I am trying to select elements in a list of files using a sequence. For this I have created a list of all my files following a pattern in a folder:
ls -d -1 /my/path/S1*.zip Where the output is:
/my/path/S1A_IW_SLC__1SDV_20180412T171648_20180412T171715_021437_024E95_BDA1.zip /my/path/S1A_IW_SLC__1SDV_20180424T171648_20180424T171715_021612_02540A_BB21.zip /my/path/S1A_IW_SLC__1SDV_20180506T171649_20180506T171716_021787_025996_98AB.zip /my/path/S1A_IW_SLC__1SDV_20180518T171649_20180518T171716_021962_025F27_A15C.zip /my/path/S1A_IW_SLC__1SDV_20180530T171650_20180530T171717_022137_0264C8_5D94.zip /my/path/S1A_IW_SLC__1SDV_20180611T171651_20180611T171718_022312_026A3D_BBFC.zip /my/path/S1A_IW_SLC__1SDV_20180623T171652_20180623T171719_022487_026F7C_450E.zip /my/path/S1A_IW_SLC__1SDV_20180623T171652_20180623T171719_543456_64324F_452W.zip I would like to create a forloop in which for every iteration, the files are processed in groups of two. So, in the first iteration, file in position 1 in my list and file in position 2 are processed. In the second interation, file in position 3 and file in position 4, etc.
To make it simpler, I will take the example of unzipping these files in grousp of 2:
for i in $(ls -d -1 /shared/Training/WROC0320_UrbanMapping_Germany/Original/S1*.zip) do unzip i unizp i+1 # Not sure if i+1 works like that in `bash` done The problem I see with this approach is that in the first iteration it will work (i will be 1 and 2) but in the secodn i will be 2 and 3 and it should be 3 and 4.
My question is then, how to iterate over the result of ls is such way?
2 Answers
In general, you should avoid iterating over the results of ls - see for example
Compared to a for loop, a while loop gives you much more control over delimiters and file selection. For example:
printf '%s\0' my/path/*.zip | while IFS= read -r -d '' f1 && IFS= read -r -d '' f2; do echo 'unzipping files'; echo unzip "$f1"; echo unzip "$f2" done unzipping files unzip my/path/S1A_IW_SLC__1SDV_20180412T171648_20180412T171715_021437_024E95_BDA1.zip unzip my/path/S1A_IW_SLC__1SDV_20180424T171648_20180424T171715_021612_02540A_BB21.zip unzipping files unzip my/path/S1A_IW_SLC__1SDV_20180506T171649_20180506T171716_021787_025996_98AB.zip unzip my/path/S1A_IW_SLC__1SDV_20180518T171649_20180518T171716_021962_025F27_A15C.zip unzipping files unzip my/path/S1A_IW_SLC__1SDV_20180530T171650_20180530T171717_022137_0264C8_5D94.zip unzip my/path/S1A_IW_SLC__1SDV_20180611T171651_20180611T171718_022312_026A3D_BBFC.zip unzipping files unzip my/path/S1A_IW_SLC__1SDV_20180623T171652_20180623T171719_022487_026F7C_450E.zip unzip my/path/S1A_IW_SLC__1SDV_20180623T171652_20180623T171719_543456_64324F_452W.zip Here printf '%s\0' my/path/S*.zip expands the shell glob into a list that is unambiguously delimited by the NULL character (the only character that may not appear in a Unix filepath), and then each read command reads one of them. The IFS= unsets the field separator - the only effect of this is to prevent the reads from stripping leading whitespace (in the very unlikely event that you need to handle such names).
A possibly cleaner way, using the set builtin with a shell glob to read the file names unambiguously into the positional parameter array $@:
shopt -s nullglob set -- my/path/S*.zip while (($# > 1)); do echo "unzipping files"; echo unzip "$1"; echo unzip "$2"; shift 2 done Loop a version sorted array with step of two.
#!/bin/bash eval \ "a=($(ls -v1 --quoting-style=shell-escape files/*))" for ((i=0; i<${#a[@]}; i+=2)); do [[ -r ${a[$i]} ]] && \ unzip "${a[$i]}" [[ -r ${a[(i+1)]} ]] && \ unzip "${a[(i+1)]}" done 
