University:
Boston University
Course:
MET CS 201 | Introduction to Programming
Academic year:

2022
Views:

441

Pages:

22
Author:

Gironetzbk3n

Control Structures Loops, Conditionals, and Tests Loops ● for ○ Expand expr and execute commands once for each member in the resultant list, with name bound to the current member. ● while ○ Execute consequent-commands as long as test-commands has an exit status of zero. ● until ○ Execute consequent-commands as long as test-commands has an exit status which is not zero. for (( expr )) ; do commands done while test-commands; do consequent-commands done until test-commands; do consequent-commands done For Loop (Simple) ● A simple countdown ● Components: ○ ○ ○ ○ The “i” becomes our iterating variable “$i” List expansion of {5..1} is 5 4 3 2 1 “echo” command prints line “sleep” command waits for 1 second ● Take each item, one at a time, perform operation in loop. Advance until end of list scc1 $ \ for i in {5..1}; do echo "$i seconds left" sleep 1s done 5 4 3 2 1 seconds seconds seconds seconds seconds scc1 $ left left left left left For Loop (In Practice) Let’s iterate on something more interesting ● Input Items can be called with $@ scc1 $ bash forloop1.sh a b c a b c #!/bin/bash # This loop iterates over input items for input in "$@"; do echo "$input" done scc1 $ bash forloop1.sh a "b c" d a b c d For Loop (In Practice) #!/bin/bash # This script takes one argument, a # directory, and prints the basename of # contents. echo $0 echo "" echo $1 for doc in "$1"/*; do shortname=$(basename $doc) # now that we have the name, we # could do something interesting echo " $shortname" done scc1 $ bash forloop2.sh ~/bash forloop2.sh /usr3/bustaff/cjahnke/bash forloop1.sh forloop2.sh myscript.sh Syntax - Best Practice for content in *; do echo “$content” done for content in * do echo “$content” done For content in * do echo “$content” done For content in *; do echo “$content” ; done Conditional Constructs ● test “[[ .. ]]” ○ Evaluates expression inside brackets and returns 0 (TRUE) or 1 (FALSE) ● if ○ Executes commands following conditional logic. ● case ○ Selectively execute commands corresponding to pattern matching. ○ Like if/then statements, but usually used for parsing inputs and determining flow. ● select ○ Used for creating user input/selectable menus, executes commands on selection. ● Arithmetic “(( .. ))” ○ Will perform arithmetic. Use caution, precision can be tricky. Tests “[[ .. ]]” Double square brackets return an exit status of 0 (true) or 1* (false) depending on the evaluation of the conditional expression inside. ● Standard Test ○ [[ expression ]] ● Negative Test ○ [[ ! expression ]] ● AND Test ○ [[ expression1 && expression2 ]] ● OR Test ○ [[ expression1 || expression2 ]] scc1 $ [[ 1 == 1 ]] ; echo $? 0 scc1 $ [[ 1 == 2 ]] ; echo $? 1 scc1 $ [[ ! cow == dog ]]; echo $? 0 scc1 $ [[ 1 == 2 && cow == cow ]]; echo $? 1 scc1 $ [[ 1 == 1 || cow == dog ]]; echo $? 0 * Anything >=1 is considered false. Programs may have many possible exit codes. 0 is success, everything else is a descriptive error. If Statement (Simple) ● An “if“ statement executes commands based on conditional tests. ● The “then” keyword begins commands to execute if conditional is true. ● An “elif” keyword can extend an if statement for multiple conditions. ○ The tests are performed in order. Only the first true test is run. ● A catch-all “else” keyword is used to execute commands if no conditions are met. ● The “fi” keyword closes the statement if test-commands; then consequent-commands; elif more-test-commands; then more-consequents; else alternate-consequents; fi If-Then in Practice Let’s say we are in a directory with the following objects: ● ● ● ● TheJungleBook.txt d newfile.sh test.qsub I can iterate through all the files. scc1 $ ls TheJungleBook.txt d newfile.sh scc1 $ \ for contents in *; do if [[ -f "$contents" ]] ; then echo "$contents" is a file elif [[ -d "$contents" ]]; then echo $contents is a dir else echo "not identified" fi done If it is a file, echo that it is a file If it is a directory, echo that it is a directory test.qsub TheJungleBook.txt is a file d is a dir newfile.sh is a file test.qsub is a file practice some loops First get the sample files $ cp /projectnb/scv/bash_examples.tar . $ tar xf bash_examples.tar $ cd bash_examples $ ls answer_scripts numbers rebuildSentence Each file has a word from a sentence, try to reconstruct the sentence Each file has a word from a sentence, try to reconstruct the sentence for task in {0..13}; do cat "$task".txt >> file done tr '\n' ' ' < file Each file has a word from a sentence, try to reconstruct the sentence for task in {0..13}; do cat “$task”.txt >> file done tr '\n' ' ' < file returns: Scripting in bash makes many many things much easier, like putting this sentence together. SCC Job Submission Example using a loop to submit jobs on SCC with names. step 1 create a file with the names $ for file in *_1.txt; do echo "$file" >> filenames.txt; done $ cat filenames.txt AG_1.txt aA_1.txt ab_1.txt ac_1.txt ad_1.txt af_1.txt ag_1.txt ah_1.txt ai_1.txt aj_1.txt order_1.txt outof_1.txt using a loop to submit jobs on SCC with names. step 1 create a file with the names step 2 get the number of filenames $ for file in *_1.txt; do echo "$file" >> filenames.txt; done $ cat filenames.txt AG_1.txt aA_1.txt ab_1.txt ac_1.txt ad_1.txt af_1.txt ag_1.txt ah_1.txt ai_1.txt aj_1.txt order_1.txt outof_1.txt $ wc -l filenames.txt 12 filenames.txt using a loop to submit jobs on SCC with names. step 1 create a file with the names step 2 get the number of filenames step 3 create a submission script that accepts inputs (remember to chmod +x) #!/bin/bash -l #$ -P tutorial value1=$(cat "$1") value2=$(cat "$2") valueNew=$(( $value1 + $value2 )) echo "$1" Has a value of $value1 echo "$2" Has a value of $value2 echo These sum to $valueNew using a loop to submit jobs on SCC with names. step 1 create a file with the names step 2 get the number of filenames step 3 create a submission script that accepts inputs (remember to chmod +x) step 3a (if practical) test it locally step 3b test a single qsub $ ./fileadder.qsub aA_1.txt aA_2.num aA_1.txt Has a value of 30565 aA_2.num Has a value of 16775 These sum to 47340 $ qsub ./fileadder.qsub aA_1.txt aA_2.num Your job 6853253 ("fileadder.qsub") has been submitted using a loop to submit jobs on SCC with names. step 1 create a file with the names step 2 get the number of filenames step 3 create a submission script that accepts inputs (remember to chmod +x) step 3a (if practical) test it locally step 3b test a single qsub step 4 Create a file to loop the submission step 4a set up for a test the loop #!/bin/bash -l for i in {1..12}; do name=$(sed -n -e "$i p" filenames.txt) base=$(basename "$name" _1.txt) #qsub fileadder.qsub "$base"_1.txt "$base"_2.num fileadder.qsub "$base"_1.txt "$base"_2.num echo $i "$base" done using a loop to submit jobs on SCC with names. step 1 create a file with the names step 2 get the number of filenames step 3 create a submission script that accepts inputs (remember to chmod +x) step 3a (if practical) test it locally step 3b test a single qsub step 4 Create a file to loop the submission step 4a set for a test loop step 4b reset for submissions #!/bin/bash -l for i in {1..12}; do name=$(sed -n -e "$i p" filenames.txt) base=$(basename "$name" _1.txt) qsub fileadder.qsub "$base"_1.txt "$base"_2.num # fileadder.qsub "$base"_1.txt "$base"_2.num # echo $i "$base" done using a loop to submit jobs on SCC with names. step 1 create a file with the names step 2 get the number of filenames step 3 create a submission script that accepts inputs (remember to chmod +x) step 3a (if practical) test it locally step 3b test a single qsub step 4 Create a file to loop the submission step 4a set for a test loop step 4b reset for submissions step 5 submit $ ./submit_fileadder Your job 6853078 ("fileadder.qsub") Your job 6853079 ("fileadder.qsub") Your job 6853080 ("fileadder.qsub") Your job 6853081 ("fileadder.qsub") Your job 6853082 ("fileadder.qsub") Your job 6853083 ("fileadder.qsub") Your job 6853084 ("fileadder.qsub") Your job 6853085 ("fileadder.qsub") Your job 6853086 ("fileadder.qsub") Your job 6853087 ("fileadder.qsub") Your job 6853088 ("fileadder.qsub") Your job 6853089 ("fileadder.qsub") has has has has has has has has has has has has been been been been been been been been been been been been submitted submitted submitted submitted submitted submitted submitted submitted submitted submitted submitted submitted