'
cat /some/dir/with/files/file2_name.tsv|awk -F "\\t" '{print $2$13$3$15}'
cat /some/dir/with/files/file3_name.tsv|awk -F "\\t" '{print $22$19$3$15}'
When i loop through the file to run the command, i get below error
cat file | while read line; do $line; done
cat: invalid option -- 'F'
Try `cat --help' for more information.
You are not executing the command properly as you intended it. Since you are reading line by line on the file (for unknown reason) you could call the interpreter directly as below
#!/bin/bash
# ^^^^ for running under 'bash' shell
while IFS= read -r line
do
printf "%s" "$line" | bash
done <file
But this has an overhead of creating a forking a new process for each line of the file. If your commands present under file are harmless and is safe to be run in one shot, you can just as
bash file
and be done with it.
Also for using awk, just do as below for each of the lines to avoid useless cat
awk -F "\\t" '{print $21$19$23$15}' file1_name.tsv
You are expecting the pipe (|) symbol to act as you are accustomed to, but it doesn't. To help you understand, try this :
A="ls / | grep e" # Loads a variable with a command with pipe
$A # Does not work
eval "$A" # Works
When expanding a variable without using eval, expansion and word splitting occurs after the shell interprets redirections and pipes, so your pipe symbol is seen just as a literal character.
Some options you have :
A) Avoid piping, by passing the file name as an argument
awk -F "\\t" '{print $21$19$23$15}' /some/dir/with/files/file1_name.tsv
B) Use eval as shown below, the potential security implications of which I would suggest you to research.
C) Put arguments in file and parse it, avoiding the use of eval, something like :
# Assumes arguments separated by spaces
IFS=" " read -r -a arguments;
awk "${arguments[#]-}"
D) Implement the parsing of your data files in Bash instead of awk, and use your configuration file to specify output without the need for expanding anything (e.g. by specifying fields to print separated by spaces).
The first three approaches involve some form of interpretation of outside data as code, and that comes with risks if the file used as input cannot be guaranteed safe. Approach C might be considered a bit better in that regard, but since the command you are calling is awk, an actual program is passed to awk, so whatever awk can do, an attacker (or careless user) with write access to your file can cause your script to do anything awk can do.
I am trying to write an fgrep statement removing records with a full record match from a file. I can do this on the command line, but not inside a ksh script. The code I am using boils down to these 4 lines of code:
Header='abc def|ghi jkl' #I use the head command to populate this variable
workfile=abc.txt
command="fgrep -Fxv \'$Header\' $workfile" >$outfile
$command
When I echo $command to STDIN the command is exactly what I would type on the command line (with the single quotes) and that works on the command line. When I execute it within the ksh script (file) the single quotes seem not to be recognized because the errors show it is parsing on spaces.
I have tried back ticks, exec, eval, double quotes instead of single quotes, and not using the $command variable. The problem remains.
I can do this on the command line, but not inside a ksh script
Here's a simple, portable, reliable solution using a heredoc.
#!/usr/bin/env ksh
workfile=abc.txt
outfile=out.txt
IFS= read -r Header <<'EOF'
abc def|ghi jul
EOF
IFS= read -r command <<'EOF'
grep -Fxv "$Header" "$workfile" > "$outfile"
EOF
eval "$command"
Explanation :
(Comments can't be added to the script above because they would affect the lines in the heredoc)
IFS= read -r Header <<'EOF' # Line separated literal strings
abc def|ghi jul # Set into the $Header variable
EOF # As if it were a text file
IFS= read -r command <<'EOF' # Command to execute
grep -Fxv "$Header" "$workfile" > "$outfile" # As if it were typed into
EOF # the shell command line
eval "$command" # Execute the command
The above example is the same as having a text file called header.txt, which contains the contents: abc def|ghi jul and typing the following command:
grep -Fxvf header.txt abc.txt
The heredoc addresses the problem of the script operating differently than the command line as a result of quoting/expansions/escaping issues.
A Word of caution regarding eval:
The use of eval in this example is specific. Please see Eval command and security issues for information on how eval can be misused and cause potentially very damaging results.
More Detail / Alternate Example:
For the sake of completeness, clarity, and ability to apply this concept to other situations, some notes about the heredoc and an alternative demonstration:
This implementation of the heredoc in this example is specifically designed with the following criteria:
Literal string assignment of contents, to the variables (using 'EOF')
Use of the eval command to evaluate and execute the referenced variables within the heredoc itself.
File or heredoc ?
One strength of using a heredoc combined with grep -F (fgrep), is the ability to treat a section of the script as if it were a file.
Case for file:
You want to frequently paste "pattern" lines into the file, and remove them as necessary, without having to modify the script file.
Case for heredoc:
You apply the script in an environment where specific files already exist, and you want to match specific exact literal patterns against it.
Example:
Scenario: I have 5 VPS Servers, and I want a script to produce a new fstab file but to ensure it doesn't contain the exact line:
/dev/xvda1 / ext3 errors=remount-ro,noatime,barrier=0 0 1
This scenario fits the type of situation addressed in this question. I could use the boilerplate from the above code in this answer and modify it as following:
#!/usr/bin/env ksh
workfile=/etc/fstab
IFS= read -r Header <<'EOF'
/dev/xvda1 / ext3 errors=remount-ro,noatime,barrier=0 0 1
EOF
IFS= read -r command <<'EOF'
grep -Fxv "$Header" "$workfile"
EOF
eval "$command"
This would give me a new fstab file, without the line contained in the heredoc.
Bash FAQ #50: I'm trying to put a command in a variable, but the complex cases always fail! provides comprehensive guidance - while it is written for Bash, most of it applies to Ksh as well.[1]
If you want to stick with storing your command in a variable (defining a function is the better choice), use an array, which bypasses the quoting issues:
#!/usr/bin/env ksh
Header='abc def|ghi jkl'
workfile=abc.txt
# Store command and arguments as elements of an array
command=( 'fgrep' '-Fxv' "$Header" "$workfile" )
# Invoke the array as a command.
"${command[#]}" > "$outfile"
Note: only a simple command can be stored in an array, and redirections can't be part of it.
[1] The function examples use local to create local variables, which ksh doesn't support. Omit local to make do with shell-global variables instead, or use function <name> {...} syntax with typeset instead of local to declare local variables in ksh.
I have a script named password-for-object which I normally run like that:
$ password-for-object example.com
sOtzC0UY1K3EDYp8a6ltfA
I.e. it does an intricate hash calculation and outputs a password that I should use when accessing an object (for example, website) named example.com. I'll just double click the whole password, it gets copied into my buffer and I'll paste it into the form.
I've also learnt a trick on how to use such a script without making my password visible:
$ password-for-object example.com | xclip
This way output of a script ends up in X's primary buffer and I can insert it right into password field in the form and it's not shown on the screen.
The only problem with this way is that password-for-object outputs a string with trailing newline and thus "xclip" always catches up an extra symbol - this newline. If I omit output of newline in password-for-object, then I'll end up with messed up string without xclip, i.e. when I'm just putting it on the stdout. I use 2 shells: zsh and bash, and I'll get the following in zsh (note the extra % sign):
$ password-for-object example.com
sOtzC0UY1K3EDYp8a6ltfA%
$
Or the following in bash (note that prompt would be started on the same line):
$ password-for-object example.com
sOtzC0UY1K3EDYp8a6ltfA$
Any ideas on how to work around this issue? Is it possible to modify the script in a way so it will detect that xclip is in the pipeline and only output newline if it isn't?
If you change password-for-object so that it doesn't output a newline, you can call it with a script like:
#!/bin/bash
password-for-object "$1"
if [ -t 1 ]
then
echo
fi
The -t condition is described in the bash manual as:
-t fd
True if file descriptor fd is open and refers to a terminal.
See the following question:
How to detect if my shell script is running through a pipe?
Give this a try:
$ password-for-object example.com | tr -d '\n' | xclip
tr -d '\n' deletes the newline