Yet again I managed to cut and paste something from a source then execute that paste with modifications, but once again I had an error that made the issue truly opaque - the non-ascii double quotes were generated by the intermediary application that wanted to be "fancy", resulting in a derailed thought process for a few minutes while I studied the command line for the changes I made - which are usually the cause of problems.
Is there a script, terminal mode or some other solution that detects bash/zsh input and gives a nice clear error message if non-ascii input is detected in the command line? Granted, it will need an override, but the usual case is that non-ascii characters are not desired on the command line.
Say you have a shell command like
cat file1 | ./my_script
Is there any way from inside the 'my_script' command to detect the command run first as the pipe input (in the above example cat file1)?
I've been digging into it and so far I've not found any possibilities.
I've been unable to find any environment variables set in the process space of the second command recording the full command line, the command data the my_script commands sees (via /proc etc) is just _./my_script_ and doesn't include any information about it being run as part of a pipe. Checking the process list from inside the second command even doesn't seem to provide any data since the first process seems to exit before the second starts.
The best information I've been able to find suggests in bash in some cases you can get the exit codes of processes in the pipe via PIPESTATUS, unfortunately nothing similar seems to be present for the name of commands/files in the pipe. My research seems to be saying it's impossible to do in a generic manner (I can't control how people decide to run my_script so I can't force 3rd party pipe replacement tools to be used over build in shell pipes) but it just at the same time doesn't seem like it should be impossible since the shell has the full command line present as the command is run.
(update adding in later information following on from comments below)
I am on Linux.
I've investigated the /proc/$$/fd data and it almost does the job. If the first command doesn't exit for several seconds while piping data to the second command can you read /proc/$$/fd/0 to see the value pipe:[PIPEID] that it symlinks to. That can then be used to search through the rest of the /proc//fd/ data for other running processes to find another process with a pipe open using the same PIPEID which gives you the first process pid.
However in most real world tests I've done of piping you can't trust that the first command will stay running long enough for the second one to have time to locate it's pipe fd in /proc before it exits (which removes the proc data preventing it being read). So if this method will return any information is something I can't rely on.
I'm trying to write a simple shell script to start a program running in the background and redirect all its output to a log file, but I've come across a case where it silently fails and kills the program.
It happens if the file that's being redirected into is modified elsewhere. I've tried recreating the situation using the ping command so I can narrow the problem down to not being my program's code, but that fails in a different way - it stops writing to the file, but the terminal still acts as though the ping is running.
This is the essence of my code:
sh "$program" > "$file"
I'm trying to create a shell script via cygwin that will automatically build an executable and run it. It's a very simple format of
#!/bin/bash
gcc test.c -o hello
./hello.exe
When I enter the 2nd and 3rd lines separately, everything works normally. However, if I save those 3 lines into a .sh file, the resulting .exe built has some extra character added in that will always throw off the last line.
hello.exe
I can't even replicate the file name because no tool, including the character map/MS word/other ASCII tools online will give me any result. Some online tool gave me the ASCII result , but as far as I can tell that doesn't correspond to anything meaningful. How can I avoid this problem in my shell script?
Very likely you have Windows linefeeds in the .sh file. Make sure you have Unix linefeeds.
I ask because I recently made a change to a KornShell (ksh) script that was executing. A short while after I saved my changes, the executing process failed. Judging from the error message, it looked as though the running process had seen some -- but not all -- of my changes. This strongly suggests that when a shell script is invoked, the entire script is not read into memory.
If this conclusion is correct, it suggests that one should avoid making changes to scripts that are running.
$ uname -a
SunOS blahblah 5.9 Generic_122300-61 sun4u sparc SUNW,Sun-Fire-15000
No. Shell scripts are read either line-by-line, or command-by-command followed by ;s, with the exception of blocks such as if ... fi blocks which are interpreted as a chunk:
A shell script is a text file containing shell commands. When such a
file is used as the first non-option argument when invoking Bash, and
neither the -c nor -s option is supplied (see Invoking Bash), Bash
reads and executes commands from the file, then exits. This mode of
operation creates a non-interactive shell.
You can demonstrate that the shell waits for the fi of an if block to execute commands by typing them manually on the command line.
http://www.gnu.org/software/bash/manual/bashref.html#Executing-Commands
http://www.gnu.org/software/bash/manual/bashref.html#Shell-Scripts
It's funny that most OS'es I know, do NOT read the entire content of any script in memory, and run it from disk. Doing otherwise would allow making changes to the script, while running. I don't understand why that is done, given the fact :
scripts are usually very small (and don't take many memory anyway)
at some point, and shown in this thread, people would start making changes to a script that is already running anyway
But, acknowledging this, here's something to think about: If you decided that a script is not running OK (because you are writing/changing/debugging), do you care on the rest of the running of that script ? you can go ahead making the changes, save them, and ignore all output and actions, done by the current run.
But .. Sometimes, and that depends on the script in question, a subsequent run of the same script (modified or not), can become a problem since the current/previous run is doing an abnormal run. It would typically skip some stuff, or sudenly jump to parts in the script, it shouldn't. And THAT may be a problem. It may leave "things" in a bad state; particularly if file manipulation/creation is involved.
So, as a general rule : even if the OS supports the feature or not, it's best to let the current run finish, and THEN save the updated script. You can change it already, but don't save it.
It's not like in the old days of DOS, where you actually have only one screen in front of you (one DOS screen), so you can't say you need to wait on run completion, before you can open a file again.
No they are not and there are many good reasons for that.
One of the things you should keep in mind is that a shell is not an interpreter even if there are some similarities. Shells are designed to work with a stream of commands. Either from the TTY ,a PIPE, FIFO or even a socket.
The shell reads from its resource line by line until a EOF is returned by the kernel.
The most shells have no extra support for interpreting files. they work with a file as they would work with a terminal.
In fact this is considered to be a nice feature because you can do interesting stuff like this How do Linux binary installers (.bin, .sh) work?
You can use a binary file and prepend shell scripts. You can't do this with an interpreter. because it parses the whole file or at least it would try it and fail. A shell would just interpret it line by line and doesnt care about the garbage at the end of the file. You just have to make sure the execution of the script gets terminated before it reaches the binary part.