这是在外壳程序脚本中使用cd和cd-命令在另一个目录中进行某些计算的好方法吗

Question

I use cd in my shell script to enter into a directory for some computation and again use cd - to come back. Is it a good way to use? My script is:

input_dir=/home/abc/2001/01/
cd $input_dir
#Execute some programm with ifile.txt e.g.
awk '$1 > 99 {printf "%.2f" "$1"}' ifile.txt > ofile.tx 
cd -

or we should always mention the path name? like:

input_dir=/home/abc/2001/01/
awk '$1 > 99 {printf "%.2f" "$1"}' $input_dir/ifile.txt > $input_dir/ofile.txt

Can you please suggest if there is anything easy way to reduce the texts?

Answer 1

Put it in a subshell:

(cd "$input_dir" && exec awk '$1 > 99 {printf "%.2f" "$1"}' ifile.txt > ofile.tx)

Thus, when the subshell exits, you're automatically back to your original directory, because the cd only applied to that subshell (containing only the awk command).

The exec ensures that you're not incurring extra overhead, as it causes the subshell to replace its process table entry with the awk . (Some shells will do this automatically for the last command inside a subshell).

Answer 2

A common technique to limit the scope of cd for a given computation is to run the command in a subshell, like this:

(cd "${inputdir}" && awk '…')

This is however not always applicable, because the subshell has limited ways to to communicate with the parent shell. The subshell introduce a frontier confining manipulations involving the following resources:

• job control
• variables
• of SIGNAL handlers

If you need the computation occurring in the directory to interact with these resources of the parent script , you still can still delimit the scope of the cd with a subshell provided that you:

• either use a named FIFO to let the child notify required job control, variables, signal manipulation to the parent script

• or rewrite the computation so that only input and simple processing occur in the subshell, while the parent shell reads and interpret the results through an anonymous pipe.